ch4751_lecture notes

8/10/2019 CH4751_Lecture Notes

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Introductory Chemistry B CH4751

Lecture Notes 11-20

Dr. Erzeng Xue

CH4751 Lecture Notes 11-20 (Erzeng Xue)


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Chemical Reaction - Observation

Reaction (1) CH4+ 2O2 CO2+ 2H2O

Reaction (2) CH4+ CO2 2CO + 2H2

When carrying out these reactions we found that

at 400K (123C), the reaction (1) will proceed and reaction (2) will not

at 1000K(723C), reactions (1) & (2) both proceed; but

rxn (1) can go complete (until CH4orCO2consumed completely)

rxn (2) wont complete (with a feed CH4=CO2=1 & CO=H2=0, max. conv.=63% at 1000K)

The reaction (1) will give out heat, but the reaction (2) will require heat.

Why?

Chemical React ions

CH4751 Lecture Notes 11 (Erzeng Xue)


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Chemical Reaction Thermodynamics

Each molecule contains certain types and quantity of chemical energy

There is always energy change In a chemical reaction because of

breaking / reformation of chemical bonds

out-giving or in-taking heat

There are different energies associated with a substances & a reaction

(A systematic study of various forms of energy & their changes is called

Thermodynamics)

We will learn some of these energies

The meanings

How to get values / do simple calculate

How to use them as a tool to study chemical reactions

Chemical React ions


C 1 11 ( )


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The heat of form ation, H, (also called Enthalpy of Formationor Enthalpy)

His an energy associated with heat

His specific for each substance and is dependent of temperature & pressure

e.g. at 1000K: HCH4=-89, HO2=0, HCO2=-394, HH2O=-241, HCO=-111, HH2=0 (kJ/mol)

(Hvalues for various substances can be found in physical chemistry/Chem Eng handbooks)

In a reaction we are interested in the enthalpy change, DH,which is calculated using

For Rxn(1) CH4+ 2O2 CO2+ 2H2O DH1000=-801 kJ/mol

Rxn (2) CH4+ CO2 2CO + 2H2 DH1000=+260k J/mol

The meaning

When DH0, a reaction requires heat reaction is endothermic, as in rxn (2)

Chemical React ions

refers to standard pressure (1 atm.)

Temperature

reacT,iiprodT,iiT )Hv()Hv(H000 D


CH4751 L t N t 11 (E X )


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The Gibb s Free Energy , G, (also called Free Energy)

Gis a thermodynamic function related to a reaction. It is a function of H, T& S(entropy)

Gis specific for each substance & is a function of H, T& S(entropy)

e.g. at 1000K: GCH4=-+30, GO2=0, GCO2=-395, GH2O=-192, GCO=-200, GH2=0 (kJ/mol)

(Gvalues for various substances can be found in physical chemistry/Chem Eng handbooks)

The Gibbs Free energy change, DG,in a reaction can be calculated using

For Rxn(1) CH4+ 2O2 CO2+ 2H2O DG400DG1000=-801 kJ/mol

Rxn (2) CH4+ CO2 2CO + 2H2 DG400=+145, DG1000=-24 kJ/mol

Use of DG- Rxn(1) DG


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Example of DHTcalculation

CH4(g)+ 2O2(g) CO2(g)+ 2H2O(g) CH4+ CO2 2CO + 2H2

Coeff. 1 2 1 2 1 1 2 2

H400( -77 0 -393 -242 -77 -393 -110 0 kJ/mol

H1000 -89 0 -394 -248 -89 -394 -111 0 kJ/mol

Equation to use

D

H400 =[1x(-393)+2x(-242)]-[1x(-77)+2x(0)]= -800kJ/mol

D

H1000=[1x(-394)+2x(-248)]-[1x(-89)+2x(0)]=-801kJ/mol

Reaction (1) DH400 =[2x(-110)+2x(0)]-[1x(-77)+1x(-393)]=+250kJ/mol

Reaction (2) DH1000=[2x(-111)+2x(0)]-[1x(-89)+1x(-393)]= +260kJ/mol

Note: The heat of formation of single element gases (O2, H2, N2etc) is defined as zero.

Chemical React ions





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Example of DHTcalculation

CH4(g)+ 2O2(g) CO2(g)+ 2H2O(g) CH4+ CO2 2CO + 2H2

Coeff. 1 2 1 2 1 1 2 2

G400( -42 0 -394 -224 -42 -394 -146 0 kJ/mol

G1000 +19 0 -396 -193 +19 -396 -200 0 kJ/mol

Equation to use

DG400 =[1x(-394)+2x(-224)]-[1x(-42)+2x(0)]= -800 kJ/mol

D

G1000=[1x(-396)+2x(-193)]-[1x(+19)+2x(0)]= -801 kJ/mol

Reaction (1) D

G400 =[2x(-146)+2x(0)]-[1x(-42)+1x(-394)]=+144 kJ/mol

Reaction (2) DG1000=[2x(-200)+2x(0)]-[1x(19)+1x(-396)]= -23 kJ/mol

Note: The Gibbs Free Energy of single element gas (O2, H2, N2etc) is defined as zero.

Chemical React ions

reacT,iiprodT,iiT )Gv()Gv(G000 D




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The values of DGTand DH T

Equations

In both cases G and H values for the reactants and products have to be those at the

reaction temperature, indicated by the subscript.

For common substances, G and H values are given as a function of Tin handbooks - okay

For some less common substances, you may only find values at 298K, G298and H298

How do you convert values of G298and H298to those of GTand HT?

Here is the equations you can use to calculate the values of GTand HTfrom G298and H298

in which, STis the entropy and Cpis the heat capacity at constant pressure

Chemical React ions

reacT,iiprodT,iiT )Gv()Gv(G000 DreacT,iiprodT,iiT )Hv()Hv(H 000 D

298

0

298

0

DD i,changephase

T

i,p,iT,i HdTCHH

j

phaseT

i,pT,iT,iT,iT,iT

Q

T

dTCSSSTHG D 298

0

298

0000where-




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Summary

Will a reaction proceed in the direction specified?

Check DGTvalue of the reaction. The DGTvalue of a reaction can be calculated by

The G values of reactants / products can be found in literature. Remember

Is a reaction exothermic or endothermic?

Check DHTvalue of the reaction. The DHTvalue of a reaction can be calculated by

The H values of reactants / products can be found in literature

Chemical React ions

reacT,iiprodT,iiT )Gv()Gv(G000 D


reaction can proceed (but we dont know how fast it will be!)

reaction at equilibrium(no further change possible-dead state)

reaction will NOT proceed (or can proceed backward!)

0

0

0

T

T

T

G

G

G

for a reaction at

constant T, P,

C 475 ectu e Notes ( eng ue)



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Chemical Reaction Equilibrium

Until now we assume reaction A + B C + D goes to complete

Meaning a reaction only stops when either A or B is consumed completely

Experimental observations

Some reactions will cease without complete consumption of limiting reactant

Without altering reaction conditions (T, P, [ ]etc.) the ratio of conc. remains constant

After a change (T, P, [ ] etc.), the ratio of concs changes to another constant value

Example 1: NH3(aq)+ H2O(l) NH4+(l) + OH-(aq)

Follow concentrations of each component with time at constant Tand P,

at t

After changing T, a new constant ratio is established

If [NH3] is reduced the amount [NH4] decrease accordingly

while the ratio above remains constant

We say the reaction has reached equilibriumstate

Chemical React ions

constantO]][H[NH

]][OH[NH

23

4

t

[NH4]1

[NH3]1

[NH3]2

[NH4]2

( g )



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Experimental observations

Example 2: 2NO(g)+ O2(g) 2NO2(g)

Again at constant Tand P, when t

Reaction equilibrium is achieved when t

We say the reaction has reached equilibriumstate

The ratio of product concentration to reactants, with the stoichiometry coefficient as the

power index, is called react ion quot ient

When reaction quotient= constant value reaction reaches equilibrium

The value of reaction quotient at equilibrium, is called equi l ibr ium constant , Keq

At equilibrium, reactants may or may not be consumed completely

e.g. A feed gas mixture: NO=500ppm, O2=10%, N2=89.95%, achieves equilibria at the following Ts

Temperature / C 50 325 500

NO remaining at equil / ppm 0 96.5 390

NO conversion at equil / % 100 80.3 22

Chemical React ions

t

NO2

NO

O2

constant

2

2

2

2

ONO

NO

PP

P



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Important concepts of reaction equilibrium 2NO(g) + O2(g) D

2NO2(g)

Is the reaction between reactants still going on?

YES. Reaction goes forward as well as reverses.

At equilibrium: Rforward= Rreverse

though there is no NETchange of all conc.s

The equilibrium constant, Keq, has a meaning of

Keq=Rfo rward/ Rreverse

Changing Tcauses both Rfo rward& Rreverseto change, leading to a new Keq.

If Keq>>1, which means Rfo rward >> Rreverse, thereaction tends to go forward

If Keq>1).

Chemical React ions

2

2

2

2

ONO

NO

pPP

PK

t

NO2

NO

O2



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Equilibrium constant and Gibbs Free Energy

For reaction vAA + vBBDvCC + vDD

Remember: The value of DGdetermines the direction of reaction

No more change possible reaction in equilibrium DG= 0

Is the DGvalue related to the equilibrium constant?

YES. DG and Keqare related by the equation below (calculate one from the other)

DGT= - RTln(Keq)

Chemical React ions

reaction is spontaneous

reaction at equi l ibr ium(no further change possible)

reverse reaction is spontaneous

0

0

0

T

T

T

G

G

G

for a reaction at

constant T, P,



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Equilibrium constant - for different type of rxns

General form: vAA + vBB DvCC + vDD

gas phase 2NO(g) + O2(g) D2NO2(g)

gas-solid phase CaCO3(s) DCaO(s)+CO2(g)

liquid phase NH3(aq)+H2O(l) DNH4+(l)+OH-(aq)

liquid-solid Cu(OH)2(s) DCu2+(aq)+2OH-(aq)

gas-liquid NH3(g)+H2O(l) DNH4OH(aq)

Chemical React ions

)(][[A]

[D][C]Tf

PP

PP

BK

BA

DC

BA

DC

v

B

v

A

v

D

v

C

vv

vv

eq

for liquid phase rxn

for gas phase rxn

2

2

2

2

ONO

NO

pPP

PK

O]][H[NH

]][OH[NH

23

4

cK

2COp PK

22 ]][OH[Cu cK

31 NHp P/K

For reactions that have gas components, we normally use pressure to represent the concs

For reactions involves gas+liquid or gas+solid, only gas terms appear in the Keqexpression



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Le Chateliers Principle

When a system in equilibrium is subjected to an external stress, the system willestablish a new equilibrium, when possible, so as to minimise the external stress

Stresses: Changes in [ ], temperature or pressure

Example: N2(g) + 3H2(g) 2NH3(g) + heat (exothermic)

a) Effect of ([ ]). Increasing [ ] of substance shifts equil. in the direction of the long arrow

N2 + 3H2 2NH3 + heat



b) Effect of heat. Addition or removal of heat at constant temperature

Addition of heat: N2 + 3H2 2NH3 + heat

c) Effect of Pressure. (only affects reactions that have volume change before & after).

Increase in pressure: N2 + 3H2 2NH3 + heat

4 volumes 2 volumes

Factors Affecting Reaction EquilibriumChemical React ions

&

orconst

223

322

22

3

HNNH

NHHN

3

HN

2

NH

PPP

PPP

PP

PKp



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Write the equilibrium expression for the following reactions and

determine the units for Keq:1)2O3(g)D3O2(g)

2)Ag+(aq)+ 2NH3(aq)DAg(NH3)2+(aq)

3)2NaN3(s)D3Na(s)+ 3N2(g)

4)2Na(s)+ Cl2(g)D2NaCl(s)

5)2NaCl(s)D2Na(s)+ Cl2(g)

6)N2(g)+ 3H2(g)D2NH3(g)

Note 1. The Keqexpression depends on how the rxn equation is written (compare rxns 4&5).

2). The unit of Keqdepends on the way how the rxn eqn is written & the unit used of each.

Chemical React ions

][atm][[atm]

[atm]

2

3

2

3

3

2 ,P

PK

O

O

p

]][atm[33

2 ,PK Np

][1/atm][1

2

,P

KCl

p

][atm][2

,PK Clp

]][1/atm[][atm][atm

][atm][ 23

2

3

HN

2

NH

22

3 ,PP

PKp

]/l[mol][]/ll[mol/l][mo

[mol/l]][

]][NH[Ag

])[Ag(NH 22222

3

23

,Kc



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1. Analysis shows that a mixture of N2(2.46 atm), H2(7.38 atm) and NH3(0.116 atm) at

472C in reaction (N2(g) + 3H

2(g) D2NH

3(g)) is in equilibrium state.

Calculate: 1) Keq; 2) DG; 3). The total pressure. 4) Will the rxn be push to the product by

decreasing the reaction pressure? Give reason why? 5) Will the removal of NH3from

reaction mixture promote the product formation? Explain why.

1)

2)D

GT= - RTln(Keq)=-8.314x(472+273)ln(2.79x10-5)=65 kJ/mol

3) Ptotal= PN2+PH2+ PNH3=2.46+7.38+0.116=9.956 atm

4) A decrease reaction Pfavours the reverse rxn because Vo lreactant> Vo lproduct.

5) Yes. As Keq=P2NH3/(PN2xP

3H2)=constant, the removal of NH3 will reduce PNH3, to

compensate the change, more N2and H2will be converted to NH3 in order to keep

the same Keq(reaction quotient).

Chemical React ions

25

3

2

3

HN

2

NH /atm10792

.38)7.46)(2(

)1660(

22

3 ..

PP

PKp



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Chemical Reaction

What we know about a chemical reaction so far

Reaction equation - quantitative representation of a chemical reaction

Reaction stoichiometry coefficients and balancing reaction equations

We can judge if a give chemical reaction would proceed in the direction specified

A reaction has a tendency if DGof a reaction is smaller than zero

We can decide if a given reaction gives out or takes up heat

If DH< 0, reaction is exothermic; if DH> 0, reaction is endothermic

For reactions that are feasible, to what extent they will complete

Chemical reaction equilibrium, equilibrium constant

Now we know a rxn would proceed in the direction specified, questions are:

- how fast that reaction is going to proceed under give conditions?- how to quantitatively describe the rate of a reaction and make comparison?

- how can we explain that some reactions occur faster than others?

e.g. 2NO(g) + O2(g) = 2NO2(g) (slow); 2NaN3 = 2Na+ 3N2(very fast)

Chemical React ions



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Chemical Reaction Kinetics

Chemical reaction kinetics study the rate of chemical reactions

Definition of chemical reaction rate The number of moles of a reactant converted (consumed) in a reaction per unit time

for a reaction A + B C + D (mol/s)

When we say rate we always refer to ONE of the components in the reaction

The minus sign refers to that the concentration of reactant decreases in reaction Reaction rate equation (or kinetic equation)

Many forms exist

The most common one

where k reaction rate constant

A0 pre-exponential factorEa reaction activation energy

a,b,l,d reaction orders with respect to A, B, C, D, respectively

R gas constant

T reaction temperature in Kelvin scale

Chemical React ions

[A]or

[A][A]

12

12 dt

dr

ttr AA

RT

EAkk

dt

dr aA

-expin which[D][C][B][A]

[A]0

dlba

Ar rhenius equation

Kineticparameters



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Meanings of kinetic parameters, k, A 0, Ea, a b l d

Reaction rate constant, k It tells how fast a reaction can occur

It is a constant dependent on temperature but independent of concentrations

Pre-exponential factor,A0 It refers to the frequency of collision between molecules, the higher frequency, the faster rxn

Reaction activation energy, Ea

It can be understood as the energy

barrier for a reaction to overcome

The higher Eavalue is, the more

difficult for a reaction to occur

Reaction orders w.r.t. each component, a, b, l, d

The magnitude of these values reflects the effectiveness of each component in the reaction

The values of a, b, l, dcan be positive or negative or zero

The values of a, b, l, dcan be integrals or fraction

All these kinetic parameters have to be determined experimental ly

Chemical React ions

RTEAk a-exp0

reaction process

Ea reactant

productenergy



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Factors affecting the rate of a chemical reaction

Reaction temperature, T

An increase Twill lead to increasing k, thus reaction rate.

The dependence of kon Tis given by differentiating kexpression,

the higher Ea value is, the more significant of the effect of increasing T on the reaction rate

Concentration of reactants / products

The effect of increasing a concentration is positive if the respective order is positive

The larger the value of order is the stronger the effect of increasing conc on the rate

When order equals to zero, there is no effect of concentration on the rate.

The presence of a catalyst

A catalyst can alter reaction rate (speeding up desired rxns or slowing down undesired rxns)

Note: we assume rate of mass transfer (to meet) is sufficient high comparing to rA.

Chemical React ions

[D][C][B][A]-exp0dlba

RTEAr aA

200

ln

-

lnln

-

exp RT

E

dt

kd

RT

E

AkRT

E

Ak

aaa

Ch i l R t i



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The reaction rate and mass transfer rate

When we discuss the reaction rate, it only makes sense if there are sufficient number

reactant molecules can be delivered to the reaction site.

We say a reaction is kinetic controlwhen the rate of mass transfer > the rate of rxn rA.

This means that molecules being transported

to the reaction site are queuing for reaction

If the mass transfer rate is slower than the reaction rate, the overall rate we observed

will be the rate of mass transfer, not the reaction rate - diffusion control

This means that molecules are waiting to be delivered

before reacting queuing for reaction

The concept of rate determining step (r.d.s.)

The slowest step in a reaction process determine the overall rate of a reaction

Chemical React ions

masstransfer reaction

masstransfer

reaction

Ch i l R t i



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Catalysis and catalysts

Catalyst is a substanc e which can alter react ion rate w ithou t i tsel f beingdestroyed or consumed(many other definitions and this is one of them)

95% of chemical industries apply one or more catalysts in their processes

e.g. polymerisation, air/water depolution, ammonia synthesis, cracking heavy oil to

LPG, etc

A catalyst can be an acid, a base; can be a liquid or a solid. Most industrial catalysts

are metals, metal oxides or a mixture of them formulated & made in special ways

Use of catalysts in industry

Speeding up desired reactions thus increase the process output

Slowing down undesired reaction thus reduce the unwanted waste products

Altering reaction route by changing the relative speed of certain steps in a reaction network

therefore realising certain products which would not be possible without catalysts.

Allowing some rxns to occur under mild conditions e.g. working with heat sensitive materials

Enzymes are catalysts that participate in bio-active processes

etc

Chemical React ions

Chemical React ions



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Calculation of reaction rate

Example: a gas phase reaction 2N2O5=4NO2+O2occurs at 300C. The concentrations of N2O5foundin the reaction mixture at different time intervals are given below:

t h 0 1 2 3 5 7 9

[N2O5] mol/L 1.40 1.07 0.80 0.58 0.34 0.18 0.09

Calculate the rxn rates w.r.t. N2O5, NO2& O21) betw. 0-1h; 2) betw. 3-5h; 3) average betw 0-9h.

N2O5consumption rate NO2formation rate O2formation rate

Eqns to use:

1) 0-1 h

2) 3-5h

3) aver.

Chemical React ions

12

1221

12

1224

12

12[A][A]

;[A][A]

;[A][A]

2252 ttr

ttr

ttr ONOON

hmol/L1650

01

4010710.66;

01

4010710.33;

01

401071 21

24

2252

.

..r

..r

..r ONOON

hmol/L060

01

5803400.24;

01

5803400.12;

35

580340 21

24

2252

.

..r

..r

..r ONOON

hmol/L0730

01

4010900.291;

01

4010900.146;

09

401090 21

24

2252

...

r..

r..

r ONOON

Chemical React ions



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More about reaction rate 2N2O5 = 4NO2 + O2

1) 0-1 h rN2O5=0.33 rNO2=0.66 rO2=0.165 mol/Lh

2) 3-5h rN2O5=0.12 rNO2=0.24 rO2=0.06 mol/Lh

3) aver. rN2O5=0.146 rNO2=0.291 rO2=0.073 mol/Lh

For the same reaction, the reaction rate expressed by different components varies

with their stoichiometry coefficients

rN2O5: rNO2: rO2 = 2 : 4 : 1 or

Given reaction rate for one of the rxn components you should be able to calc others.

For the same reaction the reaction rate may vary with the time

because of change of reactant concs with time & rate in general is proportional to [ ]s.

When rxn orders w.r.t. reactant > 0 (usually they are) the rxn rate rbeginning> rlater

As reaction rate is a function of temperature, the determination of reaction of reaction

rate must be done at a constant temperature (you may need to determine the rate at

a different T, or you may need to vary temperature to determine such as Ea

Dont forget to put the correct unit to the reaction rate you determined

Chemical React ions

1

4

22252 ONOON rrr

Chemical React ions



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Chemical Reaction Calculations

Question 1. How many grams of water are produced in the oxidation of 1.0g of glucose,

C6H12O6? Reaction equation: C6H12O6+ 6O2= 6CO2+ 6H2O

Step 1: Use molar mass of glucose to convert g to moles

1 mole C6H12O6=6x12(C)+12x1(H)+6x16(O)=180g/mol

number of moles C6H12O6=1.0g x (1mol/180g)=5.55x10-3mol

Step 2: Use balanced equation to determine no. of moles of H2O produced

1 mole C6H12O6produces 6 moles H2O

the no. of moles of H2O produced: 5.55x10-3moles C6H12O6x6=0.033 moles H2O

Step 3: Convert moles of H2O to grams using molar mass

1 mole of H2O=2x1(H)+1x16(O)=18 g/mol

Grams of H2O produced: 0.033 mol of H2Ox(18g/mol)x = 0.6g H2O (answer)

Note: You cannot use the weight directly in the calculation. It has to be converted to moles.

Chemical React ions

Chemical React ions



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Question 2. In a reactor one put 180g of glucose (C6H12O6) and 160g of O2. Can you

produce 108g of H2O. Why? What is the maximum amount of H2O which canbe produced? Reaction equation: C6H12O6+ 6O2= 6CO2+ 6H2O

Step 1: Convert all components from grams to moles:

No.moles of C6H12O6=180g/190g/mol=1 mole of C6H12O6

No.moles of O2= 160/32g/mol=5 mole of O2

No.moles of H2O= 108/18g/mol=6 mole of O2

Step 2: Find out how much glucose AND O2you need to produce 108g H2O.

To produce 108g which is 6 moles of H2O, you will need 1mole glucose AND 6 moles of O2.

Do we have enough glucose? - Yes. Do we have enough O2? - No.

Step 3: Every 6 molecules of O2will burn 1 molecule of glucose, this will proceed UNTIL

one of the reactant consumed completely, in this case O2.

When all O2is consumed the reaction will stop and the max. amount of H2O which can be

produced can be calculated from O2available: 5moles of O2gives 5moles (or 90g) of H2O

Note: When one of reactants is consumed completely the reaction will stop.

Chemical React ions

Chemical React ions



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Question 3. As in Question 2, one puts 180g of glucose (C6H12O6) and 160g of O2. When

O2is completely consumed, what is the glucose left & what is the percentageconversion of glucose? Reaction equation: C6H12O6+ 6O2= 6CO2+ 6H2O

Step 1: Convert all components from grams to moles:

No.moles of C6H12O6=180g/190g/mol=1 mole of C6H12O6

No.moles of O2= 160/32g/mol=5 mole of O2

No.moles of H2O= 108/18g/mol=6 mole of O2

Step 2: Find out how much glucose left after all O2has consumed.

The molar ratio of glucose and O2in the reaction=1:6. For a consumption of 5 moles of O2,

the amount glucose reacted will be 1x5/6=5/6 moles or 0.83 moles, or 0.83x180=150g.

The amount glucose left over=1-0.83=0.167moles or 180-150=30g

Step 3: The percentage conversion of glucose at complete conversion of O2

(try the weight base)

Note: The conversion (%) calculated based on moles is the same as that based on weight .

Chemical React ions

%.%.

%conversion 3831001

83301100

[A]

[A][A](%)

in

outin

Chemical React ions



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Question 4. The brown gas NO2can form colorless gas N2O4, 2NO2DN2O4. At 25C the

concentrations of NO2& N2O4are 0.018 M & 0.055 M respectively when atequilibrium. 1) Calculate the equilibrium constant Keqat 25C. 2) If in another

equilibrium system of the same gases at the same temperature, the NO2

concentration is found to be 0.08 M, what is the concentration of N2O4?

Step 1: Determine the equilibrium constant

From equilibrium constant definition:

Step 2: When at equilibrium

Chemical React ions

1700.018

0550

][NO

]O[N22

2

42

.Keq

M08811700.08]O[N1700.08

]O[N

][NO

]O[N 2422

42

2

2

42 .Keq

Chemical React ions



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Question 5. The rate constants of a reaction are determined to be 3x10-5mol/L.h at

200C and 4x10-4mol/L.h at 250C. Estimate the reaction activation

energy.

Arrhenius eqn relates the rate constant to activation energy

Mthd.1: lnk=lnA0+(-Ea/R)(1/T), A plot of lnkagainst 1/Twill produce a

straight line, the slope of which is -Ea/R. So that Ea=slope x R

Ea=-12750 x 8.314=106,000 J/mol= 106 kJ/mol

Mthd 2:

Let A 0,1=A 0,2

T1=273+200 K, T2=273+250 K, k1=3x10-5& k2=4x10

-4mol/L.h, R=8.314 JK/mol

RT/EaeAk 0

2120

10

20

10

2

1

20

10

2

1

202101

lnlnln

&

2

1

2

1

21

RT

E

RT

E

A

A

eA

eA

k

k

eA

eA

k

keAkeAk

aa

,

,

RT/E

,

RT/E

,

RT/E

,

RT/E

,

RT/E

,

RT/E

,

a

a

a

a

aa

2

1

21

21

21

21

212

1 lnorln

k

k

TT

TRTE

TRT

TTE

RT

E

RT

E

k

ka

aaa

kJ/mol106104

103ln

250273200273

2502732002733148ln

4

5

2

1

21

21

.

k

k

TT

TRTEa

1/T

ln kslope= -12750

Chemical React ions



31/81

31


Question 6. Two catalysts A & B are compared for their catalytic activity for reaction RP.When A is present it takes 10s for Rto change from 2 to 0.5 moles and when

B is present it takes 20s for R to decrease from 5 to 2 moles at the same

temperature and with the quantities of catalyst.

Which catalyst is more active for the reaction concerned?

Answer: The activity of two catalysts can be compared based on the average reaction

rate when A & B presence separately.

The A catalyst is more active for the reaction concerned.

mol/s10020

5-3][-][

mol/s15010

2-.50][-][

12

12

.t

RRr

.t

RRr

B

B,B,

B,R

A

A,A,

A,R

D

D

Chemical React ions



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Question 7. Verify that the rate constant of a reaction following second order rate law

rA=-k[A]

2

can be determined from the slope of a line obtained by plotting 1/[A]tagainst reaction time t, where [A]tis the concentration of A measured at time t.

Answer: Second order rate law: (1)

rearrange: (2)

Define boundary conditions: at t=0, [A]=[A]0and at t=t, [A]=[A]tintegrate eqn (2),tfrom 0-tand [A] from [A]0to [A]t

(3)

compare eqn (3) with linear eqn Y=aX+B, which is a straight line with slope a

Let

A plot of vs. twill give a straight line with slope=k.

2[A][A]

kdt

drA

kdtd

kdt

d

2

2

[A]

[A][A]

[A]

0t0t

t

0

[A]

[A] 22 [A]

1

[A]

10

[A]

1

[A]

1[A]

[A]

1-

[A]

[A] t

0

kttkdtkdkdt

d

0t [A]

1and

[A]

1 btX,ka,Y

t[A]

1

t

slope=k

1/[A]

Chemical React ions



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Question 8. Reaction RPfollows the second order rate law rR=-k[R]2. Verify that the time

required for the reactant Rto fall to a half of its initial value is t1/2=1/(k[R]).Answer: Second order rate law: (1)

After integration of eqn (1) with the boundary conditions:

at t=0, [R]=[R]0& at t=t1/2, [R]=[R]t1/2=0.5[R]0

2[R][R]

kdt

drR

0

21

0000

21

00

21

0

21

00t

[R]

1

[R]

1

[R]

21

[R]

1

0.5[R]

11

[R]

1

0.5[R]

1

[R]

1

0.5[R]

1

[R]

1

[R]

1

kt

kktkt

ktkt

/

//

/

Chemical React ions



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Homogeneous and Heterogeneous Reactions

In a chemical reaction, the reactants can be in various physical states

Homogeneous - All of reactants & products in the same phase & no phase boundary

Heterogeneous - Involving multi-phases & phase boundary crossing

Phase Type Example

gas - gas Homog. 2NO + O2= 2NO2

gas - liquid Hetrog. CO2 + H2O = H2CO3

gas - solid Hetrog. O2 + Fe = Fe2O3

liquid - liquid (miscible)* Homog. NaOH + HCl = NaCl + H2O

liquid - solid Hetrog. CaO + H2O = Ca(OH)2

solid - solid Hetrog. CaCO3= CaO + CO2

* When two immiscible liquid, such as oil and water is regarded as heterogeneous type.

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Phase & Phase Change

A substance can exist in different physical states

Gas / vapour

Liquid

Solid

Note: Some other states may sometime mentioned.

such as liquid-crystal, super-critical state, gel. etc.

The temperatures at which phase changes occur

vary with substances and circumstances (e.g. P)

The energy required for phase change varies with

substances and type of phase change.

The energy possessed by molecules of the same

substance at different state are different

liqui

d

solid

sublimation d

epositio

n

condensatione

vaporation

melting

freezingE

nergy

leve

l

gas

Temperature

Energy (heat) added

ice

liquid

water

water

vapour

melting

evaporating

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Reactions In Liquid Phase - Solution

Solution = solute + solvent

Solute is a solid or a gas - solute is dissolved in solvent(e.g. NaCl + H2O, O2+ H2O)

Solute is another liquid - solute and solvent are miscible (e.g. C2H6O + H2O)

solute in a solution can exist as molecules or ions, or both (such as weak acid)

some solutes are dissolved in a solvent in any proportions (e.g. C2H6O in water); others are

only dissolved in a solvent in certain proportion - solubility limitation (e.g. NaCl or N2in H2O)

Concentration of a solution - the amount of solute in the solution

Molar concentration (molarity) - number of moles solute in ONE litre solution

Molarity is the most commonly used concentration unit in chemistry

Weight percentage (wt.%) of solute in solution

definition: solute wt%=100% x (wt of solute)/(wt. of solute + wt. of solvent)

e.g. A solution contains 20g solute & 30g solvent

solute wt%=100% x 20/(20+30)=40wt% (the wt% of solvent =100%-40%=60%)

solute- substance that is dissolved

solvent- dissolving medium

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Reactions In Liquid Phase - Solution

Some important notes on the chemical reaction in solution

When a solute dissolve in solution, solute can either be present as molecules or ions

as molecules. e.g. dissolving sugar in water - not electronic conductivity.

as ions. e.g. dissolving salt (NaCl) in water - Na+& Cl-both conduct electricity.

Strong acids or base, when dissolved in H2O, form only ions in solution

Weak acid or base, when dissolved in H2O, form mixture of molecules and ions (partial dissociatn)

The solubility of some gas solutes, when dissolved in a solvent, depends on thepressure of the solute gas above the solution

ideal solution: Pi=xiPi* or xi=Pi/ Pi* in which

Solvent molecules may form weak bond with solute molecules (e.g. Hydrogen-bond),

which may to a certain degree change the reactivity of solute.

The presence of solute may affect certain properties of the resultant solution

e.g. boiling point elevation, freezing point depression, osmosis pressure, etc.

Pi- vapour pressure of solute i

xi- mole fraction of solute iin solution

Pi* - equil. vapour pressure ofpuresolute i

Chemical React ions



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Gas Phase Reactions

Distinctive features of molecules in gas phase in relation to reaction

Having high energy Sometime a liquid or even a solid substance is heat to gas phase to react

e.g. steam reforming hydrocarbons, heavy oil cracking,

Moving freely within the space of reaction

High mass transfer rate - General magnitude of mass transfer rate in solid, liquid and gas

solid 100 liquid 103 gas 105

High heat transfer rate - This is very important for reactions involving heating/cooling

In practice, many reactions in which the reactants are liquid or solid at normal temperature

are carried out at elevated temperatures in order to convert the reactants to gas

Compressible therefore sensitive to the reaction pressure

This has implication on the reactions involving the change of number of moles before and

after reaction. The main disadvantages of the gas phase reactions are

Usually high volume (large reactor)

not suitable for heat sensitive substances if heating to high temperature is required.

Chemical React ions



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Solid Phase Reactions

Solid phase reactions are usually slow due to limited mobility of molecules

When a solid reacts with another reactant which is liquid or gas, the

reaction starts from outer surface of the solid

In industry if a reaction involves a solid reactant (at ordinary temperature)

what we usually do is

dissolving solid in solvent

heating it to above the melting point so that it takes part in reaction as a liquid

Many catalytic reactions use solid catalysts

The reactant in this case can be a liquid or a gas or liquid-gas mixed phase

Solid catalysts are very easy to separate from liquid, gas or a liquid-gas mixture

It is easy to handle solid catalysts from practical point of view (loading, discharge etc)

Chemical React ions



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Reactions involve multi-phases

Many reactions involve multi-phase in one reactor

reactants and products can be presented as any combination of two or three phases.

When multi-phases present in a rxn following issues become important:

Relative rate of reactant and/or product molecules diffusion within each phase as well

as through phase boundaries must match the rate of reaction

Solubility of solids and/or gases in liquid phase When a porous solid is involved the liquid/gas molecules transport within the pore

Both pressures (for gas phase) and concentrations (for liquid) are interlinked in the

reaction network therefore these have to be considered systematically.

When reaction involves heating or cooling, as most of reactions do, this has to be

dealt with by considering both mass transfer and heat transfer within and betweendifferent phases.

The main advantage of multi-phase reactions is the easiness for separation

Chemical React ions



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Example of Reactions involving multi-phases

The long journey for reactant molecules

j. travel within gas phasek. cross gas-liquid phase boundary

l. travel within liquid phase

m. cross liquid-solid phase boundary

n. reach outer surface of solid

o. travel with pore

p. reach reaction site

q. be adsorbed on the site and activated

r. react with other reactant molecules (eitheradsorbed or approached from surface above

Product molecules must follow the reverseprocess to return to gas phase

Heat transfer follows similar process

j

r

gas phase

poreporoussolid

liquid

phase

k

l

mn

o

pq

gas phase

reactant molecule

Chemical React ions



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42

Acids and Bases

Acids & Bases are one of the most important classes of chemicals

Acids and bases have been know to human for a long time

Acids taste sour (in fruit), change colour of certain dye

Bases taste bitter and feel slippery (like in soap, lime water)

Acids and bases are widely present in nature,

especially in plants, electrolyte balance in life system cycle etc

Acids and bases are widely used in industry for various purpose

Dissolving chemicals, e.g. HF, aqua regia(HCl:HNO3=3:1)

Reagents for producing various chemicals

Catalysing various types of reactions

Titration in volumetric analysis

etc

A id d B D fi iti

Chemical React ions



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Acids and Bases - Definition

Classical definition

Acids- Substances that, when dissolved in water, increase the concentration of H+ions

e.g. HCl(g) H+(aq)+ Cl-(aq)

Note: H+, which is a proton only (no e- ), is actually bond with water molecule forming H3O+, the rxn is

HCl(g)+ H2O (l) H3O+(aq)+ Cl

-(aq)

For simplicity, we often use H+instead of H3O+.

Bases - Substance that, when dissolved in water, increase the concentration of OH-ions

e.g. NaOH OH-(aq)+ Na+(aq)

NH3+ H2O NH4++ OH

-

Brnsted-Lowry definitionAcid isproton donorand Base isproton acceptor

(because H+is a proton and OH-of a base reacts with H+giving water)

H2O

H2O

C j t A id d B P i

Chemical React ions



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Conjugate Acid and Base Pairs An acid & a base always work together to transfer proton (donate-accept). A substance

can function as an acid only if another substance behaves simultaneously as a base.

When an acid or a base is dissolved in water, ions are released - this process involvesproton transfer. To mark the process and link the ions with its original acid or base,

conjugate acid-base pairs are defined.

Acid and conjugate base always appear in pair; likewise base and conjugate acid appear in pair

When an acid losses proton (H+) it becomes the conjugate base of that acid (e.g. HX to X -)

when a base receives a proton (H+) it becomes the conjugate acid of that base (H 2O to H3O+)

If an acid dissolves in water, H2O is a base; if a base dissolves in water, H2O becomes an acid.

remove H+

HX(aq) + H2O (l) X-(aq) + H3O

+(aq)

acid base conjugate base conjugate acidadd H+

remove H+

HCl(aq)+ H2O(l) Cl-(aq) + H3O

+(aq)acid base conjugate conjugate

base acid

add H+

add H+

NH3(aq)+ H2O(l) NH4+(aq) + OH-(aq)

base acid conjugate conjugateacid base

remove H+

Strengths of Acids and BasesChemical React ions



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Strengths of Acids and Bases

The strength of acids and bases

The strength of an acid is the ability to donate proton,

or increase [H+] when acid is dissolved in water. likewise, the ability to accept proton, or [OH-],

determine the strength of a base

Common acids and their relative strengths

Strong acids,paired with bases with negligible basicity

-Able to completely transfer their proton to water

- Their conjugate bases are the weakest, with negligible

tendency to accept proton

Weak acids,paired with week bases

- These acids are partially dissociated to ions

- Their conjugate bases are also weak, with limited ability of

accepting proton

Acidswith negligible acidity,paired with strong bases

- These class of acids, though carrying H, give out no [H+]

- Their conjugate bases, however, are strong bases

Water can act as acid as well as base

acid base

HCl Cl-

H2SO4 HSO4-

HNO3 NO3-

H3O H2O

HSO4 SO42-

H3PO4 H2PO4HF F

-

HC2H3O2 C2H3O2-

H2CO3 HCO3-

H2S HS-

H2PO4 HPO42-

NH4 NH3HCO3 CO3

2-

HPO4

PO4

3-

H2O OH

-

OH O2-

H2 H-

CH4 CH3-

acid

strength

in

crease

base

s

trengthincrease

negligible

weak

strong

negligible

weak

strong

A id d B E ilib i

Chemical React ions



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46

Acid and Base Equilibrium

The extent of ionisation of an acid or a base in water

Some acids (or bases) ionise in water completely, leaving no molecules behind

Other acids (or bases) ionise partially in water, forming an equilibrium between

molecules and ions

e.g. HF(aq) + H2O (l) D F-(aq) + H3O

+(aq) (1)

NH3(g) + H2O (l) DNH4+(aq) + OH

-(aq) (2)

The tendency of ionisation of an acid (or a base) varies with the type of acids, we

can use the concept of reaction equilibrium to indicate the degree of ionisation.

The equilibrium constant used to describe the degree of ionisation of an acid is

called acid-disso ciat ion con stant, Ka, which is defined as

for equili. (1)

for equili. (2)

[HF]

]][H[For

[HF]

]O][H[F-

3

-

aeqa KKK

][NH

]][OH[NH

3

4

eqa KK

ions

molecule

The higher the Kavalue, the higher

ion conc., the higher acidity/basicity

Q tif i th St th f A id d B

Chemical React ions



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Quantifying the Strength of Acids and Bases

[H+] and [OH-] are the measure of the strengths of acids and bases

We know that an acid when dissolved in water releases [H+] and a base gives [OH-] We also know that the strengths of an acid or a base depend on the [H+] and [OH-]

It comes naturally that [H+] & [OH-] are used to indicate the strengths of acids/bases

The range of [H+] and [OH-]

Dilute aqueous solutions at 25C always give,

Kw=[H+][OH-]=1.0x10-14

For an acid [H+]>[OH-], Kw=[H+][OH-]=1.0x10-14

For a base [OH-]>[H+], Kw=[H+][OH-]=1.0x10-14

For pure water, which is neutral

[H+]=[OH-]=1.0x10-7, Kw=[H+][OH-]=1.0x10-14

pH scale

For convenience the low value of [H+] and [OH-], we use the scale of log10[H+]

define pH= -log10[H+] Scale: 1-14. Acid pH=0-7 [H+]>[OH-]; strong acids have low pH

Base pH=7-14 [OH-]>[H+]; strong bases have high pH

Note: When using [OH-] (which is less used), we have pOH= -log10[OH-] (=14-pH)

water can act as an acid as well as a base

at equilibrium H2ODH++ OH-

Equili. constant at 25 C is found to be

Further examine other aqueous solution

the same relation holds

14-

2

101.0]][OH[HO][H

]][OH[H

wK

In pure water [H2O] is constantKnown [H+], [OH-] can be calculated by this eqn.

Calculation of pHChemical React ions



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Calculation of pH

Example 1: Calculate pH of 0.05M HNO3solution

HNO3 + H2O H3O+

+ NO3-

HNO3is a strong acid, HNO3ionizes completely in water, i.e. [H3O+]= 0.05M

pH = - log10[0.05] = 1.3

Example 2: Calculate pH and pOH of 0.05M NaOH solution

NaOH + H2O Na+ + OH-

NaOH is a strong base, NaOH ionizes completely in water, i.e. [OH-]=0.05M,

Kw= [H3O+][OH-] = 1 x10-14M2

[H3O+] = 1 x10-14M2/ [OH-] = 1 x10-14M2/ 0.05 M = 2 x 10-13M

pH = - log10[2 x 10

-13

] = 12.7 pOH = 14 - pH = 14 - 12.7 = 1.3

(why is this result the same as that of example1?)

Calculation of pHChemical React ions



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Calculation of pH

Example 3: What is the [OH-], in mol/L, in a solution whose pH is 9.72?

Known: pH = - log10[H3O+

] = 9.72

[H3O+

] = 1.9 x 10-10

(mol/L)for any aqueous solution Kw= [H3O

+][OH- ] = 1.0 x 10-14 (mol/L)2

[OH- ] = Kw/ [H3O+] = 11.0 x 10-14 (mol/L)2/ 1.9 x 10-10(mol/L) = 5.3 x 10-5 (mol / L)

Example 4: The acid-dissociation constant, Ka, of hydrofluoric acid is 6.8x10-4. What is

the [H3O+] in a 2M HF solution? What is the pH of the solution?

HF(aq) + H2O(l) F-(aq) + H3O+(aq)

initial 2 0 0

at equili. 2 - x x x

By definition

Solve the eqn forx( = [H3O+])

pH = - log10[H3O+] = - log10(0.0365) = 1.44

43

-

1086-2

[HF]

]O][H[F

.x

xxKK eqa

M03650108621086 442 .x.x.x

A E ilib i d S A li ti

Chemical Equil ibr ia



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Aqueous Equilibria and Some Applications

In chemistry many aqueous systems involve equilibria

Human body fluids are in electrolyte equilibria in order to function properly Electrolyte: aqueous solutions that contain ions

Plants contain weak acids, which maintain right balance for plants to grow

Many properties of a solution that has ions are affected by its equilibrium state.

etc. (In a broad sense, harmony=balance=equilibria)

Many phenomena in chemistry can be studied by means of equilibria.

We will look at:

The behaviour of an equilibrated electrolyte solution when other ions are added

Applications

Buffer effect

Acid-base titration

Solubility of ionic substances and the factors affecting it

The Common Ion Effect from Equilibrium




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The Common-Ion Effect from Equilibrium

Considering the following two cases

Case 1. What is the pH of 0.3M acetic acid HC2H3O2solution, (Ka=1.8x10-5)?

HC2H3O2(aq) DH+ (aq) + C2H3O2

-(aq)

initial 0.3 0 0

at equilibrium 0.3-x x x

By definition

Solve eqn for x

5

232

-232 1081

-0.3

]OH[HC

]OH][C[H .x

xxKa

6421032-log]-log[HpH

M1032][H

3

3

..

.x

Note: HC2H3O2is a weak acid (Ka


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52

The Common-Ion Effect from Equilibrium (contd)

Case 2. What is the pH of solutncontains 0.3M acetic acid HC2H3O2& 0.3M NaC2H3O2?

HC2H3O2(aq) DH+(aq) + C2H3O2

-(aq)

initial 0.3 0 0

at equilibrium 0.3-x x 0.3+x

By definition

Solve equ for x

Compare cases 1 & 2:

The extent of ionisation of HC2H3O2is reduced by the presence of NaC2H3O2(which has

C2H3O2-ion in common with HC2H3O2)

This is called the Common- ion Effect. It works in many equilibrated electrolyte solutions

such as buffer solutions, solubility of ionic compounds etc.

5

232

-

232 1081

-0.3

30

]OH[HC

]OH][C[H

.

x

x.xKa

Note:

NaC2H3O2ionises in water completely

NaC2H3O2(aq) DNa+(aq) + C2H3O2

-aq)

7441081-log]-log[HpHM1081][H

5

5

..

.x

Note:The presence of NaC2H3O2&

Na+does not change Kavalue

Note:C2H3O2-is the conjugate base

of HC2H3O2

Buffered Solutions




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53

Buffered Solutions Behaviour of a solution containing a weak conjugate acid-base pair

equilibrium of weak acid HX(aq) DH+(aq) + X-(aq)

acid-dissociation constant

If a base, OH-, is added, OH-(aq) + HX(aq) DH2O(aq) + X-(aq) [HX] & [X-]

If an acid, H+, is added, H+(aq) + X-(aq) DHX(aq) [X-] & [HX]

When the addition of OH-or H+is small compared to [HX] & [X-], the change to [HX] & [X-] is very

small, so does the ratio [HX] / [X-] the [H+] thus pH will remain almost constant.

A Bu ffered Solut io n(also called Buffer) contains a weak conjugate acid-base

pair. It can resist drastic change of pH upon the adding strong acid or base.

Buffers solutions are widely used in biology and biochemistry because of the

need of maintaining certain pH for some reactions/process to occur properly.

Note: Buffer solutions can be made for all pH ranges. The amount of acid or base it can

neutralise before pH begins to change (called buffer capacity) depends on the [HX] & [X-].

As the HC2H3O2and

C2H3O2-pair in case 2

][X

[HX]][H

[HX]

]][X[H-

-

aa KK

Solubility of Ionic Compounds




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54


Equilibrium between solid of ionic compound and its ions when dissolved

dissolv ingCaCO3(s) Ca

2+(aq) + CO32-(aq)

precipi tat ion

Equilibrium constant:

solubi l i ty -produ ct constantKsp=[Ca2+] [CO3

2-]

When adding another strong electrolyte Na2CO3, which dissociates completely in

water solution and contains common ions CO32-, into the above equilibrated solution

The above equilibrium will shift to the left, meaning that the CaCO3solubility

Reason?- Common-ion effect(Kspis constant, [CO32-] [Ca2+]

[CaCO3] )

Addition of common ions alter the equilibrated solubility of an ionic compound.

(you may like to link this with the cases such as scale formation in kettle, kidney stone, etc.)

Note:The solid CaCO3does not

appear in the expression





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Equilibrium between solid and its constituent ions containing OH-

dissolv ing

Mg(OH)2(s) Mg2+(aq) + 2OH-(aq)

precipitat ion

Many metal hydroxides are partially dissolved in solutn(or precipitated when formed)

solubi l i ty -produ ct constantKsp=[Mg2+] [OH-]2

When adding an acid, H+, into the above equilibrated solution, a reduction of solution

pH occur due to the following reaction: H++ OH-DH2O, OH-in the solution is

consumed thus reduced [OH-]

[Mg2+] [Mg(OH)2] .

This is also a kind of common-ion effect but working in reverse direction.

Due to consumption of one of ions in the equilibrated solid-ions solution, theequilibrated solubility of an ionic compound is increased.

(you may like to link this with the cases such as kettle de-scaling, tooth decay etc.)

Note:the stoichiometric number raised

to power in Kspexpression

Titration

Chemical App l icat ions



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Titration

A method to determine the concentration of a particular solute in a solution

Titration reactions

Acid base reactions (acid / base indicators, etc)

Oxidation-reduction reactions (colour change, etc

Precipitation (cloud appearance, etc)

Standard solution - a solution with known concentration which is used to titrate.

Equivalent point - The point at which stoichiometrically equivalent quantities are

brought together. It is a theoretical point of reaching stoichiometry

End point - The point at which a pre-determined indication of reaching the equivalent

point effects. It is practically the point one stops adding standard solution

and is usually very close to the Equivalent point

Usual means of indicating the arrival at the end point

Colour (indicators or the colour change of the substance itself before and after equiv. point

Conductivity if the quantities of ions is used as an indication

Others such as precipitation formation

Acid - Base Titration

Chemical App l icat ions



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57

Acid Base Titration

Typical titration curves of strong

& weak acids by Strong base

pH change of weak acids is less

drastically as that of strong acids

because of equilibrium shifting

0 10 20 30 40 50 60

pH

14

12

10

8

6

4

20

0 20 40 60 80 100mL NaOH

H3PO3 H2PO3-

HPO32-

mL NaOH

pH

14

12

10

8

6

4

20

strong acid

Ka=10-2

Ka=10-4

Ka=10-6

Ka=10-10

Ka=10-8

Equivalent point

Typical titration curve of polyprotic

acids by Strong base

Different equilibria of ions with

different charges

Introductory to Organic Chemistry

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Organic chemistry

A branch of chemistry devoted to the study of carbon-containing (organic) molecules All life forms on earth have organic molecules as their basic building blocks

The most important carbon-containing molecules are hydrocarbons (HCs)

General characteristics of hydrocarbons (HCs)

Hydrocarbons (HCs) - molecules contains mainly carbon (C), hydrogen (H)

Bonds and general structures The bonds of HCs are mainly covalent, formed between C and C (C-C, C=C or CC), H (C-

H), O (C-O or C=O) and others such as N (C-N).

The C-C bonds forms the backbone or skeleton of HCs and Hs are at surface

Functional groups (FGs)

Many FGs attached to C-C skeleton give HCs various unique function and properties.

Stability

All HCs can burn in oxygen easily giving heat.

C-C single bonds are most stable (- > = > ), C-H & C-FGs are easy to break


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There are four types of HCs (based on the kinds of C-C bonds)

Alkanes (C-C)

contain only C-C bonds in this group of molecules, also called saturated HCs

Alkenes (C=C)

contain C=C bonds

Alkynes (CC)

contain CC bonds unsaturated HCs

Aromatics

carbon atoms are connected in a planar ring structure

usually possess special odour


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Alkanes

Most stable HCs Its molecular formula can generally be written as CnH2n+2, where nis the number of

carbon atoms in molecule.

Most common alkanes

Name Molecular Condensed Lewis structure Boiling

formula structure formulary point

methane CH4 CH4 -161C

ethane C2H6 CH3CH3 or CH3-CH3 -89C

propane C3H8 CH3CH2CH3 -44C

butane C4H10 CH3CH2CH2CH3 -0.5C

pentane C5H12 CH3CH2CH2CH2CH3 36C

H

H

H-C-HH

H

H-C-C-HH

H H

H

H-C-C-C-HH

H

H

HH

H

H-C-C-C-C-HH

H

H

H

H

H H

H

H-C-C-C-C-C-HH

H

H

H

H

H

H

H


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Introductory to Organic Chemistry Some general points about HCs

Straight-chainHCs: All carbon atoms are joined in a non-branched chain

C-C C-C-C -C-C-C-C- C-C-C-C-C=C-C-C-C

Branched-chainHCs C

-C-C-C- -C-C-C C-C-C-C-C=C-C-C C-C-C-C-C=C

C C C C C

Structural isomers

compounds that have the same C C

molecules formulas but with C-C-C-C-C=C C-C-C-C-C=C

different bonding arrangement C C C C

There is a system way of naming HCs

to differentiate the isomers with different structures

We can sometimes write only carbon atoms to show the structure & bonds, as

shown above

All of these are isomers of C9H18


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Alkenes

Less stable than alkanes Its molecular formula can generally be written as CnH2n, where nis the number of

carbon atoms in a molecule.

The double bond C=C can locate between any two C (n equal to or larger than 2).

There can be more than one double bonds in an alkene

Isomers exists when n equal or larger than 4.

Common alkenes

ethene or ethylene CH2=CH2or CH2CH2 (ethene is a plant hormone)

propene or propylene CH3

-CH=CH2

or CH2

CHCH2

(play key role in fruit ripening)


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y g y

Alkynes

Least stable - very active It molecular formula can generally be written as CnH2n-2, where nis the number of

carbon atoms in molecule.

The triple bond CC can locate between any two C (n equal to or larger than 2).

There can be more than one triple bonds in an alkyne

Isomers exists when n equal or larger than 4.

Common alkenes

acetylene CH

CH or CHCH very active. Burn in oxygen - oxiacetylene torch

with flame temperature 3200K.

Very important intermediates in chemical industry


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y g y Aromatic HCs

More stable than alkenes and alkynes, though there are unsaturated bonds. Most common aromatic HC is benzene

Examples of other aromatic compounds

H

H

or

C

C

C C

C

C

H

HH

HCH3

Tuluene

CH3

CH3

H3C

H3C

H3C

HH

Cholesterol

HO


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y g y

Some common functional groups

Alcohols (name suffix -ol)Soluble in H2O; use in food, medicine, cholesterol is an alcohol

Ethers (name suffix ether)

Mostly used as solvent

Aldehydes (name suffix -al)

Flavour (vanilla, cinnamon etc)

Ketones (name suffix -one)Such as acetone used extensively as solvent

Carboxylic acids (name suffix -oic acid)

Sour veg, fruits; application in polymers, fibres, paints

Esters (name suffix -oate)

very pleasant odour (fruits)

Amines and Amides (name suffix -amide)

Are key functional group in protein structure

R-OH

R-O-R

O

R-C-H

O

R-C-R

O

R-C-OH

O

R-C-O-R

O

R-C-N-R

Introductory to Biochemistry

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y y

Some general remarks on Biochemistry

Biochemistry - Biological chemistry which studies living species in chemical means Looking at the compositions and structure of biochemical molecules, and try to understand

their functions at molecular level.

Looking at the properties these molecules especially from biological point of view

The change processes of these molecules in relation to its role in life cycle

Making use of our knowledge for human benefits

General observations on biochemical molecules

The molecules are generally very large, molecular wt in the range of 1,000s -1,000,000s

They are generally very complicated in structure, yet they contains mainly C, O, H as their

main building blocks and some other atoms such N, P, S in their functional groups

The specific ways of these molecules are structured make them specific functions in

biological processes.All life processes of mammals and other animals on the Earth require energy (processes of

bio-molecule synthesis are endothermic in large), which, ultimately coming from the Sun,

are obtained indirectly through plant photosynthesis.

In t rodu ctory to Biochem ist ry- Proteins

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Proteins and Amino acids

Proteins are big molecules present in living cells (~50% dry wt of our body)animal tissues, skin, hair, nails, muscles

All proteins are chemically similar

All proteins are composed of the same building blacks - a-amino acids

a-amino acids are linked by amidegroups, which is formed by reaction between

-C-O-and H-N- groups of 2 amino acids after dropping a H2O, into proteins

H

R

+

H3N-C-C-O

-

OH

RH2N-C-C-OH

O

or

H

H

O

H

R

+H3N-C-C-O-

O

H

e.g. + = + H2O

H

R

+H3N-C-C-O-

O H

R

+H3N-C-C-N-C-C-O-

O OH

R

amide group


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Structure of proteins - 4 levels

Primary structure- amino acids sequence Secondary structure- a-helix

Tertiary structure- folded individual peptide

Quaternary structure- aggregation of 2 or more peptides


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Enzymes

Enzyme is one of the most important classes of proteins. Each enzyme is capableof catalysing very specific reactions with living organisms

More about amino acids

There are many different amino acids, the difference being the Rgroups

Our body requires 20 amino acids

Our body can synthesise 10 of these 20

The other 10 (called essential amino acids) must be ingested.

H

R

H2N-C-C-OHO

e.g.H

H

H2N-C-C-OHO H

CH3

H2N-C-C-OHO

Glycine Alanine

In t roductory to B iochemist ry- Carbohydrates

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Carbohydrates (hydrate of carbon)

It has general formula of Cx(H2O)y It is a form of sugar used for store energy by plants

Carbohydrates can be divided according to the number of units of basic sugar

Monoaccharides - containing a single unit of sugar (that cannot be broken by a acid)

The most important monoaccharides are glucoseand fructose

Glucose - the most abundant carbohydrate, C6(H2O)6or C6H11OH (aldehyde sugar)

Fructose - present in most of fruit, C6(H2O)6or C6H11OH (ketone sugar)

Diaccharides - composed of two monoaccharides

The most most important diaccharide is sucroseandlactose

Sucrose (table sugar) - is a sugar composed of 1 glucose + 1 fructose.

The invert sugar (sweeter than sucrose) is made from hydrolysis of sucrose converting part ofglucose to fructose. The sugars made from sugar beets and canes are the same.

Lactose (milk sugar) - is a sugar composed of 1 glucose + 1 galactose

Polyaccharides - composed more than two units of monoaccarides

In t roductory to B iochemist ry- Carbohydrates

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Some of most important polyaccharides

Starch Many crops contains mainly starch (corn, potatoes, wheat, rice etc). It is a major way

plants store their energy

It consists of mainly glucose. due to different way in which glucose units are joined

together, starch may be unbranched or branched in structure

Hydrolysis of starch, catalysed by enzyme within our digestive system, gives glucose

Glycogen

These type of polyaccharides can be synthesised within our body and stored in liver and

muscles. It services as immediate energy source of our body.

Cellulose

It forms major structural unit of plants (e.g. wood 50%, cotton fibres 100% cellulose).

usually unbranched chain of glucose units with average molecular weight 500,000 amu.

The enzymes within our body which help hydrolysis starch cannot digest cellulose.

In t roductory to B iochemist ry- Nucleic Acids

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Functions of nucleic acids

Nucleic acids are chemical carriers of an organisms genetic information

They are also chemical controller of cell development through controlling protein

synthesis

Composition of nucleic acids

The nucleic acids are bio-polymers (molecules are linked together through

polymerisation reaction like the formation of proteins from amino acids)

The basic building blocks of nucleic acids are called necleotides.

nucleotides are formed from the following units:

1.a phosphoric acid molecule, H3PO42. a five-carbon sugar

3. a nitrogen-containing organic base

Type of Nucleic acids DNA

RNA

In t roductory to B iochemist ry- DNA & RNA

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DNA - Deoxyribonucleic Acids

DNA has huge molecular weight ranging from 6~16 million amu

DNA is found primarily in the nucleus of living cell

DNA stores the genetic information of the cell & controls the production of protein

RNA - Ribonucleic Acids

RNA has smaller molecular weight ranging from 20,000~40,000 amu

RNA is mostly found outside the nucleus in the cytoplasma(substance around cell

membrane)

RNA carries the information stored by DNA out of nucleus of cell into cytoplasma,

where proteins are synthesised based on the instruction delivered by RNA

The differences in composition of DNA and RNA

The only difference between DNA and RNA

is the 5-carbon sugar units in the nucleotides.DNA has deoxyribose

NRA has ribose

HOCH2lClH

O

HlClOH

HlClOH

H

lClOH

HOCH2lClH

O

HlClOH

HlClH

H

lClOH

ribose deoxyribose

Introduction to SpectroscopesAnalyt ical Techniqu es

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4f



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Atoms & molecules can have different

states each having a specific E level

Ground state

The normal state (the lowest possible E level)

Excited state

When absorbing electromeganetic radiation

(e.m.r.). Atoms/molecules rise their Elevels

Atoms/molecules at excited state are not stable& tends to return to their ground state.

In the process of returning their ground states,

the energy gained earlier is release in a form of

e.m.r. (photons).

Absorption/Emission spectra

E.m.r. has energy determined by its frequency

E.m.r. can be absorbed and emitted by atoms/

molecules contain specific Information of A/M.

These e.m.r.s can be recorded and analysed

- the base of many spectroscopic techniques.

n = 1

n = 2

n = 3,

etc.

D

Energy

n=1

n=2

n=3

n=4

1s2s

2p

3s

3p

4s

3d4p

4d4f

S0

v1

v2v3v4

S2

v1

v2v3

v4

T1

v1

v2v3v4

Atomic level

Molecular level

Introduction to

UV-Visible Absorption Spectroscopy

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UV-Visible Absorption Spectroscopy

The electromnetic radiations

X-Ray UV Visible IR Microwave

200nm 400nm 800nm

Wavelength (nm)

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UV Absorption Spectrometer

Sample

90C

DetectorUV Light Source

Monochromator Monochromator

Emit fluorescent lightas energy decreases

Ground state

AntibondingAntibonding

Nonbonding

Bonding

BondingEnergy s

p

ss

pp

nsn

np

Electron's molecular energy levels

sp

UV Spectrometer Applications

Protein

Amino Acids (aromatic)

Pantothenic Acid

Glucose Determination

Enzyme Activity (Hexokinase)

Visible Spectrometer Applications

Niacin

Pyridoxine

Vitamin B12

Metal Determination (Fe)

Fat-quality Determination (TBA)

Enzyme Activity (glucose oxidase)

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UV Absorption Spectra Visible Absorption Spectra

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The working principle

1. Sample molecules are ionised in

the ion source section2. The ions with different masses

and charges travel through a

magnet field at different speeds,

arriving to detector at different

time scales

3. The charges of ions are

converted to electricity current,the intensity of which is then the

measure of the concentration of

the molecules

The measurement results are

directly linked to the atomic

mass of molecules Very useful in detecting organic

molecules & in isotopic tracing

analysisSchematic diagramme of a single-focusing mass

spectrometr with an electron-impact ion source

lon source mass analyser detector

heated filament

produce electrons

beam which

collide and ionise

sample molecules

Ions with different mass & e-

changes travel at different

speeds under the magnetic

field, reaching the detector

at different time.

The charges carried

by the ions are

converted to electricy

current and detected

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Mass spectrum of chlorine gas

m/e ratio Corresponding ion

35 35Cl+

37 35Cl+

70 35Cl+ - 35Cl+

72 35Cl+ - 37Cl+

74 37Cl+ - 37Cl+

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The working principle of Chromatography

The level of Interaction (adsorption) betw.

packing material & sample A, B differs,resulting in different speeds of travel of A &

B in a media (paper, column etc.)

Usually sample to be analysed is

injected into a carrier (gas or liquid)

Carrier is usually inert (does not react

with packing materials)

The components in sample, being

separated after chromatography, are

analysed by TCD or mass spec.

Types of chromatography

LC - Liquid (carrier & A,B) Chromatography

GC - Gas (carrier & A,B) Chromatography

HPLC - High Pressure Liquid Chromatography

t

gas or

liquid

sample (A+B)injection A B

effluent

column packing, P (stationary phase)

t

c

t

c

c

A

B

A

B

t = to

t = ti

t = te

effluent

Assuming P likes A (A stay with P longer)

carrier

(C)

effluent

effluent

v

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Typical GC setup GC chromatograph

ch4751_lecture notes

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