Bonding & Energy

Energy of Reactions

K Warne

Bonding

O

OH

H

H

H

We know that atoms form bonds. And we also know that these bonds can break and new bonds can be formed when reactions take place.

Bonding

O

OH

H

H

H

Bonding

O

H

OH

H

H

Bonding

O

H

OH

H

H

Bonding

OH

H

H2O

OH

H

H2O

OOH

HH

H

Two hydrogen molecules and one oxygen molecule react to form two water molecules.

.........H-H .......... bonds

.......... O=O ...........bond

............H-O ...........bonds

BONDS BROKEN

•....... x ......... single bonds•1x O=O double bond

BONDS FORMED

.................H single bonds

........ H2 + O2 ........ H2O

Bonding

O

H

H

H2O

O

H

H

H2O

OOH

HH

H

Two hydrogen molecules and one oxygen molecule react to form two water molecules.

Two H-H single bonds

One O=O double bond

Four H-O single bonds

BONDS BROKEN

2x H-H single bonds1x O=O double bond

BONDS FORMED

4x O-H single bonds

2 H2 + O2 2 H2O

+

e-

Bonding TheoryWhy do atoms join together?

e- e-

?

e-

The nucleus of one atom attracts the electrons from the other atom.

Bonding Theory

e- e-

e- e-

• Atoms are attracted to each other as the ________ from one atom attract the ________ from the other atom.

• As two atoms approach each other the _______ of the system _______.

• When the atoms get close enough for the ________ to begin __________ the energy begins to ___________.

• If the atoms were forced closer the energy of the system would _________.

e-e-

Bonding Theory

e- e-

e- e-

• Atoms are attracted to each other as the protons from one atom attract the electrons from another atom.

• As two atoms approach each other the energy of the system drops.

• When the atoms get close enough for the nuclei to begin repelling the energy begins to increase.

• If the atoms were forced closer the energy of the system would rise sharply.

e-e-

Bond Energy & Length

1. Atoms are attracted to each other as the ............... from one atom attract the ................ from another atom.

2. As two atoms approach each other the energy of the system ..................

3. When the atoms get close enough for the ................ to begin repelling the energy begins to ................

4. If the atoms were forced closer the energy of the system would .......................

74 100 200 H-H bond length pm (X10-12 m)

0

-100-

-200-

-300-

-400--432-

-500-

74 100 200 H-H bond length pm (X10-12 m)

0

-100-

-200-

-300-

-400--432-

-500-

Bond Energy & Length1. Atoms are attracted

to each other as the protons from one atom attract the electrons from another atom.

2. As two atoms approach each other the energy of the system drops.

3. When the atoms get close enough for the nuclei to begin repelling the energy begins to increase.

4. If the atoms were forced closer the energy of the system would rise sharply.

X10-12

The bond is stable when the minimum energy system is reached.

Bond Enthalpies

+

Whenever a bond is formed ENERGY ____________.

Whenever a bond is BROKEN ENERGY is _________.

Bond FORMING – _______________.

Bond BREAKING – ________________.

Bond Energy: The energy __________ to break a bond. (kJ.mol-1) Stronger bonds _______________ bond energy.

Bond Enthalpies

+

Whenever a bond is formed ENERGY is given out.

Whenever a bond is BROKEN ENERGY is TAKEN IN.

Bond FORMING – EXOTHERMIC.

Bond BREAKING – ENDOTHERMIC.

Bond Energy: The energy required to break a bond. (kJ.mol-1) Stronger bonds greater bond energy.

Exothermic Reactions

Reaction gives off energy to the surroundings – surroundings get HOT!

Exothermic ReactionsReaction ________________ the surroundings – _____________________!

Exothermic ReactionsReaction GIVES ENERGY to the surroundings – GET HOT.

REACTANTS

PRODUCTS

540

140

EN

TH

AL

PY

(In

tern

al h

eat

ener

gy)

kJ

Exothermic ReactionsReaction gives off energy to the surroundings – surroundings get HOT!

540 kJ

Energy

140 kJ

Energy released = 140 – 540 = -400 kJ

Energy of REACTION = ENERGY OF PRODUCTS– ENERGY OF REACTANTS

ΔHr = ΔHP - ΔHR

ΔHr for EXOTHERMIC is ALWAYS NEGATIVE!

EndothermicEndothermic ReactionsReaction takes energy from the surroundings – surroundings get cold!

EndothermicEndothermic ReactionsReaction ________________ the surroundings – _____________________!

HW: Fill in all details on this profile lable: REACTANTS, PRODUCTS , ΔHr, ACTIVATION ENERGY, BOND BREAKING, BOND FORMING…

Endothermic ReactionsReaction takes energy from the surroundings – surroundings get cold!

(a) Energy taken in when reactant bonds break.

Endothermic ReactionsReaction takes energy from the surroundings – surroundings get cold!

Reactants

Products

Activated Complex(b) Energy released when product bonds form.

(c)(a)

(b)

ΔHreaction = Eproducts– Ereactants

= 540 – 140 = +400 kJ

ΔHreaction > 0 (i.e. positive)

ΔHreaction

140kJ

540kJ

600

kJ

Activation energy

200

kJ

400

k

J ΔHr > 0

Overall more energy taken in than given out!

Exothermic ReactionsReaction gives off energy to the surroundings – surroundings get HOT!

Energy needed to ................... all bonds.

Energy released when

............... bonds

...................

ΔHreaction = E............................ – E.................................

ΔHreaction ....... 0 (i.e. ...................)

ΔHreaction

H = internal Heat Energy

or ENTHALPY

E = ................ ENERGY

ΔHr= Ebreaking – E forming = ........– ........= ......... kJ

100

kJ

640

kJ

Exothermic ReactionsReaction gives off energy to the surroundings – surroundings get HOT!

Energy needed to break all bonds.

Energy released when new bonds form

ΔHreaction = EBREAKING – EFORMING

ΔHreaction < 0 (i.e. negative)

ΔHreaction

H = internal Heat Energy

or ENTHALPY

E = BOND ENERGY

ΔHr= Ebreaking – E forming = 100 – 640 = -540 kJ

100

kJ

640

kJ

(a) Energy needed to ................ all bonds.

Endothermic ReactionsReaction takes energy from the surroundings – surroundings get cold!

Reactants

Products

Activated Complex

(b) Energy released when ..................................

(c)(a)

(b)

ΔHreaction = Eproducts– Ereactants

= .........– .........= .......kJ

ΔHreaction .... 0 (i.e. ...............)

ΔHreaction

140kJ

540kJ

ΔHreaction = Ebreaking– Eforming

= ......– ........= ..........kJ

ΔHreaction ...... 0 (i.e. ...............)

600

kJ

Activation energy

200

kJ

400

k

J

Or

ΔHr ....... 0

Overall ........... energy taken in than given out!

(a) Energy needed to break all bonds.

Endothermic ReactionsReaction takes energy from the surroundings – surroundings get cold!

Reactants

Products

Activated Complex

(b) Energy released when new bonds form

(c)(a)

(b)

ΔHreaction = Eproducts– Ereactants

= 540 – 140 = +400 kJ

ΔHreaction > 0 (i.e. positive)

ΔHreaction

140kJ

540kJ

ΔHreaction = Ebreaking– Eforming

= 600 – 200 = +400 kJ

ΔHreaction > 0 (i.e. positive)

600

kJ

Activation energy

200

kJ

400

k

J

Or

ΔHr > 0

Overall more energy taken in than given out!

Bond Length (pm) Energy

(kJ/mol)Bond Length (pm)

Energy (kJ/mol)

H--H 74 436 H--C 109 413

C--C 154 348 H--N 101 391

N--N 145 170 H--O 96 463

O--O 148 145 H--F 92 568

F--F 142 158 H--Cl 127 432

Cl-Cl 199 243 H--Br 141 366

Br-Br 228 193 H--I 161 298

I--I 267 151 C--C 154 348

C--C 154 348 C=C 134 614

C--N 147 308 CC 120 839

C--O 143 360 O--O 148 145

C--S 182 272 O=O 121 498

C--F 135 488 N--N 145 170

C--Cl 177 330 C=O 120 799

C--Br 194 288

Can you identify any TRENDS or PATTERNS in this data?

Bondlength (pm) and bond energy (kJ/mol)

Bond Length Energy Bond Length Energy

H--H 74 436 H--C 109 413

C--C 154 348 H--N 101 391

N--N 145 170 H--O 96 463

O--O 148 145 H--F 92 568

F--F 142 158 H--Cl 127 432

Cl-Cl 199 243 H--Br 141 366

Br-Br 228 193 H--I 161 298

I--I 267 151 C--C 154 348

C--C 154 348 C=C 134 614

C--N 147 308 CC 120 839

C--O 143 360 O--O 148 145

C--S 182 272 O=O 121 498

C--F 135 488 N--N 145 170

C--Cl 177 330

C--Br 194 288

Bond Energy & LengthBond FORMING – EXOTHERMIC.Bond BREAKING – ENDOTHERMIC.

Bond Energy: E (kJmol-1)

• The amount of energy ____________ ______________________________

• Given out when a bond is _________.

• Taken in when bond is ___________.

Po

ten

tia

l E

ner

gy

Distance between nuclei

Bond Length: d (nm)

• The distance between ___________ _____________________________.

• _________ when bonds STRONGER.

• ___________ when bonds WEAKER.

e- e-

e- e-

Bond Energy & LengthBond FORMING – EXOTHERMIC.Bond BREAKING – ENDOTHERMIC.

Bond Energy: E (kJmol-1)

• The amount of energy needed to break a bond.

• Given out when a bond is FORMED.

• Taken in when bond is BROKEN.

Po

ten

tia

l E

ner

gy

Distance between nuclei

Bond Length: d (nm)

• The distance between the nuclei of two covalently bonded atoms.

• Shorter when bonds STRONGER.

• Longer when bonds WEAKER.

d E

e- e-

e- e-

Energy Changes During Reactions

Energy Changes During Reactions

ΔHr = Hprod - HReact

Po

ten

tial

En

erg

y

Po

ten

tial

En

erg

y

H prod

H reactH prod

H react

A CATALYST speeds up a reaction by lowering the ACTIVATION ENERGY of the reaction through providing a different reaction pathway.

ΔHΔHrr > 0 > 0 ΔHΔHrr < 0 < 0

Biological catalysts are called ENZYMES

Energy Changes During Reactions

ΔHr = Hprod – HHReact = Ebreakingbreaking– Eforming

Po

ten

tial

En

erg

y

Po

ten

tial

En

erg

y

H prod

HreactH prod

H react

You can determine ΔHr in TWO different ways. The equation you use depends on the information you are given.

ΔHr > 0 ΔHΔHrr < 0 < 0

HHbreakingbreakingHforming

HHbreakingbreakingHforming

Given ENTHALPYS given BOND ENERGIES

Bond Energy Calculations

H2 + Cl2 2H-Cl

H H

Cl Cl

H H

Cl Cl

H H

Cl Cl

Activated complexBonds Breaking/Forming

ΔHreaction = Ebreaking– Eforming

=

= …… – ……. = …….kJ

ΔHreaction > …… (i.e. ………….)

Look up the required bond energies:E (H-H) =

E (Cl-Cl) =

E (H-Cl) =

Bond Energy Calculations

H2 + Cl2 2H-Cl

H H

Cl Cl

H H

Cl Cl

H H

Cl Cl

Activated complexBonds Breaking/Forming

ΔHreaction = Ebreaking– Eforming

= E(H-H)+E(Cl-Cl) – 2E(H-Cl)

= 436 + 243–(2x432)

= -185kJ

Δhreaction < 0 (i.e. EXOTHERMIC)

Look up the required bond energies:E (H-H) = 436 kJ.mol-1

E (Cl-Cl) = 243 kJ.mol-1

E (H-Cl) = 432 kJ.mol-1

Bond Energy Calculations

H2 + Cl2 2H-Cl

H H

Cl Cl

H H

Cl Cl

H H

Cl Cl

Activated complex Bonds Breaking/Forming

E (H-H) = 436 kJ.mol-1

E (Cl-Cl) = 243 kJ.mol-1

E(H-Cl) = 432 kJ.mol-1

ΔHreaction = Ebreaking– Eforming

= E(H-H) + E(Cl-Cl) – 2E(H-Cl)

= 436 + 243 – 2(432)

= - 185 kJ

ΔHreaction > 0 (i.e. EXOTHERMIC)

Bond Energy ExamplesUse data from the bond energy table to

calculate the enthalpy of reaction for the following reactions:

1.2 H2 + O2 2H2O (no 4 TT)

2.CH4 + O2 CO2 + H2O

3.CH3CH2OH + O2 CO2 + H2O

OC C

H

H

H

H

H

H

C

H

H

H

H

Bond Energy ExamplesUse data from the bond energy table to calculate the

enthalpy of reaction for the following reactions:

1. 2 H2 + O2 2H2O (no 4 TT)

Bond Energy ExamplesUse data from the bond energy table to

calculate the enthalpy of reaction for the following reactions:

2.CH4 + O2 CO2 + H2O

C

H

H

H

H

Bond Energy ExamplesUse data from the bond energy table to calculate the

enthalpy of reaction for the following reactions:

3. CH3CH2OH + O2 CO2 + H2O

OC C

H

H

H

H

H

H

EXAM QUESTIONS

TRUE or FALSE1.The C-H bond is stronger than the C-C bond because an H-atom is bigger than a C-atom.2.Energy is released in all chemical reactions.

EXAM QUESTIONS - ANSWERSTRUE or FALSE1.The C-H bond is stronger than the C-C bond because an H-atom is bigger than a C-atom. FALSE – longer = weaker/lower bond energy (H atom is smaller)

2.Energy is released in all chemical reactions. FALSE – only released in exothermic reactions

EXAM QUESTIONS

1.

2.

EXAM QUESTIONS1.

2.

C

B

Calculate H r

Formation of Ionic Bond1. Write down all the

steps that need to take place for this reaction to take place.

2. Try and place the steps in order.

3. Decide which steps would be endothermic and which would be exothermic.

Remember NaCl

is actually Na+ Cl-

Na(s) + Cl2(g) NaCl(s)

Eg: sodium atoms need to separate from each other:

Na (s) Na (g)

Formation of Ionic BondA large amount of

energy (lattice) is released when the gaseous ions bond together into the ionic crystal lattice.

Ionic compounds are therefore very stable and require large amounts of energy to break the bonding.

Ionic compounds have HIGH MELTING POINTS we say they are thermally stable.

Na(s) + 1/2 Cl2(g) NaCl(s)

Na(g) + 1/2 Cl2(g)

Na(g) + Cl(g)

Na+(g) + e- + Cl(g)

Na+(g) + Cl-

(g)

Born Haber CycleA large amount of

energy (lattice) is released when the gaseous ions bond together into the ionic crystal lattice.

Ionic compounds are therefore very stable and require large amounts of energy to break the bonding.

Ionic compounds have HIGH MELTING POINTS we say they are thermally stable.

Na(s) + 1/2 Cl2(g) NaCl(s)

Na(g) + 1/2 Cl2(g)

Na(g) + Cl(g)

Na+(g) + e- + Cl(g)

Na+(g) + Cl-

(g) Ionisation Energy

Dissociation Energy

Sublimation EnergySublimation Energy

Electron Affinity

Lattice Energy

MUST BE LEARNT BY HEART!

ONE TWO THREE

Hydrogen H+ Beryllium Be2+ Aluminium Al3+

Lithium Li+ Magnesium Mg2+ Iron(III) Fe3+

Sodium Na+ Calcium Ca2+  

Potassium K+ Barium Ba2+  

Silver Ag+ Lead Pb2+  

Copper(I) Cu+ Zinc Zn2+  

Ammonium NH4+ Iron(II) Fe2+  

Oxonium H3O+ Copper(II) Cu2+  

VALENCY TABLE 1

VALENCY TABLE 2 Negative IonsFluoride F- Oxide O2- Nitride N3-

Chloride Cl- Sulphide S2- Phosphate PO43-

Bromide Br- Carbonate CO32-  

Iodide I- Sulphate SO42-  

Hydroxide OH-    

Nitrate NO3-    

Hydrogencarbonate HCO3-    

Hydrogensulphate HSO4-    

Permanganate MnO4-    

Ethanoate CH3COO-

   

The trivial names for HCO3- and HSO4

- are bicarbonate and bisulphate, respectively.