01) kinetics 1

8/2/2019 01) Kinetics 1

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CHEMICAL KINETICS

Billones Lecture Notes

8/2/2019 01) Kinetics 1


Chemical Kinetics

• study of rates or speeds of chemical reactionand the detailed processes by which reactantsare converted into products.

rate of rxn - change in conc. per unit time

[AB]

[A] or [B]

Concentration

Reaction Coordinate

For the reaction: A + B → AB


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Rate of reaction: Appearance of product

rate= Conc. = [AB]2-[AB]1 = [AB]

time t2-t1 t

Rate of reaction: Disappearance of reactant

rate= - [A] = - [B] (-) is introduced to make the

t t rate positive.

In General, for the rxn:

aA + bB cC + dD

rate= - 1 [A] = -1 [B] = 1 [C] = 1 [D]

a t b t c t d t


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Example:

2O3 3O2

rate = -1 [O3] = 1 [O2]

2 t 3 t

If the rate of NH3 consumption is 0.24 M/s, the rate of disappearance ofO2 would be

4NH3 + 5O2 4NO + 6H2O

rate = (1/4) r NH3 = (1/5) r O2

rate O2 = (5/4) r NH3 = (5/4) (0.24 M/s ) = 0.30 M/s


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COLLISION THEORY- based on the idea that all rxns occur as a result of collisions of

reacting molecules.

rate α no. of collisions

t

-since not all reactions are explosive, not all collisions are effective

for gases, typically 1033 collisions /s. mL

REQUIREMENTS FOR AN EFFECTIVE COLLISION

a) Minimum energy or Energy of Activation, Ea

-Ea must be satisfied!


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Note: The value of Ea has no relationship to the value of H.

E EEa

RH (-)

P

exothermic

R

Ea P

H (+)

endothermic

b) Proper Orientation

- molecules should approach one another in a manne such thatorbitals overlap effectively.

A

A+

BB

A

A

BB

A B

A B

A

A+ B B No rxn!


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TRANSITION STATE THEORY

Thus,

rate = Collision

frequency

Fraction w/

correctorientation

Fraction w/

required energy

- based on the idea that in all rxns, an intermediate complex (calledtransition state or activated complex) formed before finally forming

the product.Rate ~ 1/ energy of T.S

- The higher the E of T.S, the slower the rate.

ER

T.S

Pexothermic R

T.S

P

endothermic

Ea = ET.S - ER


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FACTORS AFFECTING REACTION RATE

- strongly bonded reactants are less reactive

Example:

C diamond + O2 (g) CO2 - high Ea very

slow

Example: In their rxn w/ H2O

Na is more reactive than Mg.

(Ea of Na lower than Ea of Mg)

2) Surface Area

- the greater the surface area; the greater the freq. ofcollision, the faster the rate.

Example: twigs burn faster than a chunk of wood.

1) Nature of Reactants


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3) Catalyst

- speeds up a rxn by providing a low-Ea route

- transformed in the process but recovered at the end

REa catalyzed

Low Ea

P

Ea uncatalyzed

a) Homogenous – catalyst has the same phase as the reactant

(CH3)3COH(l) + HBr(l) (CH3)2C=CH2 +H2O+HBr(catalyst)

w/o HBr: Ea = 65.5 kcal/mol w/ HBr: Ea = 30.4 kcal/mol


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b) Heterogenous – catalyst has different phase than that of

the reactants

Ex. 2N2O (g) + Au(s) 2N2 (g) + O2(g) + Au(s)

Ea w/ Au = 120 kcal/mol

Ea w/o Au = 240 kcal/mol

4) Temperature – the higher the temperature, the faster the rate

k

temp

k= rate constant

K ~ rate of rxn


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Arrhenius Equation

k = A e

- Ea /RT

ln k = - Ea 1 + ln A

R T

y m x b

ln k m = -EaR

1/T (K-1)

At two temperatures

ln k1

= Ea 1 - 1

k2 R T 2 T 1


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5) Concentration of reactants

- Increasing the conc. increases the rate

For eqn:

aA + bB cC + dD

Rate Law: rate= k[A]x[B] y

x,y = order of reaction

- shows the magnitude of the effect of conc.

on the rate

- determined experimentally


01) kinetics 1

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