Chemical Chemical KineticsKinetics

H2O2 decomposition in an insect

H2O2 decomposition catalyzed by MnO2

Day 2: Rate equationsDay 2: Rate equations

Collision Theory

Collisions

Collisions

EnergyCollisions

Energy

Orientation

NO

NO

YES

REACTION RATES

RR = [P ] = - [R ] t t

P =products R = reactants

Three “types” of rates Three “types” of rates initial rateinitial rateaverage rateaverage rateinstantaneous rateinstantaneous rate

Reaction Rates Reaction Rates Section 15.1Section 15.1

Relative Rates

Reactant

4A 2B + 3C

- [A ] = [B ] = [C ] 4 t 2 t 3 t

Rate Expressions

To determine a reaction To determine a reaction mechanism, mechanism,

we studywe study

the effect of concentrationthe effect of concentration

on the rate of the reactionon the rate of the reaction

To determine a reaction To determine a reaction mechanism, mechanism,

we studywe study

the effect of concentrationthe effect of concentration

on the rate of the reactionon the rate of the reaction

Rate of reaction is proportional to Rate of reaction is proportional to [reactant][reactant]

We express this as a RATE LAWWe express this as a RATE LAW

RateRate of reaction = of reaction = k k [[XX]]

where where kk = rate constant = rate constant

k is k is dependentdependent only on only on TT

Rate of reaction is proportional to Rate of reaction is proportional to [reactant][reactant]

We express this as a RATE LAWWe express this as a RATE LAW

RateRate of reaction = of reaction = k k [[XX]]

where where kk = rate constant = rate constant

k is k is dependentdependent only on only on TT

Rate contantRate contant: : Arrhenius equation Arrhenius equation Rate contantRate contant: : Arrhenius equation Arrhenius equation

k Ae -Ea/RTk Ae -E

a/RTRate

constant

Temp (K)

8.31 x 10-3 kJ/K•molActivation energy

Frequency factor

Frequency factor = frequency of collisions with correct geometry.

Rate constant is dependent on only the activation energy and temperature

REACTION ORDERREACTION ORDERIn general, In general, reaction: reaction:

a a AA + b + b BB --> x --> x X X with a catalyst “C”with a catalyst “C”

Rate = k [A]Rate = k [A]mm[B][B]nn[C][C]pp

The exponents The exponents m, n, and p m, n, and p •• are the reaction order

• can be 0, 1, 2 or fractions

•• must be determined by must be determined by experiment!experiment!

Rate = k [x]m

If m = 0,

Reaction is zero order.

Rate = k [X]0 = k *1

If [X] doubles; rate = K

if Rate = k [X]m

If m = 1,

Reaction is 1st order

Rate = k [X]1

If [3X]1, then rate goes up by 31

If [2X]1, then rate goes up by 21

14

FIRST ORDER REACTIONS

The rate law is

Rate -[A]

time = k [A]

Rate = k [X]Rate = k [X]mm

If m = 2,

Reaction is 2nd order.

Rate = k [X]2

[2X]2 increases rate by 22 = 4

[3X]2 increases rate by 32 = 9

16

SECOND ORDER REACTIONS

Deriving Rate LawsDeriving Rate Laws

Expt. [CH3CHO] Disappear of CH3CHO (mol/L) (mol/L•sec) RATE

1 0.10 0.020 2 0.20 0.081 3 0.30 0.182 4 0.40 0.318

Derive rate law and k for Derive rate law and k for CHCH33CHO(g) --> CHCHO(g) --> CH44(g) + CO(g)(g) + CO(g)

from experimental DATA for rate of disappearance of CH3CHO

Deriving Rate LawsDeriving Rate Laws

Rate = k [CHRate = k [CH33CHO]CHO]22

Here the rate goes up by Here the rate goes up by 44 when initial when initial conc. doubles. Therefore, we say this conc. doubles. Therefore, we say this

reaction is reaction is SECOND SECOND order.order.

The value of k. Use expt. #3 data—The value of k. Use expt. #3 data—

0.182 mol/L•s = 0.182 mol/L•s = k k (0.30 mol/L)(0.30 mol/L)22

k = k = 2.0 2.0 (L / mol•s)(L / mol•s)Using k you can calc. rate at other values ofUsing k you can calc. rate at other values of [CH[CH33CHO] CHO]

at same T.at same T.

SOLVED PROBLEM: pg141

21practice: pg151

22

Rate -[A]

time = k [A]

FIRST ORDER REACTIONS

The rate law is