Chemical Kinetics or

Reaction Rates

You have 6 ball bearings with identical magnetic strength

but different diameters where A > B > C > D > E > K.

Which two will hold together the most & the least.

Reaction Rate•The speed at which reactants become products in a chemical reaction

•M/s

Kinetic Theory• All matter is made up of

tiny particles

• The particles are in constant motion

• All collisions are elastic ?

Elastic Collisions•Collisions in which there is no energy change

Particles Reacting•For particles to react, they must collide

•Reaction rate is dependent on collision rate

Particles Reacting• When a reaction takes place,

an energy change occurs

• Collision in which reactions occur are not elastic

Reaction Rate•Rate = - d[Reactant]/dt

•Rate = +d[Product]/dt

•M/s

Factors Affecting Rxn Rate

• Concentration, Temperature, Agitation, Process, Orientation, Catalyst, SA of Solid, Pressure of Gas, Reaction Mechanism

List & describe all of the factors

affecting rxn rates.

Drill:•List at least 5 factors that will affect reaction rates

Rate Expression•An equation showing how rate depends on the amount of reactants in a rate determining reaction

Rate Expression

•Rxn: aA(aq) + bB(aq)

P•Expr: Rate = k[A]a[B]b

•[A] = molarity of A

Rate Expression

•Rxn: aA(g) + bB(g)

Product

•Exp: Rate = kPAaPB

b

Write Rate Expressions for:

H2(g) + N2(g) NH3(g)

HCl(aq) + Ba(OH)2(aq)

BaCl2(aq) + H2O (l)

Write Rate Expressions for:

Cl2(g) + HOOH (aq)

P(aq)

Reaction Order• The total number of reactants

• Equals total of all of the coeffecients of R in the rxn

• Equals total exponents in rate expression

Determining Rxn Rate

• Find rate expression ratios

• (Ratio of concentrations)x = rate ratio

• x = exponent

Experimental ResultsExp [A] [B] [C] Rate

1 1.0 1.0 1.0 2

2 2.0 1.0 1.0 4

3 1.0 2.0 1.0 8

4 1.0 1.0 2.0 4

Experimental ResultsExp # [A] [B] [C] Rate

1 0.1 0.1 0.1 3

2 0.3 0.1 0.1 9

3 0.1 0.3 0.1 27

4 0.1 0.1 0.3 3

Half-Life•The time required for one half of a reactant

to be decomposed in a reaction

Useful RelationshipsRxn Order Rate Exp

0 R=k

1 R=k[A]

2 R=k[A]2

Useful RelationshipsRxn Order Half Life

0 [Ao]/2k

1 0.693/k

2 1/k[A]

Useful RelationshipsRxn Order Lin Plot

0 [A] vs k

1 ln[A] vs k

2 1/[A] vs k

Useful Relationships• Rxn Order Calc R E

• 0 -d[A]/dt = k

• 1 -d[A]/dt = k[A]

• 2 -d[A]/dt = k[A]2

Experimental ResultsExp # [R] [S] [T] Rate

1) 0.25 0.20 0.10 2

2) 0.25 0.60 0.10 18

3) 0.50 0.20 0.10 4

4) 0.25 0.20 0.40 32

Clausius-Claperon Eq

Ea= R ln(T2)(T1) k2

(T2 – T1) k1

Calculate the activation energy of a reaction

whose rate constant is 2.0 x 103 at 27oC and is

2.0 x 106 at 77oC:

Experimental ResultsExp # T [A] [B] Rate

1) 27oC 0.10 0.10 2

2) 27oC 0.30 0.10 18

3) 27oC 0.10 0.20 4

4) 77oC 0.10 0.10 20

Reaction Mechanism

•The sequence of steps that make up

the reaction process

Reaction Mechanism

Step 1 A <--> B fast

Step 2 B ---> C slow

Step 3 C <--> D fast

Total A ---> D

Reaction Mechanism•The rate determining

step in a reaction mechanism is the

slowest step

Reaction Mechanism•To solve the rate expression, you

must use the slowest step

Solve Rate Expression

A + B C + D fast

2C + A G fast

2D + B K fast

2G + 2K Prod. slow

Solve Rate Expression

X + Y M + N fast

M + N 2G slow

2N + G K fast

2G + 2K Prod. fast

Solve Rate Expression

X + Y M + N fast

3M + N 2G fast

2N K fast

4G + 2K Prod. slow

Solve Rate ExpressionA + B C + D fast

4C + A 2G fast

4D + B 2K fast

G + K Q + W slow

Q + W Prod. fast

Review

Experimental ResultsExp # T [A] [B] Rate

1) 27oC 0.10 0.10 2

2) 27oC 0.30 0.10 18

3) 27oC 0.10 0.20 8

4) 77oC 0.10 0.10 20

Solve Rate ExpressionA + B C + D fast

4 C + A 2G fast

2 K 4D + B fast

G + K Q + W fast

2Q + 2W Prod. slow