Chemical Kinetics

An Introduction

S2O8

2- + 2 I- I2

+ 2 SO42-

CO(g) + NO2(g) CO

2(g) + NO

H2O2(aq) H2O(l) + O2(g)

Questions about reactions

1. What’s happening?The chemical equation

?2. How fast is it happening?

3. To what extent does it occur?

Kinetics

EquilibriumChapters 15-18

2 IO3- + 5 HSO3

- → I2 + 5 SO42- + 3 H+ + H2O

H2O2(aq) → H2O(l) + O2(g) -- slow

H2O2(aq) + catalyst → H2O(l) + O2(g) -- FAST

HC2H3O2 + H2O C2H3O2- + H3O+

Chemical Kinetics

The study of reaction rates and the sequence of steps by which a reaction

occursA Definition

Rates of Reaction• Very fast

– Explosions, neutralizations• Almost instantaneous

• Slow– Formation of diamonds, decay of 235U

• Up to millions of years

• Medium– Cooking, rusting

• minutes to years

Expressing Reaction Rates

Speed of a car:

mph = t

x

tt

xx

12

12

in time change

positionin change

For a chemical reaction we want to track concentration of products or reactants over time:

Concentration of reactants decreasesConcentration of products increases

For the reaction A → B

t

]B[

tt

B of concB of conc

in time change

B ofion concentratin change

12

12

Rate of reaction =

Or…

t

]A[

tt

A of concA of conc

in time change

A ofion concentratin change

12

12

Expressing Reaction Rate Cont’d

Note the sign: A is disappearing

a A + b B → c C + d D

Rate of reaction = rate of disappearance of reactants

=Δ[C]

Δt1c

=Δ[D]

Δt1d

Δ[A]

Δt1a

= -Δ[B]

Δt1b

= -

= rate of appearance of products

General Rate of Reaction

Time(s) [O3](mol/L)

0.0 3.20x10-5

10.0 2.42x10-5

20.0 1.95x10-5

30.0 1.63x10-5

40.0 1.40x10-5

50.0 1.23x10-5

60.0 1.10x10-5

The reaction between ethylene and ozone:

C2H4 (g) + O3 (g) C2H4O(g) + O2 (g)

Rate = - = -[C2H4] t

[O3] t

Rate = - = -3.50 x 10-7mol/L·s

(1.10x10-5mol/L) - (3.20x10-5mol/L)60.0 s - 0.0 s

[O3] tRate = -

An Example

This rate is the average rate for a time period

Does not show that rate is changing with time

Does not show rate at a given instant

Plot of [O3] vs. Time

But Wait! There’s More…

Rate is dependent on concentration!

Can see this experimentally

Plot of [C2H4] and [O2] vs. Time

Instantaneous rate

• Use smaller and smaller increments of time

• The slope of a tangent line to the curve at any point is the instantaneous rate

• Note that reaction rate usually refers to the instantaneous rate

Plot of [O3] vs. Time

Reaction Rate LawFor a chemical reaction:

aA + bB + . . . → cC + dD + . . .

The rate law for the forward reaction has the form:

Rate = k [A]m[B]n . . .

k = the reaction rate constant exponents m & n are the reaction orders defines how rate is affected by concentration

More on this next week

For example, if the rate doubles when the concentration of A doubles, the rate depends on [A]1, so m = 1; if the rate quadruples when the concentration of B doubles, the rate depends on [B]2, so n = 2.

So, what affects reaction rate?

ConcentrationMolecules must collide in order to react.

Reaction rate is proportional to the

concentration of reactants.

Rate = k (collision frequency) = k (concentration)

Factors Affecting Reaction Rate

Physical stateMolecules must mix in order to collide.

The physical state (solid, liquid, gas) will affect

frequency of collisions, as well as the physical size of droplets (liquid) or particles in the case of solids.

Factors Affecting Reaction Rate

Temperature Molecules must collide with enough energy to react.

Raising the temperature increases the reaction rate by increasing the number of collisions per time unit, and

especially, the energy of the collisions.

Factors Affecting Reaction Rate

Nature of the reactantsSome species are more reactive than others.

You have seen this with the periodicity of reactivity.

For example the reactivity of the group 1 metals.

Factors Affecting Reaction Rate

Presence of a catalystCatalysts can provide alternate, lower

energy, reaction pathways. Catalysts generally reroute the pathway of a chemical reaction so that this “alternate” path, although perhaps

more circuitous, has a lower activation energy for reaction than the un-catalyzed reaction.

Lab This Week• Explore several of the factors which

affect reaction rate• Perform in any order• Perform either B or C• CHECK REAGENT

CONCENTRATIONS!• Caution with strong acids• Use waste beakers• Part A.1 use 2M H3PO4

• Part D—Note H2O2 decomposes slowly on its own.

Evidences of a Chemical Reaction

• Color change• Precipitate formation• Temperature change• Gas evolution• Formation of a weak electrolyte