Chemical kinetics
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Chemical Kinetics. An Introduction. CO(g) + NO 2 (g)  CO 2 (g) + NO. H 2 O 2 (aq)  H 2 O(l) + O 2 (g). S 2 O 8 2- + 2 I -  I 2 + 2 SO 4 2-. Questions about reactions. ?. What’s happening?. The chemical equation.

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Chemical Kinetics

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Chemical kinetics

Chemical Kinetics

An Introduction

CO(g) + NO2(g)  CO2(g) + NO

H2O2(aq)  H2O(l) + O2(g)

S2O82- + 2 I- I2 + 2 SO42-


Questions about reactions

Questions about reactions

?

  • What’s happening?

The chemical equation

2 IO3- + 5 HSO3-→ I2 + 5 SO42- + 3 H+ + H2O

2.How fast is it happening?

Kinetics

H2O2(aq) → H2O(l) + O2(g) -- slow

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

3.To what extent does it occur?

Equilibrium

Chapters 15-18

HC2H3O2 + H2O C2H3O2- + H3O+


Chemical kinetics1

Chemical Kinetics

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

A Definition


Rates of reaction

Rates of Reaction

  • Very fast

    • Explosions, neutralizations

      • Almost instantaneous

  • Medium

    • Cooking, rusting

      • minutes to years

  • Slow

    • Formation of diamonds, decay of 235U

      • Up to millions of years


Expressing reaction rates

Expressing Reaction Rates

Speed of a car:

mph =

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

Concentration of reactants decreases

Concentration of products increases


Chemical kinetics

Expressing Reaction Rate Cont’d

For the reaction A → B

Rate of reaction =

Or…

Note the sign:

A is disappearing


Chemical kinetics

Δ[B]

Δ[A]

1

1

= -

= -

b

a

Δt

Δt

Δ[D]

Δ[C]

1

1

=

=

d

c

Δt

Δt

General Rate of Reaction

a A + b B → c C + d D

Rate of reaction = rate of disappearance of reactants

= rate of appearance of products


Chemical kinetics

An Example

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

[C2H4]

t

[O3]

t

Rate = - = -

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

[O3]

t

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

Rate = -

(1.10x10-5mol/L) - (3.20x10-5mol/L)

Rate = -

= -3.50 x 10-7mol/L·s

60.0 s - 0.0 s

The reaction between ethylene and ozone:


Chemical kinetics

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 o 3 vs time

Plot of [O3] vs. Time


But wait there s more

But Wait! There’s More…

Rate is dependent on concentration!

Can see this experimentally


Chemical kinetics

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


Instantaneous rate

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 o 3 vs time1

Plot of [O3] vs. Time


Chemical kinetics

Reaction Rate Law

For 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

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.

More on this next week


So what affects reaction rate

So, what affects reaction rate?

 Concentration

Molecules 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

Factors Affecting Reaction Rate

 Physical state

Molecules 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 rate1

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 rate2

Factors Affecting Reaction Rate

 Nature of the reactants

Some 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 rate3

Factors Affecting Reaction Rate

 Presence of a catalyst

Catalysts 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

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

Evidences of a Chemical Reaction

  • Color change

  • Precipitate formation

  • Temperature change

  • Gas evolution

  • Formation of a weak electrolyte


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