Reaction Rates

Chapter 17

Honors Chemistry

Red Blue

Reaction Rates

Reactions can be…

FAST!

Liquid hydrogen and oxygen reacting to launch a shuttle

..or

S

L

O

WConcrete hardening

..or

S

L

O

W

Watching paint dry

Expressing rxn rates in quantitative terms

quantityAverage rate

t

Example: Reaction data for the reaction between butyl chloride (C4H9Cl) and water is given below. Calculate the average reaction rate over this time period expressed as moles of C4H9Cl consumed per liter per second.

quantityAverage rate

t

[C4H9Cl] at t=0.00 s

[C4H9Cl] at t=4.00 s

0.220 M 0.100 M

mol molL L(0.100 - 0.220 )

(4.00 s - 0.00 s)

Table 17-1: Molar Concentration

initialfinal

initial94final94

tt

Cl]H[C - Cl]H[C

molL-0.120

4.00 s

molL s -0.0300

Collision Theory Atoms, ions and molecules must

collide in order to react. Reacting substances must collide with

the correct orientation. Reacting substances must collide with

sufficient energy to form the activated complex.

Orientation and the activated complex

Analogy: if you start with two separate paperclips (reactants) and you wish to link them together (products), not only must the paperclips come into contact, but they also must collide with a specific orientation.

Orientation and the activated complex

Biological example: ENZYMES

You’ll learn much more about this next year

Activation energy and reaction

Only collisions with enough energy to react form products

Activation energy and reaction

Activation energy and reaction

Another example

Activated complex (also called transition state)

reactants

products

Does ∆G tell us anything about rxn rate?

NO If a reaction is spontaneous, it does not

follow that it is fast or slow. Thus, a new branch of chemistry…

kinetics

Factors affecting reaction rates

1) The nature of the reactants

2) Concentration

3) Pressure (gases only)

4) Surface area

5) Temperature

6) Catalysts

NATURE OF REACTANTS

Some elements/compounds are more reactive than others

sodium in water (alkali metals are VERY reactive)

FAST

NATURE OF REACTANTS

Some elements/compounds are more reactive than others

Rusting of iron (it takes time for moisture in the air to oxidize the metal… process can be sped up if salt is present, but will still not react as fast as sodium and water)

SLOW

CONCENTRATION

As concentration ↑, frequency of collisions ↑, and therefore rxn rate ↑

PRESSURE (gases)

For gases, increasing pressure creates the same effect as increasing concentration

SURFACE AREA

← slow

fast

As surface area ↑, rxn rate ↑

TEMPERATURE:

Generally, ↑ temp = ↑ rate Why? Higher temp = faster molecular motion

= more collisions and more energy per collision = faster rxn

Analogy: imagine that you are baby-sitting a bunch of 6 year olds. You put them in a yard and you let them run around. Every now and then a couple of kids will run into each other. Now imagine that you decide to feed them some sugar. What happens? They run around faster and of course there are many more collisions. Not only that, the collisions are likely to be a lot harder/more intense.

CATALYST

Provides an easier way to react

Lowers the activation energy

Enzyme = biological catalyst

Catalyst: a substance that speeds up the rate of a reaction without being consumed in the reaction.

CATALYST

Adding a catalyst speeds up the rxn by lowering the activation energy

Reaction Rate Laws

The equation that expresses the mathematical relationship between the rate of a chemical reaction and the concentration of reactants is a rate law.

k[A]Rate

BA :reaction the For

*where k is a constant specific to this reaction

Reaction Order

The reaction order for a reactant defines how the rate is affected by the concentration of that reactant.

The overall reaction order of a chemical reaction is the sum of the orders for the individual reactants in the rate law.

Reaction Order

The rate law for most reactions has the general form….

Rate = k[reactant 1]m[reactant 2]n… The exponents m and n are called

reaction orders. Their sum (m + n) is called the overall reaction order.

Reaction Order For the reaction aA + bB products

Rate = k[A]m[B]n

where m and n are the reaction orders for A and B respectively

Only if the rxn between A and B happens in a single step (with a single activated complex… which is unlikely) does m=a and n=b.

Thus, the values of m and n must be determined experimentally!!!

Reaction Order

For the reaction aA + bB products

Rate = k[A]m[B]n

where m and n are the reaction orders for A and B respectively

Rate laws cannot be predicted by looking at a balanced chemical equation.

Finding the rate law

The most common method for experimentally determining the differential rate law is the method of initial rates.

In this method several experiments are run at different initial concentrations and the instantaneous rates are determined for each at the same value of time (as near t = 0 as possible)

Using Initial Rates to Determine the Form of the Rate Law

Exp # [A] [B] Initial Rate (M/s)

1 .100M .100M 4x10-5

2 .100M .200M 4x10-5

3 .200M .100M 16x10-5

A + B C

From this data, find the form of the rate law..

Rate = k[A]m[B]n

Exp # [A] [B] Initial Rate (M/s)

1 .100M .100M 4x10-5

2 .100M .200M 4x10-5

3 .200M .100M 16x10-5

5

5

2 4 10 [.100] [.200]1 4 10 [.100] [.100]

m n

m n

Rate kRate k

[.200]1

[.100]

n

n

1 2n

n = 0

Rate = k[A]m[B]n

Exp # [A] [B] Initial Rate (M/s)

1 .100M .100M 4x10-5

2 .100M .200M 4x10-5

3 .200M .100M 16x10-5

Rate = k[A]m[B]n

5

5

3 16 10 [.200] [.100]1 4 10 [.100] [.100]

m n

m n

Rate kRate k

[.200]4

[.100]

m

m

4 2m

m = 2

Exp # [A] [B] Initial Rate (M/s)

1 .100M .100M 4x10-5

2 .100M .200M 4x10-5

3 .200M .100M 16x10-5

Rate = k[A]m[B]n

Rate = k[A]2[B]0

Rate = k [A]2

Exp # [A] [B] Initial Rate (M/s)

1 .100M .100M 4x10-5

2 .100M .200M 4x10-5

3 .200M .100M 16x10-5

2 0[ ] [ ]rate

kA B

5

32

4 104 10

[.100]

Rate = 4x10-3 [A]2

Now, solve for k… Rate = k [A]2

Knowing rate laws and rate orders helps us predict how the reaction will proceed over time

Application: Radioactive decay

is a first order reaction

Half life is constant over time

Allows us to date fossils, etc.

Reaction Mechanisms Most chemical reactions consist of a

sequence of two or more steps (or simpler reactions). These add together to create the overall reaction equation.

Generally, some steps will be fast and others will be slow.

The slow step is the rate determining step.

The reaction between NO2 and CO has the overall reaction:

NO2 + CO NO + CO2

A study of the kinetics of this reaction revealed the rate law for the reaction is.

Rate = k[NO2]2

This Rate Law requires that the slow step of the reaction involves a collision between two NO2 molecules. How can this be a step in the seemingly simple reaction above?

Example:

NO2 + CO NO + CO2

NO2 + NO2 NO3 + NO

NO3 + CO NO2 + CO2

2NO2 + NO3 + CO NO2 + NO3 + NO + CO2

NO2 + CO NO + CO2

Further study of this reaction established that two NO2 molecules can react as follows…

NO3 is a highly reactive material which is capable of transferring an oxygen atom.

NO2 + CO NO + CO2

NO2 + NO2 NO3 + NO

NO3 + CO NO2 + CO2

NO2 + CO NO + CO2

slow

fast

The first equations sets the rate law, so it must be the slow one.

Remember, the rate law only provides information about the slowest reaction in the mechanism.