REACTION RATES

How fast do reactions happen? The rate of a reaction is stated as the

change in concentration of a reactant or product per unit of time.

Average reaction rate. Example: CO + NO2 CO2 + NO If the concentration of CO is 0.100M at

time 0.00s and 0.01M at time 2 seconds. we can calculate the average reaction rate.

Example In a reaction between butyl chloride

(C4H9Cl) and water, the concentration of C4H9Cl is 0.220M at the beginning of the reaction. At 4.00s, the concentration of butyl chloride is 0.100M. Calculate the average reaction rate as moles of C4H9Cl consumed per second.

H2 + Cl2 2HCl

Time (s) [H2]M [Cl2]M [HCl]M0.00 0.030 0.050 0.0004.00 0.020 0.040

Rate Law For every chemical reaction we can write

a Rate Law Example:

The reaction A B is a one step reaction. The rate law for this reaction is…

Rate = k [A] The symbol k is the SPECIFIC RATE CONSTANT.

This is a specific constant that is different for every reaction.

We can see that the rate is directly proportional to the concentration of A

First-Order Reaction Rate Laws In the last example the rate law was Rate = k [A] The notation [A] could also be written as

[A]1

This would be a first order reaction. The reaction order defines how the rate is

affected by the concentration of reactant A.

Real Example: H2O2 O2 + H2O

Higher-Order Rate Laws The over all reaction order of a chemical

reaction is the sum of the orders of the individual reactants.

Consider the equation aA + bB Products Where a and b are the molar coefficients The general rate law will be Rate = k[A]m[b]n

The over all reaction order will be m + n

Write the rate law for the reaction A B if the reaction is third order in A. ([B] is not part of the rate law, B is a product.

The rate law for the reaction 2NO + O2 2NO2 is first order in O2 and third order over all. What is the rate law for this reaction?

Determining Reaction Order

To determine the exponent of each reactant we need to compare the concentrations and the reaction rates.

Trial [A] (M) [B] (M) Rate 1 0.100 0.100 2.00x10

-32 0.200 0.100 4.00x10

-33 0.200 0.200 16.0x10

-3

Find the rate law for reaction A+B C

Trial [A] [B] Rate1 0.100 0.100 2.00x10-3

2 0.200 0.100 2.00x10-3

3 0.200 0.200 4.00x10-3

A + B C

Trial [A] [B] Rate

1 0.1 0.1 4.0 x 10-5

2 0.1 0.2 4.0 x 10-5

3 0.2 0.1 16.0 x 10-5

2 NO + O2 2 NO2

Trial [NO] [O2] Rate

1 0.03 0.02 0.0041

2 0.06 0.02 0.0164

3 0.03 0.04 0.0082

Collision Theory In order for a chemical reaction to occur

the reactants must come in physical contact with one another.

They also have to come in contact with one another in the right way.

When they come in contact they must have enough energy in order for the reaction to occur.

Collision Theory Summary Reacting substances must collide Reacting substances must collide in the

correct orientation. Reacting substances must collide with

enough energy for the reaction to occur.

Activation Energy The minimum amount of energy that

reacting particles must have to form an activated complex and lead to a reaction is called the activation energy. (Ea)

A high Ea means that very few of the collisions that are taking place have enough energy to produce activated complexes and a reaction.

A low Ea means that more collisions that are taking place have more than the required energy to cause a reaction.

Ways to speed up a reaction

Temperature Concentration Surface area. Catatysts

Temperature Increasing the temperature will increase the

amount of energy that the colliding particles have. Resulting in a faster reaction

Concentration: Increasing the concentration of one or both of

the reactants will increase the number of collisions that happen, thus speeding up the reaction.

Surface area: Providing more opportunities for collisions to

happen will speed up a reaction. Catalysts:

A catalysts is used to lower the activation energy of a reaction.