Chemical Kinetics Review

The study of chemical reactions

Reaction Rates

Chemical reaction rates are the rates of change in concentrations or amounts of either reactants or products.

Units for changes in amounts: mol/s, g/s, lb/s, kg/day etc.

Units for changes in concentrations: mol/(L s), g/(L s), %/s etc.

Rates can be average rates, instantaneous rates, or initial rates.

Factors Influence Reaction Rates

  1. Concentration Effect or dependence on cencentration

    The dependences of rates on concentrations are called differential rate laws: expressions of rates in terms of concentrations of reactants - order.

  2. Dependences of concentration on time are Integrated rate laws:
    First Order Reactions
    Second Order Reactions

  3. Nature of Reactants: mechanism

  4. Temperature: activation energy

  5. Heterogeneous reactions: reactants are present in more than one phase.

  6. Catalysts: substances used to change reaction rates.

Initial rate problems

Example 13.3.4 For the reaction 2 N2O5 = 4 NO2 + O2

Exprmnt Initial [N2O5] in M Rate in M/s
expnt 1 1e-2 4.8e-6
expnt 2 2e-2 9.6e-6
What is the order of the reaction?

Solution

 

 

 

Example

Derive the rate law for the reaction A + B + C => products from the following data, where is measured as soon as the reactants are mixed.
Experiment1234
[A]o0.1000.2000.2000.100
[B]o0.1000.1000.3000.100
[C]o0.1000.1000.1000.400
rate0.1000.8007.2000.400

Solution

13.4 Change of Concentration with Time - The integrated rate law

Example 13.5

The reaction 2 N2O5 = 4 NO2 + O2 is first order, with k = 4.8 s-1 at some T. If the initial concentration is 2.00e-2 M, what is the concentration after 10 min? How long will it take for the concentration of N2O5 to reduce to 1.00e-2 M from its initial concentration?