Chapter 13: Chemical Kinetics
Key Topics:
- The rate of a chemical reaction is a measure of how fast the reaction progresses. The reaction rate is one
number that applies to the entire reaction. It always has units of concentration per unit time. Sometimes it is helpful
to talk about the rate of consumption or rate of formation for a specific reactant or product. These specific rates are related
to the overall rate through stoichiometry.
- The rate of a reaction is dependent on temperature and often reactant concentration.
These relationships are considered through the rate law for a reaction. The rate law equates the rate to the product of a rate
constant "k" (the temperature dependent part) and the reactant concentrations raised to a power called the order.
- The reaction
order is not related to the stoichiometry of the reaction, but is experimentally determined.
- The method of initial rates
- The initial
concentrations of the reactants, and the rate at the start of the reaction for several different reaction conditions are compared.
This comparison allows you to calculate the reaction orders with respect to each reactant and the value of the rate constant.
- Integrated
Rate Law
- These show the relationship between the reactant concentrations and time. Plots of one experiment's worth of concentration
and time measurements provide enough information to graphically determine the order and rate constants for the reaction.
- Both of those
methods provide one value for the rate constant "k". At other temperatures, the same initial concentrations give different values
of k. The reaction order DOES NOT CHANGE as a function of temperature. The dependence of k on time is related through
the Arrenius Equation.
- There are two portions of the Arrenius Equation.
- The frequency factor (A)
- This number is based on the number of
collisions that are in a proper orientation.
- The exponential facto
- Decreases when the activation energy increases, but increases as
the temperature increases.
- The Reaction Mechanism is a series of elementary steps (one collision at a time) that sum to the
overall equation. The rate limiting step (or the slow step) in a mechanism is the step that the overall rate and the rate law
for the overall reaction depends on.
- A Catalyst or enzyme (biological catalyst) changes the reaction mechanism for a reaction, and
lowers the activation energy. By lowering the Ea, the catalyst raises the reaction rate. It must be noted that the addition
of a catalyst does not change the amount of products that you will end up with, it just gets you to that point faster.
Chapter 13 Problems: # 3, 5, 12, 13, 15, 18, 19, 25-31, 35-47, 51-54, 57-60, 63, 67-69, 71-76, 78, 82, 93, 94, 100, 105, 107}