8.1: Background

Week 1

Chemical kinetics is the study of the rates and mechanisms of chemical reactions. The rate of a chemical reaction refers to the speed of the reaction. Some reactions, such as the rusting of iron, are very slow. Other reactions, such as the reaction of an acid and a base, are very fast. To determine the rate of a reaction, the time rate of change of the concentration of some reactant or product must be measured. One method of measuring reaction rates involves preparing solutions of known concentration and then measuring the time required for the concentration to decrease by a certain amount.

The ability to prepare a solution of accurately known concentration is an important skill in many disciplines. Chemists, biologists, pharmacists, and medical technologists are just a few examples of those who need to have this skill. Solution concentrations are often expressed in terms of molarity (M).

During Part 1 you will prepare solutions that will be used in Part 2 of the Chemical Kinetics experiment.

Week 2

The rate of a chemical reaction refers to the speed of the reaction. Speed or rate is a concept familiar to everyone. For example, you can determine your average speed for a trip in an automobile by using the equation:

Rate = change in odometer / time

Using a similar method you can determine the rate of a chemical reaction except that now the rate is expressed as

Rate = change in concentration / time

The graph below shows the change in the concentration of a reactant with time. Note that the concentration decreases rapidly at first and then decreases more slowly. Reaction rates are not constant; they slow down as the reactants are consumed.

In general, the rate of a chemical reaction depends on the concentration of the reactants, but exactly how reactant concentrations affect the rate can only be determined by experiment. Consider the chemical reaction

aA + bB -> cC + dD

where the capital letters represent chemical substances and the lower case letters represent the stoichiometric coefficients needed to balance the equation. The dependence of the reaction rate on the concentrations of A and B may be expressed by the "rate law" equation

Rate = k[A]^m[B]ⁿ

where k is the rate constant and the square brackets mean “molar concentration”. The exponents m and n must be determined by experiment. The rate constant is independent of reactant concentrations, but does depend on temperature. Changing the concentration of A or B will not affect k, but increasing the temperature or adding a catalyst will cause k and hence the reaction rate to increase.

The exponents can be determined by running a series of reactions with different concentrations of A and B and measuring the rates. Consider the experimental data in the table below:

By comparing experiments 1 and 2 we see that the concentration of B was the same and, therefore, had no effect on the rate. However, the concentration of A was doubled in the second experiment (2.0 M versus 1.0 M) and we note that the rate also doubled (0.0100 M/s versus .0050 M/s). This means that the rate is directly proportional to the concentration of A or mathematically the exponent m = 1. Similarly, if we compare experiments 1 and 3, where the concentrations of A are the same, we see that doubling the concentration of B (2.0 M versus 1.0 M) results in a four-fold increase in the rate (0.0200 M/s versus 0.0050 M/s). This means that exponent n = 2.

In general, if changing the concentration of a reactant has no effect on the rate, then the reaction is zeroth order (exponent = 0) in that reactant. If doubling the concentration doubles the rate, then the reaction is first order (exponent = 1). If doubling the concentration increases the rate by four, then the reaction is second order (exponent = 2).