SAT Subject Test Chemistry




Rates of Chemical Reactions


The relationship between the rate of a reaction and the masses (expressed as concentrations) of the reacting substances is summarized in the Law of Mass Action. It states that the rate of a chemical reaction is proportional to the product of the concentrations of the reactants. For a general reaction between A and B, represented by

aA+bB→ …

the rate law expression is

or, inserting a constant of proportionality that mathematically changes the expression to an equality, we have

r = k [A]a [B]b

Here is called the specific rate constant for the reaction at the temperature of the reaction.

The exponents a and b may be added to give the total reaction order. For example:

H2(g)+I2 → 2HI(g)
r = k[H2]1[I2]1

The sum of the exponents is 1 + 1 = 2, and therefore we have a second-order reaction.

Reaction Mechanism and Rates of Reaction

The beginning of this chapter stated that the reaction rate is usually proportional to the concentrations of the reactants. This occurs because some reactions do not directly occur between the reactants but may go through intermediate steps to get to the final product. The series of steps by which the reacting particles rearrange themselves to form the products of a chemical reaction is called the reaction mechanism. For example:

Step 1

A + B

I1 (fast)

Step 2

A + I1

I2 (slow)

Step 3

C + I2

D (fast)

Net equation

2A + B + C


Notice that the reactions of steps 1 and 3 occur relatively fast compared with the reaction of step 2. Now suppose that we increase the concentration of C. This will make the reaction of step 3 go faster, but it will have little effect on the speed of the overall reaction since step 2 is the rate-determining step. If, however, the concentration of A is increased, then the overall reaction rate will increase because step 2 will be accelerated. Knowing the reaction mechanism provides the basis for predicting the effect of a concentration change of a reactant on the overall rate of reaction. Another way of determining the effect of concentration changes is actual experimentation.