Barrons SAT Subject Test Chemistry, 13th Edition

represented by

aA + bB → ...

the rate law expression is

r ∝ [A]a[B]b

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

r = k[A]a[B]b

Here k 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:

H 2 (g) + I 2 (g) → 2HI(g)

r = k[H 2 ]^1 [I 2 ]^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:

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