The Foundations of Chemistry

1.Once the reaction orders are known, experimental data must be used to deter-

mine the value of kfor the reaction at appropriate conditions.

2.The value we determine is for a specific reaction,represented by a balanced equa-

tion.

3.The units of k depend on the overall orderof the reaction.

4.The value we determine does not change with concentrations of either reactants

or products.

5.The value we determine does not change with time (Section 16-4).

6.The value we determine refers to the reaction at a particular temperatureand

changes if we change the temperature (Section 16-8).

7.The value we determine depends on whether a catalystis present (Section 16-9).

EXAMPLE 16-2 Interpretation of the Rate Law

For a hypothetical reaction

A
B
C88nproducts

the rate law is determined to be

ratek[A][B]^2

What happens to the reaction rate when we make each of the following concentration
changes?

(a) We double the concentration of A without changing the concentration of B or C. (b) We
double the concentration of B without changing the concentration of A or C. (c) We double
the concentration of C without changing the concentration of A or B. (d) We double all three
concentrations simultaneously.

Plan

We interpret the rate law to predict the changes in reaction rate. We remember that changing
concentrations does not change the value of k.

Solution

(a) We see that rate is directly proportional to the first powerof [A]. We do not change [B] or
[C]. Doubling [A] (i.e., increasing [A] by a factor of 2) causes the reaction rate to increase by
a factor of 2^1 2 so the reaction rate doubles.

(b) We see that rate is directly proportional to the second powerof [B]. We do not change [A]
or [C]. Doubling [B] (i.e., increasing [B] by a factor of 2) causes the reaction rate to increase
by a factor of 2^2 4.

(c) The reaction rate is independent of [C], so changing [C] causes no change in reaction rate.

(d) Doubling all concentrations would cause the changes described in (a), (b), and (c) simulta-
neously. The rate would increase by a factor of 2 due to the change in [A], by a factor of 4 due
to the change in [B], and be unaffected by the change in [C]. The result is that the reaction
rate increases by a factor of 2^1  22 8.

You should now work Exercises 14 and 15.

16-3 Concentrations of Reactants: The Rate-Law Expression 657