Physical Chemistry Third Edition

488 11 The Rates of Chemical Reactions

PROBLEMS

Section 11.1: The Macroscopic Description of Chemical
Reaction Rates

11.1 Consider the chemical reaction balanced in two different
ways:

2NO+Cl 2 −→2NOCl

and

NO+

1

2

Cl 2 −→NOCl

a.If the rate of the reaction according to the first version

is 7. 1 × 10 −^5 mol L−^1 s−^1 , what is the rate of the

reaction according to the second version?

b.Write an expression for the rate of change of the

concentration of each substance. Does this quantity

differ for the two versions?

11.2 Consider the chemical reaction balanced two different

ways:

2O 3 −→3O 2

and

O 3 −→

3

2

O 2

a.If the rate of the reaction according to the first version

is 5. 0 × 10 −^4 mol L−^1 min−^1 , what is the rate of the

reaction according to the second version?

b.Write an expression for the rate of change of the

concentration of each substance. Does this quantity

differ for the two versions?

11.2 Forward Reactions with One Reactant

In this section we discuss reactions of a single reactant with a negligible reverse reaction.

Inspection of Figure 11.1 shows that when a reaction approaches equilibrium, the

reverse rate cannot be neglected. We must avoid applying the results of this section to

reactions that are close to equilibrium. However, many reactions proceed essentially

to completion and in that case we can neglect the reverse reaction for nearly the entire

reaction.

The “classical” method for determining the rate law for a reaction is to mix the

reactants and then to determine the concentration of one of the reactants or products as

a function of time. A variety of methods have been used to determine concentrations,

including measurement of the following:

1. The absorbance of radiation at some wavelength at which a given product or reac-
   tant absorbs.


2. The intensity of the emission spectrum of the system at a wavelength at which a
   given product or reactant emits.


3. The volume of a solution required to titrate an aliquot removed from the system.


4. The pressure of the system (for a reaction at constant volume).


5. The volume of the system (for a reaction at constant pressure).


6. The electrical conductance of the system.


7. The mass spectrum of the system.


8. The ESR or NMR spectrum of the system.


9. The dielectric constant or index of refraction of the system.


10. The mass loss if a gas is evolved.

Once we know from experiment how the concentration of a reactant or product

depends on time we can determine the rate law. We now proceed to integrate the

rate laws for a number of cases to obtain formulas with which to compare experi-

mental data.