Comments re Chapter 16 Problems:

 

(Note that the problems assigned in this first set (16.1 – 16.45) are almost all review problems over material that was covered originally in General Chemistry.  The answers to some of the questions asked appear on the handout distributed in class).

16-1

Some T/F questions about rates and rate laws.

16-2

A question about the relationship between the form of the rate law and the units of the rate constant.

16-3

A short question about the relationship between rates for two reactants that have different stoichiometric coefficients.  The general equation for a reaction
aA + bB ® cC + dD
is
-(1/a)(d[A]/dt) = -(1/b)(d[B]/dt) = +(1/c)(d[C]/dt) = +(1/d)(d[D]/dt).

16-9

A small thought problem that points out that rate constants are not rates.

16-15

An exercise based on a first-order rate law. Applied calculus.  (See pg 521 of the textbook).

16-26

A problem that is preparation for problems in which the rate law is determined from data about initial rates.

16-33
(not part c)

Determination of a rate law from information about initial rates (see pg 529 of the textbook).. Calculate the rate constant for each set of concentrations and then average those rate constants for the best value. Note that rate constants are seldom known to much more than two digits.

16-45
(not part b)

Another problem in which a rate law is determined from information about initial rates. Very important.

16-60

Three T/F questions related to the Arrhenius equation, which is a description of how a rate constant changes with temperature:
            k = A exp (-Ea/RT), where Ea is the activation energy.

16-63

Given the rate constant at two temperatures calculate Ea [from k = A exp (-Ea/RT)] and then the pre-exponential factor A. Look at their values.

 

 

16-10

An exercise in identifying the various components of a mechanism.  Remember that an intermediate is formed and then consumed while a catalyst is the same when the reaction has gone to completion as when the reaction started.  The concept of “stoichiometric number” is not important but please work out how the different reaction steps can be combined to give an overall balanced reaction.

16-21

A small problem about an important distinction (same word used differently in two contexts)

16-41

Another short problem that emphasizes a very important distinction.

16-52

A small thought problem that emphasizes the low probability of collisions involving more than two molecules.  A collision takes place in ca 10-12 s, and the probability of three molecules hitting each other within that length of time is very low.  The probability of four molecules hitting each other within 10-12 s is essentially zero.  Also, if the reaction goes forward in a single step it must go backwards in a single step.

16-53
[part (a) only]

Practice in writing down the differential equations that correspond to a mechanism.

16-60

Three T/F questions related to the Arrhenius equation, which is a description of how a rate constant changes with temperature:
            k = A exp (-Ea/RT), where Ea is the activation energy.

16-63

Given the rate constant at two temperatures calculate Ea [from k = A exp (-Ea/RT)] and then the pre-exponential factor A. Look at their values.

16-69

The relationship of Ea for the forward reaction, Ea for the reverse reaction, and DHo (or DUo, it doesn't really matter) for the overall reaction.

16-92

A T/F question about catalysts.  Catalysts are substances that increase the rate of the reaction without appearing in the overall balanced equation.

R16-16

An exercise in deriving a rate law from a mechanism using (1) the steady-state approximation for intermediates, and (2) the rate-determining step (or, slow-step) approximation.  The two rate laws are the same if the approximations are valid.  The assumption for part (b) is that k1 and k-1 are much larger than k2 so that
k1[A][B] @ k-1[C][D].

 

 

17-102 (replaced by 16.19, which is not assigned)

A problem concerning radioactive decay, which is always a first-order reaction. Note that the value for counts/s is the rate in atoms/s. The amount is given as mass rather than as a concentration because the substance is a solid so that the volume doesn't matter. It is necessary to change that mass to atoms of 233U, which replaces the concentration (since the volume doesn't matter).

 

 

16-23

How does the concentration of A change with time if the reaction is zero order?

16-39

Problem based on the relationship Keq = kf/kr (microscopic reversibility). The problem is not as easy as it first looks because the stoichiometric coeffient is 1 for the reactant and 2 for the product. Note that the equilibrium constant must be calculated with molar concentrations rather than with pressures.

16-116

Yet another of Levine's excellent true-false problems.