CHM 2046C Module 10 Sample Exam Answers
Part I: Discussion Questions 10 points
Answer five of the following:
1. What is dynamic equilibrium?
A chemical equilibrium is a dynamic system; reacts are
combining to form products and the products are combining to form reactants.
The two reactions, the forward one and the reverse one, occur at the same rate
so that there is NO NET CHANGE in the concentrations of either the reactants or
the products.
2. What is true about the initial rate of the forward and reverse reactions in a system where only reactants are present?
The initial rate of the forward reaction is faster when the
concentration of the reactants are at a maximum, while the initial rate of the
reverse is zero when no products are present. As the reaction proceeds the rate of the forward retards and the rate of the
reverse increases until the concentrations of the reactants and products remain
unchanged. At that point equilibrium is achieved. The graph below demonstrates
the concentration change versus reaction progress:
3. a. What is true about the rates of the forward and reverse reactions at equilibrium?
They are equal.
b. What happens
to the value of K when the reaction is reversed?
The value of K when the reaction is
reversed is the reciprocal of the original value (or 1/K)
c. What happens to
the value of K when the temperature is changed for the reaction?\
K will change if the temperature changes
4. Your textbook introduces the concept of equilibrium by noting that no reaction goes fully to completion. What does this imply about the reverse reaction?
Theoretically all chemical reactions are reversible. If K
>> 1, then the reaction is product favored, that is the equilibrium concentrations of the products are very much
greater than the equilibrium concentrations pf the reactants. If there is virtually all products in the vessel and only a very
small trace of the reactants, then chemists say “the reaction goes to completion”.
5. Discuss the relationship between the magnitude of the equilibrium constants as to whether the forward or the reverse reaction is favored.
A large value for K means a proportionately
large concentrations of products, i.e. a PRODUCT-FAVORED process. A
value much greater than 1, in fact, means the concentration of the product is
higher than the concentration of the reactant at equilibrium.
When K is equal to one (very rare condition), neither
reactant or products are favored
On the other hand, a smalled value
for K means less product at equilibrium, i.e.
REACTANT-FAVORED process.
6. What effect does adding a catalyst on the concentrations of the reactants and products after equilibrium is established.
No, the position of the equilibrium will not change if a
catalyst is added. The equilibrium will undoubtedly be reached more quickly,
but the final composition of the system will be the same.
7. What disturbances are possible for a system in equilibrium?
There are three common ways to disturb a chemical system in
equilibrium:
(a)
Change the temperature
(b)
Change the concentration or partial pressure of reactant or
product.
(c)
Change the volume (or pressure) for a system involving gases.
8. When is it reasonable to neglect the change in the
concentration in the denominator of an equilibrium constant expression when
solving for the equilibrium concentrations from the initial concentrations.
(Or when does 1.0 M – x ~ 1.0)
It is reasonable to assume that the change in concentration x is negligible compared to the original concentration (i.e.(1.0 – x) = 1.0, when 100K is less than the original concentration of the reaction.