equilibrium

Question 1

List four conditions required for dynamic equilibrium.

Answer 1

The four conditions are:

1. Closed system
2. No macroscopic changes
3. Reactants and products both present
4. Rate of forward and reverse reactions are equal

If the first three conditons are true, essentially the last condtion must also be true.

Question 2

What parameter is equal at equilibrium?

Answer 2

The rate of the forward and reverse reactions are equal at equilibrium. What is not equal are the relative amounts of reactant and product.

Question 3

Define equilibrium constant and equilibrium constant expression.

Answer 3

For a given reaction (e.g. aA + bB ⇔ cC), where lower case letters are the coefficients from the balanced equation, the equilibrium constant expression is:

K = [C]^c / [A]^a[B]^b

If the appropriate numbers are inserted from an equilibrium condition, the resulting calculation will give the equilibrium constant (symbolized as K).

Note that the equilibrium constant expression is entirely derived from the balanced equation.

Question 4

Compare and contrast equilibrium constant expression and rate law.

Answer 4

For a given reaction (e.g. aA + bB ⇔ cC), where lower case letters are the coefficients from the balanced equation, the equilibrium constant expression is:

K = [C]^c / [A]^a[B]^b

This expression is entirely derived from the balanced equation.

By contrast the rate law for the same reaction would have the format:

rate = k [A]^m[B]ⁿ

The rate law only includes the reactants in the expression and the superscripts (m and n) must be determined experimentally.

The rate law show how changing concentration affects the reaction rate, where the equilibrium constant expression shows the ratio of products to reactants at equilibrium.

Question 5

Under what conditions can the K_p = K_c ?

Answer 5

• K*_p and K_c are related to one another by the equation:
• K*_p = K_c (RT)^{delta n}

These two equilibrium constants will be equal when delta n (that is, the change in the amount of gaseous molecules (moles) from reactants to products) equal zero. For example:

K_p = K_c (RT)⁰ = K_c

Question 6

For a given reaction

A + 2B ⇔ C,

K_c = 25. What is the K_c for this reaction:

C ⇔ A + 2B?

Answer 6

The second reaction is the reverse of the first reaction, therefore the new K_c = 1/25 = 0.04.

Question 7

For a given reaction

A + 2B ⇔ C,

K_c = 25. What is the K_c for this reaction:

½A + B ⇔½C?

Answer 7

The second reaction is arrived at by dividing the coefficients for the first reaction by 2. Therefore the new K_c = (25)^½ = 5.

Question 8

Write the equilibrium constant expression for the following balanced equation:

HF(aq) + H₂O(l) ⇔ H₃O⁺(aq) + F^-(aq)

Answer 8

the equilibrium constant expression for the following balanced equation:

HF(aq) + H₂O(l) ⇔ H₃O⁺(aq) + F^-(aq) is:

K_c = [H₃O⁺][F^-]/[HF]

Note that the concentration of H₂O is not included given that it is also the solvent and therefore its concentration is essentially a constant.

Question 9

Write the equilibrium constant expression for the following balanced equation:

C₃H₈(g) + 5O₂(g) ⇔ 3CO₂(g) + 4H₂O(g)

Answer 9

the equilibrium constant expression for the following balanced equation:

C₃H₈(g) + 5O₂(g) ⇔ 3CO₂(g) + 4H₂O(g) is:

K_c = [CO₂]³[H₂O]⁴/[C₃H₈][O₂]⁵

Note that the concentration of H₂O IS included given that it is a product in the gaseous phase whose concentration can change.

Question 10

Compare and contrast the equilibrium constant (K_c) and the reaction quotient (Q_c).

Answer 10

For a given reaction, both parameters use the same “expression” for calculation (i.e. [products]/[reactants] with appropriate superscripts depending on the coefficients). The equilibrium constant (K_c) is a constant at a particular temperature and onlly concentrations of reactants and products at equilibrium should be used for its calculation. By contrast the reaction quotient is the ratio of products to reactants under any condition. It is usuallu calculated to “test” whether the system is at equilibrium or not. If at equilibrium, then K_c = Q_c. If K_c > Q_c, this indicates that the system is out of equilibrium and the system needs to shift right to achieve equilibium. If K_c < Q_c, then the system needs to shift left to achieve equilibrium.

Question 11

Explain the meaning of the statement: “the system is out of equilibrium and needs to shift right to achieve equilibrium”

Answer 11

The statement: “the system is out of equilibrium and needs to shift right to achieve equilibrium” means that under the current conditions of the reaction the system is not at equilbrium (K_c does not equal Q_c) and the forward rate of reaction does not equal the reverse rate.

If the system needs to “shift right”, this means temporarily the forward rate is faster than the reverse rate. Temporarily products will be made faster than reactants. As more products are made the reverse rate will increase and the forward rate will decrease until finally the forward and reverse rates are equal and K_c = Q_c.

Question 12

State Le Chatelier’s principle and explain why this is a useful tool for equilibrium chemistry.

Answer 12

Le Chatelier’s principle says: When system at equilibrium is subjected to a stress (change in concentration, pressure or temperature), the system “responds” to, in part, counteract this stress.

This is a useful tool because it allows a chemist, without doing a calculation, predict the effect of changing conditions in an equilibrium system.

Question 13

While Le Chatelier’s principle is a useful tool, it can also be used inappropriately. Give an example of an inappropriate use of Le Chatelier’s principle.

Answer 13

Le Chatelier’s principle states that an increase in concentration of a reactant will result in a shift right.

in some reactions, the reactant is in solid phase. Adding more solid does not change the concentration of this reactant and therefore this will not affect the equilibrium system. A student may not notice that the reactant is in solid phase and may incorrectly predict that the addition of this solid will cause a shift right.

Question 14

Which is the only stress to an equilibrium system which results in a change of the equilibrium constant? Explain why this is so.

Answer 14

A change in temperature is the only stress which results in a change in the equilibrium constant. A change in temperature alters the energy of the system and therefore alters the equilibrium ratio of products to reactants. For an endothermic reaction, an increase in temperature will result in more products and fewer reactants at equilibrium. This will result in an increase in K.

Other changes, such as a change in concentration or a change in pressure (because of the change in volume) will temporarily put the system out of equilibrium, where K will not longer equal Q. The system will then shift appropriately such that once again K = Q.

Question 15

In terms of Le Chatelier’s principle, list several false stresses.

Answer 15

A false stress is a change to an equilibrium system which does not lead to a shift right or left. Examples are:

1. An increase in pressure due to the addition of an inert gas.
2. The addition or removal of solid reactant or product.
3. A change in the surface area of a reactant or product.

All of these changes will have no impact on the equilibrium position - no shift right or left.