39: An Introduction to Chemical Equilibrium

Question 1

What is the chemical equation of the reaction between hydrogen gas and iodine gas?

Answer 1

H₂ (g) + I₂ (g) ⇌ 2HI (g)

Question 2

How can it be deduced that the reaction between hydrogen gas and iodine gas is reversible?

Answer 2

When colourless hydrogen and purple iodine are mixed in a closed container, the colour becomes paler as colourless hydrogen iodide gas is produced. However, the purple colour will not disappear and the deepness of the colour stays constant after a while. This shows that equilibrium is attained and the reaction does not go to completion, indicating that it is a reversible reaction.

Question 3

What are three features that define a chemical system that is in dynamic equilibrium?

Answer 3

1. The concentrations of reactants and products remain constant.
2. The forward and backward reactions are both happening.
3. The rate of the forward reaction is equal to the rate of the backward reaction, and they are both not zero.

Question 4

Explain why equilibrium in the decomposition of calcium carbonate cannot be attained in an open system.

Answer 4

In an open system, substances can enter or leave. The carbon dioxide produced in the decomposition escapes to the atmosphere and thus no carbon dioxide is available to react with the calcium oxide and re-form the calcium carbonate. Therefore, the reaction goes to completion and equilibrium cannot be attained.

Question 5

List 6 characteristics of a chemical equilibrium system.

Answer 5

1. The concentrations of reactants and products remain constant. (not necessarily equal to each other)
2. The forward and backward reactions are both happening.
3. The rate of the forward reaction is equal to the rate of the backward reaction, and they are both not zero.
4. It requires a closed system.
5. It can be attained from either direction (either starting with the substances on the left or the right the equation)
6. It may be affected by a change in condition.

Question 6

What is K_c?

Answer 6

K_c is the equilibrium constant for a reaction.

Question 7

State 2 substances not to include when calculating K_c.

Answer 7

1. Do not include the solvent if it is produced or consumed in the reaction.
2. Do not include solids.

Question 8

An equilibrium mixture contains substances A, B, C, D according to the equation:
aA + bB ⇌ cC + dD
Calculate K_c.

Answer 8

K_c = ([C]^c[D]^d)/([A^a[B]^b)

• the concentrations of A, B, C, D need to be the equilibrium concentration.

Question 9

An equilibrium mixture contains substances A, B, C, D according to the equation:
aA + bB ⇌ cC + dD
Determine the unit of K_c.

Answer 9

Unit of K_c = (mol dm^{-3 (c+d-a-b)})

Question 10

The K_c of the reaction
aA + bB ⇌ cC + dD
is x.
Determine the K_c of the reaction:
cC + dD ⇌ aA + bB

Answer 10

1/x

Question 11

The K_c of the reaction
aA + bB ⇌ cC + dD
is x.
Determine the K_c of the reaction:
2cC + 2dD ⇌ 2aA + 2bB

Answer 11

1/(x)²

Question 12

The K_c of the reaction
aA + bB ⇌ cC + dD
is x.
Determine the K_c of the reaction:
2aA + 2bB ⇌ 2cC + 2dD

Answer 12

x²

Question 13

The K_c of the reaction
aA ⇌ cC
is x, and the K_c of the reaction
2bB ⇌ 2dD
is y.
Determine the K_c of the reaction:
2aA + bB ⇌ 2cC + dD

Answer 13

x² * √(y)

Question 14

The K_c of the reaction
aA ⇌ cC
is x, and the K_c of the reaction
2bB ⇌ 2dD
is y.
Determine the K_c of the reaction:
aA + dD ⇌ cC + bB

Answer 14

x / √(y)

Question 15

What is implied if the K_c of a reversible reaction is much larger than 1?

Answer 15

There is a high proportion of product compares to the reactants. The reaction goes almost to completion.

Question 16

What is implied if the K_c of a reversible reaction is much smaller than 1?

Answer 16

There is a low proportion of product compares to the reactants. The reaction has hardly taken place.

Question 17

A reaction mixture contains substances A, B, C, D according to the equation:
aA + bB ⇌ cC + dD
Calculate Q_c.

Answer 17

Q_c = ([C]^c[D]^d)/([A^a[B]^b)

• the concentrations of A, B, C, D need not be at equilibrium.

Question 18

Predict the direction of net reaction if Q_c < K_c.

Answer 18

Q_c tends to increase until it reaches K_c. A net forward reaction occurs to generate more products until equilibrium is attained.

Question 19

Predict the direction of net reaction if Q_c > K_c.

Answer 19

Q_c tends to decrease until it reaches K_c. A net backward reaction occurs to generate more reactants until equilibrium is attained.

Question 20

Predict the direction of net reaction if Q_c = K_c.

Answer 20

The system is at equilibrium. There is no net reaction.