Chapter 19 Equilibrium

Question 1

Write the expression for Kc?

Answer 1

[C]^c [D]^d
Kc —————-
[A]^a [B]^b

Question 2

What is dynamic equilibrium?

Answer 2

Equilibrium where the rate of the forward reaction is equal to the rate of the reverse reaction so there is no overall change in the concentrations of the reactants or products
Occurs in a closed system

Question 3

What is homogeneous equilibrium?

Answer 3

When all the reactants and products in the equilibrium system are in the same physical state

Question 4

What is heterogeneous equilibrium?

Answer 4

When the reactants and products in an equilibrium system are in different physical states

Question 5

Outlines the rules of using heterogeneous equilibrium states in Kc?

Answer 5

If the system is heterogeneous, only gases and aqueous substances are substituted into the Kc expression (solids and liquids are omitted)

Question 6

State the Kc expression for
C (s) + H2O (g) <-> CO (g) + H2 (g)

Answer 6

[CO (g) ] [H2 (g) ]
Kc = —————————
[H2O (g) ]

Question 7

What kind of table is used to calculate Kc from equilibrium amounts?

Answer 7

ICE table:
Initial
Change
Equilibrium

Question 8

What is Kp?

Answer 8

The equilibrium constant in terms of partial pressures

Question 9

How is the total pressure of a gas mixture calculated?

Answer 9

Sum of all partial pressures of individual gases

Question 10

What is a mole fraction of a gas?

Answer 10

The proportion of a gas mixture that is a particular gas

Question 11

How can the mole fraction of a gas be calculated?

Answer 11

Mole fraction of gas in mixture=

Number of moles of the gas
———————————————
Total number of moles in gas mixture

Question 12

What is partial pressure?

Answer 12

The contribution that the gas makes towards the total pressure
So the sum of the partial pressures of each gas = total pressure

Question 13

How is the partial pressure of a gas calculated?

Answer 13

Partial pressure of gas p(A)=
Mole fraction of gas (A) x total pressure of mixture

Question 14

Write the expression for Kp?

Answer 14

p(C)^c x p(D)^d
Kp = ————————-
p(A)^a x p(B)^b

Question 15

When do equilibrium constants change?

Answer 15

At a set temperature, K is constant and does not change despite modifications to concentration, pressure or presence of catalyst.
However, K can change when temperature is changed.

Question 16

How does K change when the temperature is increased?
N2 + 3H2 ⇌ 2NH3
Δ H= -92 KJmol-1

Answer 16

The forward reaction is exothermic so increasing the temperature will shift equilibrium in the endothermic reverse reaction.
Therefore, less product will from, partial pressure of the products will decrease.
This causes the equilibrium constant to decrease.

Question 17

How does K change when temperature is decreased?
N2 + 3H2 ⇌ 2NH3
Δ H= -92 KJmol-1

Answer 17

The forward reaction is exothermic so decreasing the temperature will favour the forward reaction.
Therefore, more products will form so the partial pressure of the products will increase.
This causes the equilibrium constant to increase.

Question 18

How does the equilibrium position change when
Kp = 1?

Answer 18

Equilibrium position is half way between reactants and products

Question 19

How does the equilibrium position change when
Kp > 1?

Answer 19

Equilibrium position will shift in forward direction/ with products

Question 20

How does the equilibrium position change when
Kp < 1?

Answer 20

The equilibrium position will shift in the reverse reaction/ with reactants

Question 21

How does the position of equilibrium change when the concentration of the reactants is increased?

Answer 21

Equilibrium will shift in the forwards direction to form more products and reduce the concentration of reactants.
This keeps equilibrium constant the same.

Question 22

How does equilibrium change when pressure is increased?

Answer 22

Equilibrium will shift to the side with the fewest gaseous moles to reduce pressure and keep the equilibrium constant the same.

Question 23

How does equilibrium change when pressure is decreased?

Answer 23

Equilibrium will shift to the side with the most gaseous moles to raise the pressure again.
This keeps equilibrium constant the same.

Question 24

How does the presence of a catalyst affect the position of equilibrium?

Answer 24

Catalysts have no effect on the position of equilibrium or the value of Kc or Kp.
They increase the rate that equilibrium is reached.

Question 25

What is the effect of concentration and pressure on equilibrium and equilibrium constant?

Answer 25

Concentration and pressure do not affect the values of Kc or Kp but can change the concentrations of reactants and products present at equilibrium.