More about equilibria

Question 1

What does the position of an equilibrium and how fast it is obtained depend on?

Answer 1

the conditions of the reaction, such as temperature

Question 2

If the forward reaction is faster, which way has the equilibrium moved?

Answer 2

right

more reactants are used, and more products on the right are made, so it is ‘heavier’ on the right

Question 3

effect of increased temperature on the equilibrium position

Answer 3

the system works to decrease the temperature so it moves in the direction of the endothermic reaction

the rate of reaching equilibrium increases

Question 4

What happens if there is a change in the equilibrium?

Answer 4

the system works to reverse the change by causing the opposite effect

Question 5

effect of increased pressure on the equilibrium position

Answer 5

moves in the direction of the fewest molecules of gas
(e.g. right if the right side has a lower pressure)

the rate of reaching equilibrium increases if the reacting gases are present

Question 6

effect of the concentration of a reacting substance added on the equilibrium position

Answer 6

it moves in the direction away from the reacting substance

the rate of reaching equilibrium is increased

Question 7

effect of adding a catalyst on the equilibrium position

Answer 7

there is no change to the position

the rate of reaching equilibrium increases

Question 8

What do the conditions chosen for industrial processes depend on? (3)

Answer 8

• the availability of raw materials and energy supplies (in the Haber process, air and water are more easily obtained than natural gas)
• the control of temperature and pressure, many industrial processes don’t reach equilibrium as it would take too long
• the use of a suitable catalyst (e.g. iron in the Haber process), processes then work at lower temperatures, decreasing cost and increasing yield

Question 9

Ammonia is manufactured in the Haber process:
N2 + 3H2 ⇌ 2NH3

Ammonia has lower % yields at higher temperatures.
Ammonia has higher % yields at higher pressures.

a) Explain why a temperature of 450 C, which gives a lower yield than lower temperatures, is a compromise. [3]
b) Explain why the pressure 200 atmospheres, which gives lower yields than higher pressures, is a compromise. [2]

Answer 9

a) as temperature increases, the equilibrium yield of ammonia decreases, but the rate of reaction increases
450 C is low enough to obtain an acceptable yield of ammonia, but high enough to obtain it in an acceptable time

b) as the pressure increases, the equilibrium yield of ammonia and the rate of reaction increase
however, very high pressures need stronger and more expensive equipment, which uses more energy

Question 10

Methanol is manufactured using this reaction:
Co(g) + 2H2 ⇌ CH[3]OH(g)
change in hydrogen = -91 kJ mol[-1]

Predict and explain the effects on the equilibrium concentration of methanol vapour when:

a) the pressure is decreased [2]
b) the temperature is decreased [2]
c) the concentration of hydrogen is increased [2]

Answer 10

a) the equilibrium concentration is decreased because the position of the equilibrium moves to the left/in the direction of the greatest number of molecules of gas
b) the equilibrium concentration is increased because the position of equilibrium moves to the right/away from the endothermic reaction/in the direction of the exothermic reaction
c) the equilibrium concentration is increased because the position of equilibrium moves to the right/away from the substance increased in concentration