Equilibria and Kc

Question 1

How is a dynamic equilibrium reached? (3 marks)

Answer 1

1. Rate of forward reaction starts fast and decreases as reactants become products
2. Rate of backward reaction starts slow and increases as products are formed
3. Rate of forward reaction equals rate of backward reaction

Question 2

What is meant by a reaction at dynamic equilibrium? (2)

Answer 2

1. Forward and backward reactions happen at the same RATE
2. CONCENTRATION of products and reactants are CONSTANT

Question 3

CO + H2O <–> CO2 + H2
What would happen if the concentration of CO in the reaction above is increased? (3)

Answer 3

1. Equilibrium shifts right
2. To oppose increase in CO
3. Yield of CO2 and H2 increases

Question 4

CO + H2O <–> CO2 + H2
What would happen if the concentration of CO in the reaction above is reduced? (3)

Answer 4

1. Equilibrium shifts left
2. To oppose decrease in CO
3. Yield of CO2 and H2 decreases

Question 5

2NO2 <–> N2O4
What would happen if the pressure of the reaction above is increased? (3)

Answer 5

1. Fewer molecules on right side
2. Equilibrium shifts right to oppose increase in pressure
3. Yield of products (N2O4) increases

Question 6

2NO2 <–> N2O4
What would happen if the pressure of the reaction above is decreased? (3)

Answer 6

1. More molecules on left side
2. Equilibrium shifts left to oppose decrease in pressure
3. Yield of products (N2O4) decrease

Question 7

CH4 + H2O <–> CO + 3H2 where enthalpy change = -206 kJmol-1
What would happen if the temperature of the reaction is decreased? (3)

Answer 7

1. Forward reaction is exothermic
2. Equilibrium shifts to the right to oppose the decrease in temperature
3. Yield of CO and H2 increase

Question 8

CH4 + H2O <–> CO + 3H2 where enthalpy change = -206 kJmol-1
What would happen if the temperature of the reaction increase? (3)

Answer 8

1. Backward reaction is endothermic
2. Equilibrium shifts to the left to oppose the increase in temperature
3. Yield of CH4 and H2O increases

Question 9

What happens when a catalyst is added to a reversible reaction? (3)

Answer 9

1. No change on position of equilibrium
2. Increase rate of forward and backward reaction equally
3. Decrease time taken to reach equilibrium

Question 10

Describe compromise conditions (7)

Answer 10

TEMPERATURE AND PRESSURE
1. Need high yield and fast rate of reaction
2. Higher temperature = faster rate
3. High temperature is expensive
4. High pressure = faster rate
5. Higher pressure requires expensive equipment

CATALYST
6. Increases rate of reaction
7. Product is produced quicker

Question 11

2A + B <–> 2AB
Write a Kc expression for the reaction above

Answer 11

Kc = [AB]2 / [A]2 [B]

Question 12

What affects Kc?

Answer 12

Temperature