Chemical Equilibria: Le Chatelier's Principle

Question 1

Define the term dynamic equilibria

Answer 1

• When the forward and backward rate of reaction is the same
• When the concentrations of reactants and products remain
  constant

Question 2

What is needed for dynamic equilibrium to be reached?

Answer 2

• Reversible reaction
• Closed system (no reactants or products leave)
• Constant temperature

Question 3

Describe reversible reactions

Answer 3

• Reactions that have both a forward and backward reaction
• Shown with symbol ⇌
• All reversible reactions eventually reach dynamic equilibrium

Question 4

State Le Chatelier’s principle?

Answer 4

• If a factor affecting the position of equilibrium is altered, the position of equilibrium shifts to oppose the change

Question 5

Which factors affect the position of equilibrium?

Answer 5

• Temperature

- Concentration and pressure

Question 6

Consider reaction A + B ⇌ C + D. If the concentration of A increases, which direction will the equilibrium shift to?

Answer 6

• Equilibrium will shift to the right
• To oppose the change and decrease the concentration of A
• And increase the concentration of C and D

Question 7

What will you need to know about the forward and backward reaction if the temperature changes and you need to determine the shift in equilibrium?

Answer 7

• Need to know whether forward and backward reaction are either endothermic or exothermic

Question 8

What are endothermic and exothermic reactions?

Answer 8

• Endothermic

• Heat is absorbed
• (+ve ΔH)

• Exothermic

• Heat is released
• (-ve ΔH)

Question 9

Consider the reaction A + B ⇌ C + D. The forward reaction ΔH change = -250kJmol-1. What happens if the temperature is increased?

Answer 9

• Position of equilibrium would shift to oppose the change, therefore shift to the endothermic direction
• Here, the position of equilibrium will shift to the left

Question 10

Which direction will the position of equilibrium shift to if the temperature increases?

Answer 10

• Shift to endothermic reaction (+ve ΔH)

- Excess heat needs to be removed

Question 11

Which direction will the position of equilibrium shift to if the temperature decreases?

Answer 11

• Shift to the exothermic direction

- Heat needs to be gained

Question 12

What type of reactions does pressure affect the position of equilibrium of?

Answer 12

• Relevant for reactions involving gases

- Equilibrium is only affected when there is a different amount of moles of gas on each side of the equation

Question 13

Consider the reaction 3H2(g) + N2(g) ⇌ 2NH3(g). What happens if the pressure is increased?

Answer 13

• When pressure increases, system will react to oppose the change and move to the side that has the fewest moles of gas
• Here, the position of equilibrium will shift to the right

Question 14

Which direction will the position of equilibrium sift to when pressure is increased?

Answer 14

• Shift to direction with lower amount of moles

- Help releases the build up in pressure

Question 15

Which direction will the position of equilibrium shift to when the pressure is decreased?

Answer 15

• Shift to direction with more moles

- Pressure has been lost

Question 16

Do catalysts affect the position of equilibrium?

Answer 16

• Catalysts do not affect the equilibrium position as they affect the forward and backward reaction equally
• Allows equilibrium to be reached faster

Question 17

In industrial processes, why would a compromise temperature be used?

Answer 17

• In some cases

• Lower temperatures may favour direction of desired product if it’s exothermic - higher yield
• But lower temperatures mean a slower rate of reaction and product produced too slowly
• Temperature is a compromise between the yield and the rate

Question 18

In industrial processes, why would a compromise pressure be used?

Answer 18

• Higher pressures may favour direction of desired product if it has fewer moles - higher yield
• Higher pressures also increases rate of reaction
• Generating high pressures is expensive
• Pressure is a compromise between the yield, the rate and the expense of producing high pressures