AI - Rates And Equilibria

Question 1

Define a dynamic equilibrium and give the conditions required

Answer 1

Where the forwards and reverse reactions are at the same rate so the amount of reactants and products doesn’t change

Only happens in a closed system at a constant temperature

Question 2

State Le Chatelier’s principle

Answer 2

If there is a change in pressure or temperature, the equilibrium will move to help counteract the change

Question 3

Using Le Chateilier’s principle, state the effect on the position of the equilibrium from:

i) Increasing temperature
ii) Decreasing temperature

Answer 3

i) Increasing temperature means adding heat
The equilibrium therefore shifts I’m the endothermic direction to absorb the heat

ii) Decreasing the temperature removes the heat.
There equilibrium therefore shifts on the exit hermit direction to replace the heat

Question 4

Using Le Chateilier’s principle, state the effect on the position of the equilibrium of a gas from:

i) Increasing pressure
ii) Decreasing pressure

Answer 4

i) Increasing the pressure shifts the equilibrium to the side with the fewer gas molecules, this reduces the pressure
ii) Decreasing the pressure shifts the equilibrium to the side with more gas molecules, this raises the pressure

Question 5

Give the effect using a catalyst has on the position of an equilibrium

Answer 5

Catalysts have no effects on the position of equilibrium.

They can’t increase yield - but they do mean equilibrium is reached faster

Question 6

Give 3 factors that effect the rate of reaction

Answer 6

• Increasing temperature
• Increasing pressure
• Using a catalyst

Question 7

Name 3 factors of that effect the rate of reaction in industry and what must be considered with each factor

Answer 7

1) Temperature - high temp makes reactions more expensive due to fuel cost but reactions go faster
2) Pressure - increase means gaseous reactions are faster but running pumps to create pressure are both expensive and dangerous
3) Catalysts - can be expensive but the right one can make reactions go quickly at low temperatures and don’t get used up

Question 8

Give the equation for the Haber process for producing ammonia (ΔH = -92 kJmol⁻¹), giving its conditions and stating why there is a compromise between the temperature and the pressure

Answer 8

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
Carried out at 400°C and 200atm
High pressure favours the forward reaction so increases the yield of ammonia
High temperatures make the reaction faster (increasing Ek) but it lowers the product yield since the forward reaction is exothermic so the equilibrium favours the reverse reaction.
The conditions are therefore a compromise between yield and rate of reaction

Question 9

Give the Equilibrium constant for the reaction:

aA + bB ⇌ dD + eE

Answer 9

Kc = [D]ᵈ[E]ᵉ / [A]ᵃ[B]ᵇ

Question 10

Define the partial pressure of a gas

Answer 10

In a mixture of gases, each gas exerts its own pressure - this is its partial pressure

Question 11

How is the total pressure of a gas calculated from the partial pressures of the individual gases in equilibria?

Answer 11

The total pressure of a gas mixture is the sum of all the partial pressures of the individual gases

Question 12

Give the equation for the partial pressure of gas A in a mixture of gases

Answer 12

Partial pressure of A,

p(A) = (moles of A / Total moles of gas)xtotal gas pressure

Question 13

Give the equation for the equilibrium constant Kp for the reaction:
aA (g) + bB (g) ⇌ dD (g) + eE (g)

Answer 13

Kp = p(D)ᵈ p(E)ᵉ / p(A)ᵃ p(B)ᵇ

Question 14

Name the 3 factors that affect the rate of reaction and state whether they alter the Kc or Kp values

Answer 14

1) Temperature - causes the equilibrium position to shift but this alters the equilibrium constant
2) Pressure - causes the equilibrium position to shift to counteract the change but the equilibrium constant remains the same
3) Catalyst - has no effect of the equilibrium constant because it affects both the forward and reverse reaction equally