1.6 Chemical equilibria, Kc

Question 1

When is a reaction at equilibrium?

Answer 1

• When the reactants are proportional to the products.
• When the reaction is a reversible reaction.
• Equilibrium can only be reached in a closed system.
• Forwards reaction and backwards reaction happen at the same rate.
• At equilibrium, the concentration of reactants and products stay constant.

Question 2

What is La Chatelier’s principle?

Answer 2

If a system of equilibrium is disturbed, it shifts on the direction that reduces the disturbance.

Question 3

What happens if you increase the temperature of a homogenous reaction?

Answer 3

The equilibrium will shift in favour of the endothermic reaction to oppose the change.

Question 4

What happens if you decrease the temperature of a homogenous reaction?

Answer 4

The equilibrium will shift in favour of the exothermic reaction to oppose the change.

Question 5

What happens if you decrease the pressure of a homogenous reaction?

Answer 5

The equilibrium will shift in favour of the reaction that makes more moles.

Question 6

What happens if you increase the pressure of a homogenous reaction?

Answer 6

The equilibrium will shift in favour of the reaction that makes less moles.

Question 7

What happens if you change the pressure of a homogeneous system, where equal amounts of moles are made and used up?

Answer 7

The equilibrium doesn’t shift at all.

Question 8

What happens if you increase the concentration of reactants in a homogenous reaction?

Answer 8

Equilibrium will shift in favour of the forwards reaction, making more products.

Question 9

What happens if you increase the concentration of products in a homogenous reaction?

Answer 9

Equilibrium will shift in favour of the backwards reaction, making more reactants.

Question 10

What does the equilibrium constant (Kc) do?

Answer 10

Gives a value for the position of equilibrium, and how it changes with concentration.

Question 11

What is the quantitive representation of Kc?

Answer 11

The bigger the Kc value, the closer it is to the products (favours the forwards reaction).

Question 12

How do you formulate a Kc expression?

eg equ: aA + bB ⇌ cC

Answer 12

[A]ª x [B]ᵇ

Question 13

What are the units of concentration used in Kc?

Answer 13

moldm¯³

Question 14

How do you find the Kc of a reaction? (step by step)

Answer 14

1. Find initial moles
2. Find moles at equilibrium.
3. Find concentration at equilibrium.
4. Put into the Kc equation and find the correct units.

Question 15

How does temperature affect Kc?

Answer 15

• Kc is bigger when reaction shifts right.

- So depending on wether the forward reaction is exo/endo will show the effect of temperature.

Question 16

How do catalysts or concentration affect Kc?

Answer 16

No effect on the Kc value, but concentration changes will shift the equilibrium.