≫3.1 - Equilibria ✔

Question 1

What is a reversible reaction?

Answer 1

• Some reactions are reversible, where the reactants react to form products, which can then react to reform the original reactants.
• It is represented using a double arrow.
• In each reaction system, one reaction will be endothermic and the opposite reaction will be exothermic.

Question 2

Reversible reactions will eventually reach dynamic equilibrium, what are the two features of this?

Answer 2

• Forward and reverse reactions proceed at equal rates.
• The concentrations of reactants and products remain constant.

Question 3

Where can dynamic equilibrium only take place?

Answer 3

• Dynamic equilibrium can only occur in closed systems where no substances can get in or out in order to influence the reactions.

Question 4

Use collision theory to explain at a particle level equilibrium being reached:

Answer 4

• The particles move around randomly until they collide with sufficient energy to form the product.
• Product can then break down to form the reactant particles again.
• Once the number of particles of the reactants and products remain constant, an equilibrium is reached.

Question 5

What is a homogenous reaction?

Answer 5

• Chemical reactions reactant and products are all in the same physical states, one phase.

Question 6

What is a heterozygous reaction?

Answer 6

• Chemical reactions reactant and products are in different physical states, two phases.

Question 7

What can concentration-time graphs be used for?

Answer 7

• These graphs can be used to demonstrate the use of reactants and resulting products in a reaction.

Question 8

Sketch a concentration-time graph for the following reaction:

2A + B ⇌ C

Answer 8

Question 9

What does Le Chatelier’s Principle state?

Answer 9

• If an external condition is changed, the equilibrium will shift to oppose the change.

Question 10

Why is it useful to consider Le Chatelier’s principle in industry?

Answer 10

• Useful principle to consider in industry so that the reaction conditions can be altered to maximise the yield of a desired product.

Question 11

Le Chatelier’s Principle:

Describe the effect of increasing/decreasing temperature:

Answer 11

• Increasing temperature will cause the equilibrium to favour the endothermic reaction as the heat needs to be removed from the system to lessen the effect of the initial increase, thus it will increase the yield of the endothermic products.
• Decreasing temperature will cause the equilibrium to favour the exothermic reaction as heat needs to be gained and the yield of the exothermic products will be increased.

Question 12

Le Chatelier’s Principle:

Describe the effect of increasing/decreasing pressure:

Answer 12

• Increasing pressure favours the side of the reaction with fewer moles as this will help to release the build up in pressure, it will increase the yield of the products on this side of the reaction.
• Decreasing pressure favours the side of the reaction with more moles as pressure has been lost, the yield of the products on this side of the reaction will be increased.

Question 13

Le Chatelier’s Principle:

Describe the effect of increasing/decreasing concentration:

Answer 13

• Increasing product concentration will cause the reactant concentration to be increased by favouring the reverse reaction.
• Increasing reactant concentration will cause the product concentration to be increased by favouring the forward reaction.
• Decreasing product concentration will cause an increase in product particles as the forward reaction will be favoured to counteract the change.
• Decreasing reactant concentration will cause an increase in reactant particles as the backwards reaction will be favoured to counteract the change.

Question 14

Le Chatelier’s Principle:

Describe the effect of adding a catalyst:

Answer 14

• Catalysts appear not to affect the equilibrium as they affect the forward and backward reaction equally, however it does allow equilibrium to be reached faster.

Question 15

What does the equilibrium law state (Kc)?

Answer 15

• Kc = the equilibrium constant in terms of concentrations.
• [C] or [D] = equilibrium concentration of the products.
• [A] or [B] = equilibrium concentration of the reactants.
• a, b, c or d = each concentration is raised to the power of its balancing number in the overall equation.

Question 16

What does it mean if Kc is large (Kc»1) ?

Answer 16

• The equilibrium is favouring the products/RHS.

e.g: (strong) HCl → H⁺ + Cl⁻
Kc=23000

Question 17

What does it mean if the Kc is small (Kc«1) ?

Answer 17

• The equilibrium is favouring the reactants/LHS.

e.g: (weak) CH₃COOH ⇌ CH₃COO⁻ + H⁺
Kc=3x10⁻⁷

Question 18

Work out the units for Kc of the following equation:

2A + B ⇌ C

Answer 18

Question 19

What is the effect of concentration on Kc?

Answer 19

• Changes in concentration do not effect the Kc at a given temperature.
• If the concentration of one substance changes in the equilibrium mixture then the concentrations of the others must change to keep the value of Kc the same.

Question 20

What is the effect of pressure on Kc?

Answer 20

• Changes in pressure do not effect the Kc at a given temperature.
• Increasing pressure causes equilibrium to shift to decreases the pressure, so overall Kc remains the same.

Question 21

What is the effect of a catalyst on Kc?

Answer 21

• No effect on Kc.
• The catalyst cannot change the yield, it only changes the time taken to reach equilibrium.

Question 22

What is the effect on Kc if you increase the temperature of the following reaction:

2SO₂ + O₂ ⇌ 2SO₃ [∆H=-197]

Answer 22

• If you increase the temperature equilibrium shifts to favour the endothermic reaction (reverse in this case.)
• The reactant concentration increases thus the product concentration decreases so the Kc gets smaller as the value for concentrations of products is smaller.

Question 23

Example:

N₂ + 3H₂ ⇌ 2NH₃

Initially there are 1.0 moles of nitrogen and 3 moles of hydrogen, at equilibrium there are 0.24 moles of ammonia in a fixed volume of 0.5dm³ at constant temperature, calculate Kc and its unit:

Answer 23

Question 24

Example:

2A ⇌ B + C

2 moles of A are placed in a container, at equilibrium there are x moles of B present, Kc was found to be 1.5, find x:

Answer 24