Dynamic equilibrium and le Chatelier's principle

Define a closed system

A system that is isolated from its surroundings, so the temperature, pressure and concentrations of reactants and products are unaffected by outside influences

Define dynamic equilibrium

Equilibrium that exists:

• In a closed system

• When the rate of the forward reaction is equal to the rate of the reverse reaction

• And the concentrations of reactants and products do not change

What does le Chatelier’s principle state?

When a system in dynamic equilibrium is subject to a change, the position of equilibrium will shift to minimise that change

What are the colours of the compounds in this experiment?

Cu(H₂O)₆²⁺_(aq) is blue

CuCl₄^2-_(aq) is yellow

Explain the effect of:

1. Adding HCl

2. Adding H₂O

1. Adding HCl increases the concentration of Cl^-, so the position of equilibrium shifts to decrease the concentration of Cl^-, moving to the right

   • The reaction turns yellow as CuCl₄^2- is formed

2. Adding H₂O increases its concentration so the position of equilibrium shifts to the left, favouring the backwards reaction to turn the mixture blue

Explain the effect of:

1. Adding water

2. Adding concentrated HCl

1. Increased concentration of water, position of equilibrium shifts to decrease the concentration of water, equilibrium shifts left, pink

2. Increased concentration of Cl^-, position of equilibrium shifts to decrease the concentration of Cl^-, shifts right, blue

For this reaction, explain the results shown when:

1. Solution turns blue when put in water at 90C

2. Solution turns pink when put in ice

1. Increased temperature, position of equilibrium shifts to decrease temperature, reaction is endothermic forwards

2. Decreased temperature, position of equilibrium shifts to increase temperature, favouring exothermic reactiton, exothermic backwards

To which kinds of reactions will a change in pressure affect the position of equilibrium?

Reactions where gases are present

Give the equation for the Haber Process, and state whether this reaction is exothermic or endothermic

N_2(g) + 3H_2(g) ⇌ 2NH_3(g) (exothermic forward reaction)

Using the Haber Process as an example, explain the effect of:

1. Increasing the pressure

2. Decreasing the pressure

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

1. Increase pressure, position of equilibrium shifts to decrease pressure, favouring the side with less gas moles, position of equilibrium shifts to the right

2. Decreased pressure, position of equilibrium shifts to increase pressure, favours side with more gas moles, shifts to left

What effect does a catalyst have on the position of equilibrium?

A catalyst increases the rate of both the forward and reverse reaction in an equilibrium by the same amount, resulting in an unchanged position of equilibrium

What is the main aim of many important industrial chemical processes?

To make the highest possible yield of product at the least cost

What is the temperature used for the Haber Process and why is this a compromise temperature?

450C is used

• In theory the forward reaction is favoured by a high pressure and low temperature

• But a low temperature would decrease the rate, leading to a compromise temperature of 450C as this leads to a sufficiently fast rate without sending the position of equilibrium too far to the left

What pressure is used for the Haber Process and why is this a compromise?

200 atm is used

• In theory a high pressure would favour the forward reaction

• But a high pressure requires high energy to compress the gases, so a compromise 200atm is used due to the cost and safety implication

What catalyst is used for the Haber Process and why?

Finely divided iron for large surface area, and speeding up of the reaction rate

• This is a heterogeneous catalyst

Only a small percentage of nitrogen and hydrogen is converted into ammonia. What is done to get around this problem?

Unreacted hydrogen and nitrogen are recycled repeatedly, so nearly all of it is eventually converted into ammonia