7.1: chemical equilibria: reversible reactions, dynamic equilibrium

Define a reversible reaction

A reaction in which products can be turned back into reactants by reversing conditions

In dynamic equilibrium:

The rate of the forwards and backwards reaction is the same in a closed system

The concentrations of the reactants and products are constant

When does the concentration stop changing in a reaction?

When equilibrium is reached

What does Le Chatelier’s principle state?

if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change

If the concentration of a reactant is increased:

Equilibrium shifts to the right

If the concentration of a product is increased

Equilibrium shifts to left

If pressure is increased:

Equilibrium shifts in the direction that produces a smaller number of molecules of gas to decrease the pressure again

If temperature is increased

• Equilibrium moves in the endothermic direction to reverse the change

If temperature is decreased

• Equilibrium moves in the exothermic direction to oppose the change 

Explain the effect of catalysts

• Substance that increases the rate of a chemical reaction

• Increase the rate of the forward and reverse reaction equally

• Only cause a reaction to reach its equilibrium faster

• Catalysts therefore have no effect on the position of the equilibrium once this is reached

What is Kc defined as

Which state is ignored in Kc expressions?

Solids

Define partial pressure

The pressure exerted by particular gas A in a mixture of gases

Mole fraction=

The number of moles of a particular gas/ total number of moles of all gases in a mixture

Partial pressure =

Mole fraction x total pressure

What is the only factor that can change Kc?

Temperature

Explain how ammonia yield is maximised in the haber process in terms of pressure

• Increasing pressure shifts equilibrium to the right, increasing ammonia yield

• Higher pressure also increases collision frequency, enhancing the reaction rate

• However, very high pressures are costly and require strong containment

• Compromise pressure used

  • ≈ 200 atm

Explain how ammonia yield is maximised in the haber process in terms of temperature

• The forward reaction is exothermic

• Lowering temperature shifts equilibrium to the right, favouring ammonia formation

• But too low a temperature would slow the reaction rate, delaying equilibrium

• Compromise temperature:

  • 400–450 °C

Explain how ammonia yield is maximised in the haber process in terms of removing ammonia

• Ammonia is removed by cooling and condensing it to a liquid

• This shifts the equilibrium further to the right, producing more ammonia

• Stored ammonia is kept at low temperatures where decomposition is very slow, especially in the absence of a catalyst

Explain how ammonia yield is maximised in the haber process in terms of using a catalyst

• An iron catalyst is used to increase the rate of reaction without affecting equilibrium position

• Without it, the reaction would be too slow

Explain how ammonia yield is maximised in the contact process in terms of pressure

• Increasing pressure shifts equilibrium to the right, favouring SO₃ formation

• However, the equilibrium constant (K_p) is already very large at low pressures

• Industrial pressure used:

  • ~1 atm

Explain how ammonia yield is maximised in the contact process in terms of temperature

• Reaction is exothermic

• Lower temperatures would favour SO₃ production, but also reduce the rate

• Compromise temperature:

  • ≈ 450 °C

Explain how ammonia yield is maximised in the contact process in terms of removing sulfuric acid

Shifts equilibrium to the right, driving reaction forward

Explain how ammonia yield is maximised in the contact process in terms of using a catalyst

Contact process uses vanadium(V) oxide as a catalyst to increase the rate of reaction