Equilibrium

What is a reversible reaction?

A reaction in which products can react to reform the original reactants.

How is a reversible reaction represented?

Using a reversible reaction arrow: ⇌.

What is dynamic equilibrium?

The state reached in a reversible reaction when the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.

Why is equilibrium described as dynamic?

Both the forward and reverse reactions continue to occur; they do not stop.

What happens to the concentrations of reactants and products at dynamic equilibrium?

They remain constant, but they do not have to be equal.

What happens to the rates of the forward and reverse reactions at equilibrium?

They are equal.

Does equal reaction rate mean equal concentrations of reactants and products?

No. The concentrations remain constant but do not necessarily equal each other.

What type of system is required for dynamic equilibrium to be established?

A closed system, so reactants and products cannot enter or leave.

What is Le Chatelier's principle?

When a system at equilibrium is disturbed by changing concentration, temperature or pressure, the equilibrium position shifts in the direction that opposes the change.

What is meant by the position of equilibrium?

The relative amounts/concentrations of reactants and products present at equilibrium.

What happens if the concentration of a reactant is increased?

The equilibrium shifts to the side that uses up the added reactant.

What happens if the concentration of a product is increased?

The equilibrium shifts to the side that uses up the added product.

What happens if a reactant is removed?

The equilibrium shifts towards the reactants to replace some of the reactant that was removed.

What happens if a product is removed?

The equilibrium shifts towards the products to replace some of the product that was removed.

What is the equilibrium reaction in the Contact Process?

2SO₂(g) + O₂(g) ⇌ 2SO₃(g).

What happens if the concentration of SO₂ or O₂ is increased?

The equilibrium shifts to the right, producing more SO₃.

What happens if the concentration of SO₃ is increased?

The equilibrium shifts to the left, producing more SO₂ and O₂.

How does pressure affect an equilibrium involving gases?

Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. Decreasing pressure shifts it towards the side with more moles of gas.

Why does increasing pressure favour the side with fewer gas molecules?

This reduces the total number of gas particles and therefore opposes the increase in pressure.

What happens if both sides of a gaseous equilibrium have the same number of gas molecules?

Changing pressure does not affect the position of equilibrium.

For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), what happens when pressure increases?

The equilibrium shifts right because there are 3 moles of gas on the left and 2 moles on the right.

For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), what happens when pressure decreases?

The equilibrium shifts left because the left side has more moles of gas.

How does temperature affect equilibrium?

Increasing temperature favours the endothermic direction because it absorbs the added heat. Decreasing temperature favours the exothermic direction because it releases heat.

What happens when temperature increases for an exothermic forward reaction?

Equilibrium shifts to the left, favouring the endothermic reverse reaction.

What happens when temperature decreases for an exothermic forward reaction?

Equilibrium shifts to the right, favouring the exothermic forward reaction.

What happens when temperature increases for an endothermic forward reaction?

Equilibrium shifts to the right, favouring the endothermic forward reaction.

What happens when temperature decreases for an endothermic forward reaction?

Equilibrium shifts to the left, favouring the exothermic reverse reaction.

What is the Haber process equilibrium?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92.4 kJ mol⁻¹.

What happens to the Haber equilibrium when temperature increases?

It shifts left because the forward reaction is exothermic.

What happens to ammonia yield when temperature increases?

It decreases.

What happens to ammonia yield when temperature decreases?

It increases.

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

None.

Why does a catalyst not change the position of equilibrium?

It lowers the activation energy of both the forward and reverse reactions by the same amount, so both reactions speed up equally.

What effect does a catalyst have on the time taken to reach equilibrium?

Equilibrium is reached more quickly.

Why are compromise conditions used in industrial equilibrium reactions?

Industry must balance yield, rate, cost, energy use and safety.

Why isn't the condition giving the maximum equilibrium yield always used industrially?

It may give an impractically slow reaction, require excessive energy/cost, or create safety problems.

What are the compromise conditions used in the Haber process?

Approximately 450°C and 200 atm, with an iron catalyst.

Why is a relatively high temperature used in the Haber process even though lower temperature gives a higher ammonia yield?

Lower temperature gives a higher equilibrium yield but a much slower reaction. Around 450°C is a compromise between reaction rate and equilibrium yield.

Why is high pressure used in the Haber process?

It shifts equilibrium towards ammonia because there are fewer moles of gas on the product side.

Why isn't an extremely high pressure used?

Very high pressures are expensive and require stronger equipment and create greater safety risks.

What is the role of the iron catalyst in the Haber process?

It increases the reaction rate by lowering activation energy, allowing equilibrium to be reached more quickly.

What is Kc?

The equilibrium constant for a homogeneous equilibrium, calculated from the equilibrium concentrations of the reactants and products.

What does [X] mean in a Kc expression?

The equilibrium concentration of species X in mol dm⁻³.

How do you write a Kc expression for aA + bB ⇌ cC + dD?

Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ.

How do the coefficients in a balanced equation relate to the Kc expression?

They become the powers of the corresponding concentrations.

What is important when writing a Kc expression?

Use equilibrium concentrations and use the stoichiometric coefficients as powers.

What does a large Kc indicate?

Products are favoured at equilibrium; the equilibrium mixture contains relatively more products than reactants.

What does a small Kc indicate?

Reactants are favoured at equilibrium.

What does Kc ≈ 1 indicate?

Neither side is strongly favoured; reactants and products are present in comparable relative amounts.

Does a large Kc mean the reaction is fast?

No. Kc tells you about the position of equilibrium, not the rate of reaction.

Does Kc tell you how quickly equilibrium is reached?

No.

What information is needed to calculate Kc?

The balanced equation and the equilibrium concentrations of the relevant species.

What is the general method for calculating Kc?

Write the balanced equation.
Write the Kc expression.
Find the equilibrium concentrations.
Substitute the concentrations into the expression.
Calculate the value of Kc.
Give appropriate units where required.

What is an ICE table?

A table showing the Initial concentration, Change in concentration and Equilibrium concentration.

Why are ICE tables useful?

They help calculate unknown equilibrium concentrations systematically.
Worked example

For 2SO₃(g) ⇌ 2SO₂(g) + O₂(g), initially [SO₃] = 0.15 mol dm⁻³ and at equilibrium [O₂] = 0.05 mol dm⁻³, what are the equilibrium concentrations?

Since O₂ increases by 0.05 mol dm⁻³, SO₂ increases by 0.10 mol dm⁻³ and SO₃ decreases by 0.10 mol dm⁻³. Therefore:
[SO₃] = 0.05 mol dm⁻³
[SO₂] = 0.10 mol dm⁻³
[O₂] = 0.05 mol dm⁻³.

What is Kc for 2SO₃(g) ⇌ 2SO₂(g) + O₂(g) using those concentrations?

Kc = [SO₂]²[O₂] / [SO₃]² = (0.10)²(0.05)/(0.05)² = 0.20.

Why must the balanced equation be correct before calculating Kc?

The stoichiometric coefficients determine the powers used in the Kc expression.

What affects the value of Kc?

Temperature.

Does changing concentration affect Kc?

No. It changes the position of equilibrium, but Kc remains constant at a fixed temperature.

Does changing pressure affect Kc?

No. It may change the position of equilibrium, but Kc remains constant at a fixed temperature.

Does adding a catalyst affect Kc?

No.

Why does changing concentration not change Kc?

The equilibrium position changes until the original Kc value is restored at that temperature.

Why does a catalyst not change Kc?

It changes the rates of both forward and reverse reactions equally but does not change the equilibrium position.

How does increasing temperature affect Kc for an exothermic forward reaction?

Kc decreases because equilibrium shifts towards reactants.

How does decreasing temperature affect Kc for an exothermic forward reaction?

Kc increases because equilibrium shifts towards products.

How does increasing temperature affect Kc for an endothermic forward reaction?

Kc increases because equilibrium shifts towards products.

How does decreasing temperature affect Kc for an endothermic forward reaction?

Kc decreases because equilibrium shifts towards reactants.

What must I be able to do for AQA Equilibria?

Define dynamic equilibrium.

Explain why equilibrium requires a closed system.

Use Le Chatelier's principle for concentration, pressure and temperature changes. Explain catalyst effects on equilibrium.

Explain industrial compromise conditions.

Write Kc expressions from balanced equations.

Calculate equilibrium concentrations.

Calculate Kc.

Interpret Kc values.

Explain how temperature changes Kc