Higher Level Revision Notes

Leaving Cert Higher Level Chemistry

Leaving Cert Higher Level Chemistry Chapter 8: Chemical Equilibrium

You learn how reversible reactions settle into equilibrium and how pressure, temperature, and concentration change the balance.

What this page doesTurns a difficult chapter into clear notes, diagrams, and one simple interactive tool.

Student promisePlain English first, exam language second.

Curriculum

Irish Leaving Certificate (ILC)

Level

Higher Level

Subject

Chemistry

Chapter

Chapter 8 — Chemical Equilibrium

Simple English Summary

You learn how reversible reactions settle into equilibrium and how pressure, temperature, and concentration change the balance.

Teacher voice: Read the ideas first, then use the detailed notes and diagrams to lock in the exam wording.

What To Focus On

Explain dynamic equilibrium simply.
Use Le Chatelier’s principle correctly.
Understand equilibrium constants.
Connect equilibrium to industry.

DisclaimerThis publication is an independent educational resource developed by ExamsLogic and compiled for student revision. It is based on publicly available official curricula and is not endorsed by any examination board.

Open Simulators Jump To Notes

Quick Simpler Start

In one sentence

You learn how reversible reactions settle into equilibrium and how pressure, temperature, and concentration change the balance.

Exam habit

When you revise this chapter, ask yourself: "Can I explain this to a friend in one easy paragraph?" If yes, you are in a good place.

Interactive Simulator

This small tool gives you a quick visual check before you move deeper into the chapter notes.

Simulator

Equilibrium Shift Explorer

Choose the change and see which side the equilibrium moves toward.

What changed?

Choose a change to see the shift.

1. Learning Objectives

Explain reversible reactions and dynamic equilibrium.
State Le Chatelier’s Principle clearly.
Predict the effect of changing concentration, temperature and pressure.
Apply equilibrium ideas to the Haber Process and Contact Process.
Write equilibrium constant expressions correctly.
HL Calculate Kc and equilibrium concentrations.
HL Explain named equilibrium practicals and how temperature affects Kc.

2. Reversible Reactions and Dynamic Equilibrium

A reversible reaction can proceed in both the forward and reverse directions. It is shown using a double arrow.

Reactants ⇌ Products

Dynamic equilibrium is reached in a closed system when the forward reaction and reverse reaction occur at the same rate, so the concentrations of reactants and products remain constant.

Examiner Tip: Never say “the reaction stops” at equilibrium. It does not stop. Both reactions continue at equal rates.

Common Mistake: At equilibrium, concentrations are constant, but they are not necessarily equal.

3. Le Chatelier’s Principle

Le Chatelier’s Principle: If a system at equilibrium is disturbed, the system shifts in the direction that opposes the disturbance.

Change	Equilibrium shifts to...	Simple rule
Increase concentration of reactant	Products	Use up added reactant.
Increase concentration of product	Reactants	Use up added product.
Increase temperature	Endothermic direction	Absorb added heat.
Decrease temperature	Exothermic direction	Produce heat.
Increase pressure	Side with fewer gas molecules	Reduce pressure.
Decrease pressure	Side with more gas molecules	Increase pressure.

Examiner Trap: Catalysts do not change the position of equilibrium. They only help equilibrium to be reached faster.

4. Concentration, Temperature and Pressure

Concentration

If concentration is changed, the equilibrium shifts to use up what has been added or replace what has been removed.

Example: For A + B ⇌ C + D, if more A is added, the equilibrium shifts right to use up A and form more C and D.

Temperature

Temperature change depends on whether the forward reaction is exothermic or endothermic. Temperature is the most important condition here because it can change the actual value of Kc.

Exothermic forward reaction: Reactants ⇌ Products + heat

HL Note: If temperature changes, the equilibrium position may shift and the numerical value of Kc may also change. Concentration and pressure changes do not change Kc if temperature is constant.

Pressure

Pressure changes only matter when gases are involved. Count the number of gas molecules on each side of the equation.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Left side: 4 gas molecules. Right side: 2 gas molecules. Increasing pressure favours the right side because it has fewer gas molecules.

5. Industrial Applications

Haber Process

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Condition	Why it is used
High pressure	Favours the side with fewer gas molecules, so ammonia yield increases.
Moderate temperature	Lower temperature gives better yield, but reaction would be too slow; industry uses a compromise.
Iron catalyst	Speeds up equilibrium being reached without changing equilibrium position.

Contact Process

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

This is the catalytic oxidation of sulfur dioxide to sulfur trioxide in the Contact Process. The forward reaction is exothermic and the product side has fewer gas molecules, so lower temperature and higher pressure would favour products. In practice, industry again uses compromise conditions for good yield and good rate.

Point	Explanation
Catalyst	Vanadium(V) oxide is used.
Temperature	A moderate temperature is used as a compromise between rate and yield.
Pressure	Higher pressure favours SO₃, but very high pressure is not always economical.

6. Equilibrium Constant, Kc

Kc gives information about the position of equilibrium for a reversible reaction at a fixed temperature.

For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b

Kc value	Meaning
Large Kc	Products are favoured.
Small Kc	Reactants are favoured.
Kc about 1	Neither side is strongly favoured.

Examiner Tip: Use equilibrium concentrations, not starting concentrations.

Common Mistake: Do not put reactants on top. Kc is products over reactants.

7. Higher Level Kc Calculations

Writing Kc Expressions

Example 1:
H₂(g) + I₂(g) ⇌ 2HI(g)
Kc = [HI]² / ([H₂][I₂])

Example 2:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
Kc = [SO₃]² / ([SO₂]²[O₂])

Numerical Kc Calculation

Example 3:
For H₂ + I₂ ⇌ 2HI, if [H₂] = 0.10 mol L^-1, [I₂] = 0.20 mol L^-1 and [HI] = 0.80 mol L^-1, then:

Kc = (0.80)² / (0.10 × 0.20)
Kc = 0.64 / 0.02 = 32

Equilibrium Concentration Method

For harder Higher Level questions, follow this sequence:

Write the balanced equation.
Write the Kc expression.
Substitute known equilibrium concentrations.
If one value is unknown, rearrange the equation carefully.
Check units and whether the answer makes chemical sense.

HL Note: Many equilibrium concentration questions become easier if you first set up a simple change table: start → change → equilibrium.

8. What Changes Kc?

Change applied	Does Kc change?	Why?
Concentration changed	No	The system shifts until the same Kc is restored.
Pressure changed	No	The position shifts, but Kc stays the same if temperature is constant.
Catalyst added	No	It speeds forward and reverse reactions equally.
Temperature changed	Yes	Temperature changes the equilibrium constant itself.

Examiner Trap: A catalyst can change how fast equilibrium is reached, but not the equilibrium position and not Kc.

9. Named Equilibrium Practicals

Cobalt Chloride Equilibrium

Cobalt chloride equilibrium is often used to show the effect of concentration and temperature changes. The pink and blue species shift depending on conditions.

[Co(H₂O)₆]²⁺ + 4Cl^- ⇌ [CoCl₄]^2- + 6H₂O

Pink side is favoured with more water.
Blue side is favoured with more chloride ions.
Heating can also shift the colour depending on which side is endothermic.

Chromate / Dichromate Equilibrium

2CrO₄^2- + 2H⁺ ⇌ Cr₂O₇^2- + H₂O

Chromate ions are yellow.
Dichromate ions are orange.
Adding acid favours dichromate; adding alkali favours chromate.

Iron(III) Thiocyanate Equilibrium

Fe³⁺ + SCN^- ⇌ FeSCN²⁺

The complex ion is deep red.
Adding Fe³⁺ or SCN^- makes the red colour deeper.
Removing one ion or diluting the mixture can reduce the colour intensity.

Practical Skill: In all three experiments, the key exam idea is: observe the colour change, identify which side is favoured, and explain the shift using Le Chatelier’s Principle.

10. Worked Exam Examples

Example 1: Pressure change
N₂ + 3H₂ ⇌ 2NH₃
Increasing pressure shifts equilibrium right because the right side has fewer gas molecules.

Example 2: Temperature change
If the forward reaction is exothermic, increasing temperature shifts equilibrium left because the system tries to absorb the added heat.

Example 3: Kc meaning
If Kc is very large, the equilibrium mixture contains much more product than reactant.

Example 4: Practical colour shift
If extra SCN^- is added to Fe³⁺ + SCN^- ⇌ FeSCN²⁺, the red colour deepens because the equilibrium shifts right to use up the extra thiocyanate ions.

11. Examiner Secrets, Mistakes and Traps

Secret 1: Always state the direction of shift: left or right.

Secret 2: Pressure only matters when gases are involved.

Secret 3: Temperature is the standard “Kc-changing” condition.

Common Mistake: Saying equilibrium means equal amounts. It means equal rates.

Common Mistake: Using starting concentrations instead of equilibrium concentrations in Kc.

Examiner Trap: A large Kc does not mean a fast reaction. It only tells you the position of equilibrium.

12. Exam Practice Questions

Define dynamic equilibrium. [3 marks]
State Le Chatelier’s Principle. [3 marks]
For N₂ + 3H₂ ⇌ 2NH₃, explain the effect of increasing pressure. [4 marks]
Explain why a catalyst is used in the Haber Process. [3 marks]
Write the Kc expression for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). [3 marks]
HL Calculate Kc for H₂ + I₂ ⇌ 2HI if [H₂] = 0.10, [I₂] = 0.20 and [HI] = 0.80 mol L^-1. [4 marks]
Explain why temperature changes Kc but a catalyst does not. [4 marks]
Describe what happens in one named equilibrium practical when a reagent is added. [4 marks]

13. Last-Minute Revision Sheet

Equilibrium needs a closed system.
Dynamic equilibrium means forward rate = reverse rate.
Le Chatelier: the system shifts to oppose a change.
Increase pressure → favours fewer gas molecules.
Increase temperature → favours endothermic direction.
Kc = products over reactants.
Use powers from the balanced equation.
Kc changes with temperature.
Catalysts do not change equilibrium position or Kc.
Colour changes in named practicals must be explained by equilibrium shifts.

14. Self-Assessment Checklist

I can define dynamic equilibrium correctly.
I can apply Le Chatelier’s Principle to concentration, temperature and pressure.
I can explain the Haber Process and Contact Process.
I can write Kc expressions correctly.
I can calculate simple Kc values.
I can explain what changes Kc.
I can describe cobalt chloride, chromate/dichromate, and iron(III) thiocyanate equilibrium experiments.