Chemical Equilibria

Learn and apply Chemical Equilibria in the published Chemistry course sequence.

GCE A-Level H1 Chemistry 8873-2027

Learning goals

explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium
state Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in concentration, pressure or temperature, on a system at equilibrium
deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction
deduce expressions for equilibrium constants in terms of concentrations, Kc
calculate the values of equilibrium constants in terms of concentrations from appropriate data
calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations)
describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of chemical equilibrium in the chemical industry

H1 Chemical Equilibria learning outcomes

Chemical Equilibria connects particle-level dynamic behaviour to composition predictions and quantitative Kc evidence. Strong H1 answers separate rate, position and equilibrium-constant claims.

Follow the four lessons in order for first learning. During revision, classify each error as definition, disturbance logic, Kc expression, ICE-table stoichiometry or industrial compromise.

What You’ll Learn

Explain dynamic equilibrium as equal forward and reverse rates in a closed system.
Predict concentration, pressure and temperature effects using Le Chatelier’s Principle.
Separate changes in equilibrium composition from changes in the equilibrium constant.
Write Kc expressions from balanced equations using equilibrium concentrations.
Complete Kc equilibrium-composition calculations that do not require solving a quadratic equation.
Evaluate Haber-process yield, rate, safety and economic compromises.

Lessons (Recommended Order)

Build the dynamic model first, express it through Kc, calculate equilibrium composition, then apply both kinetics and equilibrium to the Haber process.

Dynamic Equilibrium and Le Chatelier’s Principle
Explain dynamic equilibrium and predict composition changes from controlled disturbances.
Open lesson
Equilibrium Constant Kc
Write and interpret Kc expressions without importing Kp.
Open lesson
Kc Equilibrium Calculations
Use coefficient-aware ICE tables for non-quadratic Kc calculations.
Open lesson
Haber Process Case Study
Balance equilibrium yield, reaction rate, safety and industrial cost.
Open lesson

Quick Reference

Idea	Exam-ready reminder
Dynamic equilibrium	forward and reverse rates are equal; concentrations are constant, not equal
Kc	use equilibrium concentrations raised to stoichiometric powers; omit pure solids and liquids
Large Kc	products predominate at equilibrium; it does not mean the reaction is fast
Concentration	the system consumes some added species; Kc is unchanged at fixed temperature
Pressure	higher pressure favours the side with fewer moles of gas; no effect when gas totals are equal
Temperature	the only listed condition that changes Kc
Catalyst	speeds forward and reverse reactions equally; position and Kc are unchanged
ICE table	apply stoichiometric coefficients to one change variable and use equilibrium concentrations in Kc
Haber compromise	moderate temperature, high but safe pressure, catalyst and ammonia removal balance yield, rate and cost

Use concentration expressions and Kc. Do not import Kp, partial-pressure calculations or equilibrium calculations requiring a quadratic equation.

Practice and Check Your Understanding

H1 Chemical Equilibria Topic Practice
Course-specific 8873 practice across dynamic equilibrium, Le Chatelier, Kc, composition and Haber compromises.
Start topic practice
H1 Chemical Equilibria: Check Your Understanding
Check the reasoning links, read the targeted feedback, then try a fresh question independently.
Practice onlyCheck understanding

Practise

Work through questions with marking and feedback as you learn.

About 10 minutes

Questions are picked at random each time you start. You'll see the answer after each question. It's for practice only and doesn't count towards mastery.

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This check needs JavaScript to record answers. The reviewed questions are listed below for study.

A little radioactive iodine is added to an equilibrium mixture of H₂(g), I₂(g) and HI(g) at constant temperature. Later, radioactivity is found in both I₂ and HI, but the concentrations of all three substances are unchanged. What does this show?

The forward and reverse reactions are both still happening, at equal rates.
The position of equilibrium has shifted to the right.
The mixture had not yet reached equilibrium.
The radioactive iodine acts as a catalyst for the reaction.

CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹. The temperature of the equilibrium mixture is raised at constant pressure. What happens to the equilibrium amount of CH₃OH?

It decreases, because the position shifts in the endothermic (reverse) direction to absorb the added heat.
It increases, because both reactions become faster.
It is unchanged, because temperature affects only the rate.
It increases, because the position shifts to the side with fewer gas molecules.

N₂O₄(g) ⇌ 2NO₂(g), ΔH = +58 kJ mol⁻¹. Which change increases the value of Kc?

Raising the temperature.
Lowering the pressure at constant temperature.
Removing NO₂ as it forms at constant temperature.
Adding a catalyst.

2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The equilibrium concentrations are [SO₂] = 0.20, [O₂] = 0.10 and [SO₃] = 0.60 mol dm⁻³. What is the value of Kc?

90 mol⁻¹ dm³
30 mol⁻¹ dm³
90 mol dm⁻³
0.011 mol dm⁻³

0.80 mol H₂S is heated in a sealed vessel: 2H₂S(g) ⇌ 2H₂(g) + S₂(g). At equilibrium 25% of the H₂S has dissociated. What amount of S₂ is present at equilibrium?

0.10 mol
0.20 mol
0.40 mol
0.60 mol

In the Haber process a pressure of about 200 atm is used. A higher pressure would give a higher equilibrium yield of ammonia. Why is a much higher pressure not used?

The extra yield does not justify the higher cost of stronger plant and of the energy for compression, and the greater safety risk.
Higher pressure would decrease Kc, lowering the yield.
Higher pressure would shift the position to the left, towards more gas molecules.
Higher pressure would poison the iron catalyst.

Practise after feedback

After a check, practise the skills it showed you need to work on.

About 10 minutes

Questions are picked at random each time you start. You'll see the answer after each question. It's for practice only and doesn't count towards mastery.

Recent attempts

History is stored only in this browser.

No completed attempts are saved yet.

This check needs JavaScript to record answers. The reviewed questions are listed below for study.

The forward and reverse reactions are both still happening, at equal rates.
The position of equilibrium has shifted to the right.
The mixture had not yet reached equilibrium.
The radioactive iodine acts as a catalyst for the reaction.

CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹. The temperature of the equilibrium mixture is raised at constant pressure. What happens to the equilibrium amount of CH₃OH?

It decreases, because the position shifts in the endothermic (reverse) direction to absorb the added heat.
It increases, because both reactions become faster.
It is unchanged, because temperature affects only the rate.
It increases, because the position shifts to the side with fewer gas molecules.

N₂O₄(g) ⇌ 2NO₂(g), ΔH = +58 kJ mol⁻¹. Which change increases the value of Kc?

Raising the temperature.
Lowering the pressure at constant temperature.
Removing NO₂ as it forms at constant temperature.
Adding a catalyst.

2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The equilibrium concentrations are [SO₂] = 0.20, [O₂] = 0.10 and [SO₃] = 0.60 mol dm⁻³. What is the value of Kc?

90 mol⁻¹ dm³
30 mol⁻¹ dm³
90 mol dm⁻³
0.011 mol dm⁻³

0.80 mol H₂S is heated in a sealed vessel: 2H₂S(g) ⇌ 2H₂(g) + S₂(g). At equilibrium 25% of the H₂S has dissociated. What amount of S₂ is present at equilibrium?

0.10 mol
0.20 mol
0.40 mol
0.60 mol

In the Haber process a pressure of about 200 atm is used. A higher pressure would give a higher equilibrium yield of ammonia. Why is a much higher pressure not used?

The extra yield does not justify the higher cost of stronger plant and of the energy for compression, and the greater safety risk.
Higher pressure would decrease Kc, lowering the yield.
Higher pressure would shift the position to the left, towards more gas molecules.
Higher pressure would poison the iron catalyst.

Check what I know

Start here to see which parts you already know.

About 8 minutes

Answer 6 short questions. It shows what to work on next and doesn't count towards mastery.

Recent attempts

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This check needs JavaScript to record answers. The reviewed questions are listed below for study.

Hydrogen and iodine only are sealed in a flask at 700 K and react: H₂(g) + I₂(g) ⇌ 2HI(g). Which statement describes the rates as the mixture approaches equilibrium?

The forward rate falls and the reverse rate rises from zero until the two rates are equal; both reactions then continue.
The forward rate falls to zero at equilibrium, and the reverse rate stays at zero.
Both rates rise until the concentrations of H₂, I₂ and HI are equal.
The reverse reaction begins only after all the H₂ and I₂ has reacted.

N₂O₄(g) ⇌ 2NO₂(g), ΔH = +58 kJ mol⁻¹. The equilibrium mixture is compressed to a smaller volume at constant temperature. How does the position of equilibrium change?

It shifts to the left, towards the side with fewer gas molecules, which partly opposes the pressure increase.
It shifts to the right, because the higher pressure makes collisions more frequent.
It does not change, because the temperature is constant.
It shifts to the right, because the forward reaction is endothermic.

2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −197 kJ mol⁻¹. Which change alters the value of Kc?

Raising the temperature, which decreases Kc.
Raising the pressure, which increases Kc.
Adding a catalyst, which increases Kc.
Removing SO₃ as it forms, which decreases Kc.

2NO(g) + O₂(g) ⇌ 2NO₂(g). At equilibrium a 1.00 dm³ vessel contains 0.20 mol NO, 0.10 mol O₂ and 0.40 mol NO₂. What is the value of Kc?

40 mol⁻¹ dm³
20 mol⁻¹ dm³
0.025 mol dm⁻³
40 mol dm⁻³

1.00 mol CO and 3.00 mol H₂ are sealed in a vessel: CO(g) + 2H₂(g) ⇌ CH₃OH(g). At equilibrium the vessel contains 0.40 mol CH₃OH. What amounts of CO and H₂ are present at equilibrium?

0.60 mol CO and 2.20 mol H₂
0.60 mol CO and 2.60 mol H₂
0.60 mol CO and 2.80 mol H₂
0.40 mol CO and 0.80 mol H₂

N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. Why is the Haber process operated at about 450 °C rather than a much lower temperature?

A lower temperature would give a higher equilibrium yield, but equilibrium would be reached too slowly even with the iron catalyst; 450 °C is a compromise between yield and rate.
A higher temperature gives a higher equilibrium yield of ammonia.
The iron catalyst increases the equilibrium yield only when it is hot.
Below 450 °C the forward reaction becomes endothermic.

Check my progress

When you feel ready, answer on your own to show what you can do.

About 10 minutes

Answer 6 questions. You'll see your score, the answers and explanations at the end. Your result can count towards your course progress.

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Check again

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About 10 minutes

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Review

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About 10 minutes

Answer 6 questions. You'll see your score, the answers and explanations at the end. A scheduled review counts towards your course progress only when it is due.

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Chemical Equilibria

H1 Chemical Equilibria learning outcomes

What You’ll Learn

Lessons (Recommended Order)

Dynamic Equilibrium and Le Chatelier’s Principle

Equilibrium Constant Kc

Kc Equilibrium Calculations

Haber Process Case Study

Quick Reference

Practice and Check Your Understanding

H1 Chemical Equilibria Topic Practice

H1 Chemical Equilibria: Check Your Understanding

Practise

Practice complete

Your answers

Practise after feedback

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Check what I know

Starting check complete

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Check my progress

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Check again

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Review

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