Grade 12 Physical Sciences · Chemical Change

Apply Le Chatelier's principle

Predict and explain how an equilibrium shifts when concentration, pressure or temperature changes (adding more reactant pushes it towards products).

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Le Chatelier's Principle grade 12: IntroductionKevinmathscience

Chemical Equilibrium Le Chatelier's Principle: Changing concentrationMiss Martins Maths and Science

Chemical Equilibrium Le Chatelier's Principle: Changing pressureMiss Martins Maths and Science

Chemical Equilibrium Le Chatelier's Principle: Changing temperatureMiss Martins Maths and Science

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Question 1.6 · May/June 2025 · Paper 2 · National (DBE) · 2 marks

1.6 Carbonyl chloride gas,

COC ℓ_{2}

(g), decomposes in a closed container and reaches equilibrium according to the following equation:

COC ℓ_{2} (g) ⇌ CO(g) + C ℓ_{2} (g)

Δ H > 0

Consider the following statements.

An increase in temperature will:

(i) Favour the reverse reaction
(ii) Increase the concentration of the products
(iii) Increase the equilibrium constant

Which of the statements above are TRUE?(2)

A (i) and (ii) only
B (i) and (iii) only
C (ii) and (iii) only
D (i), (ii) and (iii)

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: apply le chatelier's principle

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3. Question 6.1.3 · November 2025 · Paper 2 · National (DBE) · 2 marks
6.1 Equilibrium is established at $25$ °C in a saturated calcium hydroxide solution according to the reaction:

$Ca(OH)_{2} (s) ⇌ Ca^{2 +} (aq) + 2 OH^{-} (aq)$

Answer the questions that follow.
6.1.3 Explain the answer to QUESTION 6.1.2 by using Le Chatelier's principle.(2)
Show the memo (2 ticks)Memo
1. ✓
  Disturbance and responseDecrease in amount of $OH^{-}$ ions / concentration of $OH^{-}$ ions, favours the reaction that increases the amount/concentration of $OH^{-}$ ions.
  OR Acid/HC $ℓ$ / $H^{+}$ reacts with $OH^{-}$ ions.
  1 mark · Apply Le Chatelier's principle
2. ✓
  Forward reaction favouredThe forward reaction is favoured.
  OR The amount/concentration of the products increases.
  1 mark · Apply Le Chatelier's principle
4. Question 6.8 · May/June 2024 · Paper 2 · National (DBE) · 2 marks
Consider the balanced equation for a hypothetical reaction that takes place in $2$ dm $^{3}$ sealed containers.

$P_{2} Q(g) + 3 Q(s) ⇌ 2 PQ_{2} (g)$

The graphs below, not drawn to scale, are obtained for the same reaction at two different temperatures.

Graph A is obtained at $298$ K and graph B at $398$ K.
6.8 Explain by using Le Chatelier's principle how the system reacts to the change in QUESTION 6.7.(2)
Show the memo (2 ticks)Memo
1. ✓
  Reaction favouredFavours the reaction that increases the number of moles (of gas).
  1 mark · Apply Le Chatelier's principle
2. ✓
  Effect on concentration $[PQ_{2}]$ increases.
  1 mark · Apply Le Chatelier's principle
5. Question 6.1.8 · November 2024 · Paper 2 · National (DBE) · 3 marks
6.1 The reaction of carbon monoxide gas, CO(g), with oxygen gas, $O_{2}$ (g), is investigated. The reaction reaches equilibrium in a closed container at constant temperature T °C, according to the balanced equation:

$2 CO(g) + O_{2} (g) ⇌ 2 CO_{2} (g)$ $Δ H < 0$

Changes to the conditions of equilibrium are made at different times. The graph shows the results obtained. X, Y and Z represent the gases in the above reaction.

Answer the questions that follow.
6.1.8 Use Le Chatelier's principle to explain the answer to QUESTION 6.1.7.(3)
Show the memo (3 ticks)Memo
1. ✓
  Forward reaction favouredConcentration of products / Y / $CO_{2}$ increases.
  
  OR: Concentration of reactant / Z / X / CO / $O_{2}$ decreases.
  OR: The forward reaction is favoured.
  1 mark · Apply Le Chatelier's principle
2. ✓
  Forward reaction is exothermicThe forward reaction is exothermic.
  1 mark · Apply Le Chatelier's principle
3. ✓
  Decrease in temperature favours exothermic reactionA decrease in temperature favours the exothermic reaction.
  1 mark · Apply Le Chatelier's principle
6. Question 1.5 · May/June 2024 · Paper 2 · National (DBE) · 2 marks
1.5 The balanced equation below represents a reaction at equilibrium.

$2 CrO_{4}^{2 -} (aq) + 2 H_{3} O^{+} (aq) ⇌ Cr_{2} O_{7}^{2 -} (aq) + 3 H_{2} O (ℓ)$
( $CrO_{4}^{2 -}$ is yellow; $Cr_{2} O_{7}^{2 -}$ is orange.)

Which statement(s) is/are TRUE when a few drops of concentrated hydrochloric acid, HCℓ(conc), are added to the mixture?

(i) The reverse reaction will be favoured.
(ii) The concentration of $Cr_{2} O_{7}^{2 -}$ (aq) increases.
(iii) The colour of the solution changes from yellow to orange.(2)
1. A (i) only
2. B (i) and (ii) only
3. C (i) and (iii) only
4. D (ii) and (iii) only
Show the memo (1 tick)Memo
1. ✓
  Choose the correct optionAdding HCℓ increases $[H_{3} O^{+}]$ , a reactant. The system uses up some of the added $H_{3} O^{+}$ by favouring the FORWARD reaction. So (i) is false, $[Cr_{2} O_{7}^{2 -}]$ increases (ii) and the solution turns from yellow to orange (iii).
  
  Answer: D
  2 marks · Apply Le Chatelier's principle

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