Grade 12 Physical Sciences · Chemical Change

Interpret equilibrium graphs

Read concentration-time or rate-time graphs to find when equilibrium is reached and which change was made at a given time.

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Equilibrium Graphs grade 12: IntroductionKevinmathscience

Equilibrium Graphs grade 12: ConcentrationKevinmathscience

Equilibrium Graphs Grade 12| How to interpret concentrations time graphAyanda Godo

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Question 6.5 · May/June 2025 · Paper 2 · National (DBE) · 2 marks

The reaction between powdered carbon, C(s), and carbon dioxide gas,

CO_{2}

(g), takes place in a closed

3

^{3}

container according to the balanced equation below.

C(s) + CO_{2} (g) ⇌ 2 CO(g)

Δ H < 0

Equilibrium is reached at temperature T °C.

The graph below, not drawn to scale, shows the curves for the mass of the reactants in the container against time. Diagram for this question

The balanced equation for this reaction is rewritten below for easy reference.

C(s) + CO_{2} (g) ⇌ 2 CO(g)

Δ H < 0

The graph below shows the equilibrium number of moles of CO(g), C(s) and CO

_{2}

(g) in the flask at time t

_{1}

6.5 At t

_{2}

, some CO(g) is added to the flask and the reaction is allowed to reach equilibrium. Curve Z correctly shows the change in the amount of CO(g) between t

_{2}

and t

_{3}

.

A learner draws corresponding curves, labelled X and Y, for C(s) and CO

_{2}

(g) respectively.

Which of these curves is the CORRECT representation for these changes?

Choose from: X or Y or X and Y(2) Diagram for question 6.5

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

More past paper questions: interpret equilibrium graphs

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3. Question 6.6 · November 2023 · Paper 2 · National (DBE) · 3 marks
Consider the following hypothetical reaction reaching equilibrium in a $4$ dm $^{3}$ closed container at $150$ °C.

$2 AB(g) ⇌ A_{2} (g) + B_{2} (g)$

The graph below shows the changes in the amounts of reactants and products over time.
The initial reaction now takes place in the presence of a catalyst at $150$ °C.
6.6 Describe the changes that will be observed on the graph between t = $0$ s and t = $60$ s.(3)
Show the memo (2 ticks)Memo
1. ✓
  Gradients steeperGradients (of all three curves) will be steeper.
  2 marks · Interpret equilibrium graphs
2. ✓
  Same equilibrium valuesand reach the same equilibrium values (sooner).
  1 mark · Interpret equilibrium graphs
4. Question 6.1.2 · November 2024 · Paper 2 · National (DBE) · 1 mark
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.
Use the graph to answer the questions below.
6.1.2 At $t_{1}$ , oxygen, $O_{2}$ (g), was added to the container. Write down the letter that represents $O_{2}$ (g). Choose from X, Y or Z.(1)
Show the memo (1 tick)Memo
1. ✓
  Letter for O $_{2}$ X
  1 mark · Interpret equilibrium graphs
5. Question 6.7 · May/June 2024 · Paper 2 · National (DBE) · 1 mark
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.
6.7 Describe the change made to the equilibrium system at t = 8 minutes, as shown in graph B, at a constant temperature.(1)
Show the memo (1 tick)Memo
1. ✓
  Change madePressure was decreased / volume of the container was increased.
  1 mark · Interpret equilibrium graphs
6. Question 6.6.2 · November 2022 · Paper 2 · National (DBE) · 1 mark
Carbon, C(s), reacts with sulphur, S(g), according to the following balanced equation:

$C(s) + 2 S(g) ⇌ CS_{2} (g)$ $Δ H > 0$

The system reaches equilibrium at temperature T in a sealed $2$ dm $^{3}$ container.

The $K_{c}$ value is $9, 4$ at temperature T.
6.6 The reaction rate-time graph below represents further changes made to the equilibrium mixture. The volume of the container is kept constant.
6.6.2 What change was made to the equilibrium mixture at $t_{B}$ ?(1)
Show the memo (1 tick)Memo
1. ✓
  Change at tBIncrease in the amount/concentration of S/reactant OR S was added.
  1 mark · Interpret equilibrium graphs

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