Reversible Reactions and Equilibrium (qualitative)
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Reversible Reactions and Equilibrium (qualitative) — Free MYP5 Chemistry Practice Questions

1QuestionClosed system requirement for equilibriumConcept Practice
3 marks~5 minCriterion A
The diagram shows two sealed flasks and one open flask containing the reversible reaction:

NO2NO_2 (brown) ⇌\rightleftharpoons N2O4N_2O_4 (colourless)

Flask A is sealed with a stopper. Flask B is open to the atmosphere.
a
Identify which flask can achieve dynamic equilibrium. [1]
b
Explain why Flask B cannot achieve dynamic equilibrium, using the concept of a closed system. [2]
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Solutions

2QuestionDefinition of reversible reactionsAssessment Practice
3 marks~5 minCriterion C
The graph below shows how the forward and reverse reaction rates change over time for a closed system at constant temperature, based on the equation:

N2O4(g)⇌2NO2(g)\text{N}_2\text{O}_4\text{(g)} \rightleftharpoons 2\text{NO}_2\text{(g)}

At t=0t = 0 s, the forward rate is 0.8 mol L−1s−10.8 \ \text{mol L}^{-1} \text{s}^{-1} and the reverse rate is zero. Both rates converge over approximately 20 s until they are equal and constant at 0.4 mol L−1s−10.4 \ \text{mol L}^{-1} \text{s}^{-1}, where they remain.
a
Justify the time at which the system first reaches equilibrium. [1]
b
Explain what the equal, constant rates after 20 s indicate about the concentrations of N2O4\text{N}_2\text{O}_4 and NO2\text{NO}_2 in the system. [1]
c
Discuss why the forward reaction rate does not drop to zero when the system reaches equilibrium, even though there is no net change in concentration. [1]
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3QuestionEffect of changes in conditions (temperature, concentration – basic idea)Assessment Practice
5 marks~8 minCriterion D
The reversible reaction between dinitrogen tetroxide and nitrogen dioxide is:

N2O4(g)⇌2NO2(g)\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)

The forward reaction produces NO2\text{NO}_2, a brown gas. A student investigates the effect of temperature on equilibrium by heating sealed tubes of the gas mixture and measuring the concentration of NO2\text{NO}_2 once equilibrium is reached. Results are shown in the graph below.

The student hypothesises that the forward reaction is exothermic.
a
State Le Chatelier's principle as it applies to a change in temperature. [1]
b
Interpret the graph to describe how the equilibrium position changes as temperature increases. [2]
c
Evaluate the student's hypothesis using your answers to (a) and (b). [2]
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4QuestionClosed system requirement for equilibriumAssessment Practice
7 marks~11 minCriterion B
A student investigates the equilibrium between nitrogen dioxide and dinitrogen tetroxide in a sealed syringe:

2 NO2(g)⇌N2O4(g)ΔH=−58 kJ mol−12\,\text{NO}_2\text{(g)} \rightleftharpoons \text{N}_2\text{O}_4\text{(g)} \quad \Delta H = -58 \text{ kJ mol}^{-1}

Different amounts of NO2_2 are injected and, after equilibrium is reached at constant temperature, concentrations are recorded:

Injection12345
[NO2]eq[\text{NO}_2]_{eq} (M)0.100.200.350.550.80
[N2O4]eq[\text{N}_2\text{O}_4]_{eq} (M)0.050.100.180.280.41
a
Construct a graph of [N2O4]eq[\text{N}_2\text{O}_4]_{eq} (y-axis) against [NO2]eq[\text{NO}_2]_{eq} (x-axis) and interpret the relationship shown. [2]
b
Deduce the equilibrium concentration of N2_2O4_4 when [NO2]eq=1.00[\text{NO}_2]_{eq} = 1.00 M. Show your method clearly. [2]
c
Analyse why [N2O4]eq[\text{N}_2\text{O}_4]_{eq} does not increase in direct proportion to [NO2]eq[\text{NO}_2]_{eq}, using both the equilibrium expression for this reaction and collision theory. [3]
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