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IB Chemistry SL 2.3 Extent of Chemical Change Questions

Practise IB Chemistry SL 2.3 by writing Kc expressions, interpreting equilibrium positions and predicting shifts when reaction conditions change.

Syllabus
First assessment 2025
Course
Chemistry SL
Level
SL

Exam points

  • Describe dynamic equilibrium in a closed system by equal forward and reverse reaction rates.
  • Write Kc expressions with stoichiometric powers; calculate Kc for reversed or scaled reactions.
  • Interpret Kc magnitude and reciprocal values to judge whether reactants or products predominate.
  • Predict concentration or pressure shifts and explain how equilibrium composition changes.
  • Predict temperature effects on equilibrium position and Kc from the reaction enthalpy direction.

Covered syllabus points

2.3.1—Dynamic equilibrium2.3.2—Equilibrium law2.3.3—Equilibrium constant (K)2.3.4—Le Châtelier's principle

Question 1

[Maximum number: 5]

Question (a)

(a)

Describe two characteristics of a reaction at equilibrium.

[ 2 ]

Question (b)

(b)

State and explain the effect of a catalyst on the position of equilibrium.

[ 2 ]

Question (c)

(c)

Ethanoic acid reacts with ethanol to form the ester ethyl ethanoate.

CH3COOH(l)+CH3CH2OH(l)⇌H+CH3COOCH2CH3(l)+H2O(l)\mathrm{CH}_{3} \mathrm{COOH}(\mathrm{l})+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{l}) \stackrel{\mathrm{H}^{+}}{\rightleftharpoons} \mathrm{CH}_{3} \mathrm{COOCH}_{2} \mathrm{CH}_{3}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

The esterification reaction is exothermic. State the effect of increasing temperature on the value of the equilibrium constant ( KcK_{\mathrm{c}} ) for this reaction.

[ 1 ]

Question 2

[Maximum number: 2]

The reaction between ethanoic acid and ethanol is homogeneous and reversible.

C2H6O(l)+C2H4O2(l)⇌C4H8O2(l)+H2O(l)ΔHθr=−4 kJ mol−1\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}(\mathrm{l})+\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}(\mathrm{l}) \rightleftharpoons \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \quad \Delta H^{\theta}{ }_{\mathrm{r}}=-4 \mathrm{~kJ} \mathrm{~mol}^{-1}

Question (a)

(a)

Deduce the expression for the equilibrium constant, K, for this reaction.

[ 1 ]

Question (b)

(b)

Explain the effect of reducing the temperature on the value of the equilibrium constant.

[ 1 ]
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