IB Chemistry SL Reactivity 2.3 Extent of Chemical Change

Practise writing equilibrium expressions, interpreting K and predicting concentration, pressure or temperature effects on composition and equilibrium position.

Syllabus
First assessment 2025
Topic
2.3
Level
SL

Exam points

  • recognise dynamic equilibrium from equal forward and reverse rates with constant macroscopic properties
  • write K from a balanced equation and interpret its magnitude, temperature dependence or reverse value
  • predict concentration, pressure or temperature effects on composition and K using Le Châtelier's principle

Covered syllabus points

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

2.3 Extent of chemical change question 1

[Maximum number: 3]

Question (a)

(a)

Describe two characteristics of a reaction at equilibrium.

[ 2 ]

Question (b)

(b)

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 ]

2.3 Extent of chemical change 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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