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CAIE A-Level Chemistry 7.1.4 Equilibrium Constant Kc Expressions

Practise constructing Kc expressions from balanced equations using equilibrium concentrations and correct powers.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • place product concentrations over reactant concentrations and use stoichiometric coefficients as powers
  • omit pure solids and liquids from Kc while retaining aqueous and gaseous species in the expression
  • derive the units from the net concentration powers instead of assuming Kc is dimensionless

7.1.4—Expressions for equilibrium constants question 1

[Maximum number: 1]

The elements sodium to chlorine, in the third period, all form oxides.

2.00 moles of SO2( g)\mathrm{SO}_{2}(\mathrm{~g}) and 2.00 moles of O2( g)\mathrm{O}_{2}(\mathrm{~g}) are sealed in a container with a suitable catalyst, at constant temperature and pressure. The resulting equilibrium mixture contains 1.98 moles of SO3( g)\mathrm{SO}_{3}(\mathrm{~g}).
The total volume of the equilibrium mixture is 40.0dm340.0 \mathrm{dm}^{3}.

2SO2( g)+O2( g)⇌2SO3( g)2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g})

Write the expression for the equilibrium constant, KcK_{\mathrm{c}}, for the reaction between SO2( g)\mathrm{SO}_{2}(\mathrm{~g}) and O2( g)\mathrm{O}_{2}(\mathrm{~g}) to produce SO3( g)\mathrm{SO}_{3}(\mathrm{~g}).

Kc=K_{c}=
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