Changing the pressure

2 marks · Normal

State why changing the pressure affects only equilibria involving gases, and only when the numbers of gaseous molecules on each side of the equation are different.

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Changing pressure only affects equilibria involving gases because solids and liquids are not compressible. It only matters when there is a different number of gaseous molecules on each side of the equation, as this changes the total number of particles affected by pressure.

Also accept: Pressure only affects gas equilibria.; Solids and liquids are not affected by changes in pressure.; Affects only if different numbers of gas molecules are present on each side.

Common mistakes: Saying all equilibria are affected; Ignoring the comparison of numbers of gas molecules

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Marking points

B1 · 1 mark · States that only equilibria involving gases are affected because solids and liquids do not significantly change volume/are not compressible.

B2 · 1 mark · Mentions that there must be a different number of gaseous molecules on each side for a pressure change to affect equilibrium.

Worked solution

Changing the pressure affects only equilibria involving gases because solids and liquids do not change volume when pressure is changed.

This only affects equilibria if there are different numbers of gaseous molecules on each side of the equation, since pressure affects the relative amounts of gaseous molecules.