The position of equilibrium

3 marks · Normal

Hydrogen iodide is formed in the reversible reaction: The forward reaction is endothermic. A closed container starts with 0.50 mol of hydrogen, 0.50 mol of iodine, and no hydrogen iodide. The mixture is allowed to reach equilibrium. State and explain what will happen to the amount of hydrogen iodide present at equilibrium if the pressure is doubled (assume temperature is kept constant and all substances are gases, with equal volumes occupied).

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Also accept: Doubling the pressure has no effect on the position of equilibrium or the amount of HI present, because there are equal moles of gas on each side of the equation.; The amount of hydrogen iodide at equilibrium stays the same because the total number of gas molecules on both sides is the same.

Common mistakes: Saying yield increases or decreases with pressure here.; Ignoring the number of gaseous moles on each side.

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Marking points

M1 · 1 mark · States that there is no effect on the position of equilibrium (or amount of HI at equilibrium) when pressure is changed.

M2 · 1 mark · Refers to the number of moles of gas (or molecules) being equal on both sides of the equation.

A1 · 1 mark · Gives a clear explanation linking no effect to equal moles/volumes of gas on each side.

Worked solution

Increasing the pressure will not affect the position of equilibrium because there are 2 moles of gas on both sides: H2 + I2 (1 + 1 = 2) and 2HI (2). Since pressure changes only influence equilibrium when the number of gas molecules differs on each side, the amount of HI formed will remain unchanged.