The position of equilibrium
2 marks · HardFor the reversible reaction in a sealed container: \[ \ce{C(g) <=> D(g) + E(g)} \] Give one change that would cause the position of equilibrium to shift to the right. Explain your answer.
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An increase in temperature would shift the equilibrium to the right if the forward reaction is endothermic, or an increase in pressure would shift it to the right if there are more moles of gas on the right. Accept either, with correct reasoning.
Must show: States a valid condition (increase temp if forward is endothermic, or increase pressure if more moles right), Explains in terms of endothermic reaction or more moles
Also accept: Decrease pressure if fewer moles on right; Remove C(g) from the system (with brief explanation)
Common mistakes: Saying concentrations become equal; Ignoring the direction of enthalpy change
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Marking points
B1 · 1 mark · Identifies one change that could shift equilibrium right (increase temperature if forward is endothermic, or increase pressure if more moles gas on right).
B2 · 1 mark · Explains effect in correct terms (e.g. favours endothermic direction or side with more moles of gas).
Worked solution
If the forward reaction is endothermic, increasing temperature shifts equilibrium to the right because it favours the endothermic direction.
Alternatively, because 1 mole of C(g) forms 2 moles on the right, increasing pressure would favour the side with fewer moles (so here, decreasing pressure favours right).