Reversible Reactions and Equilibria
6 marks · NormalThis question is about how both the position of equilibrium and changes in pressure affect reversible reactions involving gases.
For the reaction below in a sealed vessel, is dynamic equilibrium reached? Explain your reasoning.
State why changing the pressure only affects the position of equilibrium in reactions with different numbers of gaseous molecules on each side of the equation.
If the pressure is increased in the vessel containing the equilibrium mixture above, state and explain which direction (left or right) the position of equilibrium will shift.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Yes, because all reactants and products are gases in a sealed container so the reaction can proceed in both directions at equal rates and concentrations become constant. · must show sealed/closed vessel, equal rates of forward and reverse, concentrations remain constant
(b) Pressure only changes the amounts of gases, not solids or liquids; only reactions with unequal numbers of gaseous molecules on each side are affected, because the shift reduces the total number of gas molecules when pressure increases. · must show only gases affected, difference in number of gas molecules
(c) The equilibrium will shift to the left (towards PCl5), because there are fewer gas molecules on the left (1) than on the right (2) and increasing pressure favours the side with fewer gas molecules. · must show shift to left, explanation about fewer gas molecules on one side
Common mistakes: Thinking equilibrium means equal amounts of all substances; Stating pressure always affects equilibrium
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Marking points
B1 (a) · 1 mark · States equilibrium is reached in a sealed vessel
B2 (a) · 1 mark · Gives correct reasoning – both reactions happen and concentrations become constant at equal rates
B3 (b) · 1 mark · States only gases affected by pressure
B4 (b) · 1 mark · Explains that only reactions with different numbers of gaseous molecules are affected
B5 (c) · 1 mark · Correctly predicts shift to the left (towards PCl5)
B6 (c) · 1 mark · Explains shift is due to fewer gaseous molecules on that side
Worked solution
a) Yes, dynamic equilibrium will be reached because all substances are gases in a sealed vessel, so both the forward and reverse reactions happen at the same rate, and the concentrations of , , and become constant.
b) Pressure only directly changes the amount of gaseous molecules; solids and liquids are unaffected because their particles are already tightly packed. Reactions are only affected if there is a different number of gas molecules on each side, as changing pressure changes how many molecules fit in the space.
c) Increasing the pressure shifts the equilibrium to the left (towards ). This is because there is 1 molecule of gas on the left and 2 on the right; an increase in pressure favours the side with fewer gaseous molecules to reduce pressure.