Reversibility and dynamic equilibria
5 marks · HardA reversible reaction between gases takes place in a sealed container.
State two characteristics of the system when it has reached dynamic equilibrium.
Explain what would happen to the amount of if the pressure on the system is increased, and relate your answer to the number of moles of gas on each side of the equation.
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) — · must show rates equal, concentrations constant
(b) — · must show position shifts to side with fewer gas moles, this is N2O4 (1 mole) vs 2NO2 (2 moles), amount of N2O4 increases
Must show: equal rates, constant concentrations, position of equilibrium shifts, fewer moles of gas favoured, amount of N2O4 increases
Also accept: forward and backward reactions continue at the same rate, amount of N2O4 increases when pressure increases due to fewer gas moles on the right
Common mistakes: saying equilibrium stops; ignoring effect of moles; stating concentrations change but not explaining direction
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that the rate of the forward reaction equals the rate of the backward reaction.
B2 (a) · 1 mark · States that concentrations of reactants and products remain constant.
M1 (b) · 1 mark · Recognises that an increase in pressure shifts the equilibrium to the side with fewer moles of gas.
A1 (b) · 1 mark · Identifies that is formed from fewer moles (1 mol on right, 2 mol on left).
A2 (b) · 1 mark · States that the amount of increases.
Worked solution
a) At dynamic equilibrium, (1) the rate of the forward reaction equals the rate of the backward reaction. (2) The concentrations of and remain constant.
b) Increasing the pressure favours the side with fewer moles of gas. On the left, there are 2 moles of , while on the right, there is 1 mole of . Therefore, the position of equilibrium shifts to the right, and the amount of increases.