Reversible Reactions and Equilibria
8 marks · HardAn experiment is set up in a sealed container for the reaction below, which can reach dynamic equilibrium:
The concentrations of all substances are measured over time.
Describe the conditions required for the system to reach dynamic equilibrium. Explain why these conditions are necessary.
The concentration of EF increases while E2 and F2 decrease for some time, then all concentrations level off. Explain what this shows about the rate of the forward and reverse reactions at this point.
If more F2 is added to the system at equilibrium, predict and explain what happens to the concentrations of E2, F2, and EF before the system settles at the new equilibrium.
Explain, in terms of equilibrium, why increasing the concentration of F2 can help industries to increase the yield of EF in this reaction.
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.
Show answer
(a) Dynamic equilibrium requires a closed system with no substances being added or removed, and the reaction mixture must be at constant temperature and pressure if appropriate. These are necessary so reactants and products cannot escape and forward and reverse reactions can balance. · must show Conditions listed and explained clearly.
(b) When concentrations level off, it means the rate of the forward reaction (E2 + F2 → 2EF) equals the rate of the reverse reaction (2EF → E2 + F2). No net change occurs in concentrations, though both reactions continue. · must show Relationship between rates explained when concentrations constant.
(c) Adding more F2 increases its concentration, so the system shifts right (towards more EF), so E2 decreases and EF increases until a new equilibrium is reached. F2 itself will partially decrease as it reacts to form more EF. · must show Predicted changes for all three substances.
(d) Increasing F2 pushes the equilibrium towards formation of more EF, increasing EF yield. This helps industries make more product without needing to add more E2. · must show Industrial advantage explained with reference to equilibrium shift.
Also accept: Well-reasoned alternatives accepted for industrial or equilibrium explanations.
Common mistakes: Saying 'no reaction' at equilibrium, rather than 'rates are equal'.; Assuming concentrations must be equal at equilibrium.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that a closed system and constant conditions are necessary for dynamic equilibrium.
M2 (a) · 1 mark · Explains why these conditions allow equilibrium (no loss of reactants/products, balance of rates).
M3 (b) · 1 mark · States that forward and reverse reaction rates are equal at equilibrium.
M4 (b) · 1 mark · Notes that concentrations remain constant, but both reactions continue.
M5 (c) · 1 mark · Predicts that increasing F2 will increase EF and decrease E2 as equilibrium shifts right.
M6 (c) · 1 mark · Correctly describes the changes in all three substances, including temporary rise in F2 followed by partial decrease.
M7 (d) · 1 mark · States that increasing F2 increases EF yield industrially via equilibrium shift.
M8 (d) · 1 mark · Explains clearly why yield increases (more product forms through equilibrium movement).
Worked solution
a) Dynamic equilibrium requires a closed system (nothing enters or leaves) and constant conditions (temperature and pressure). This ensures that both forward and reverse reactions can occur and reach a balance without loss of substances.
b) When concentrations of all substances become constant, the rate at which EF is produced equals the rate at which it is broken down. Both reactions continue, but there is no net change in amounts.
c) Adding more F2 causes the system to shift right to use up extra F2, so EF increases and E2 decreases until equilibrium is restored.
d) By increasing F2, industries can shift equilibrium towards more EF, maximising the amount of desired product made efficiently.