Reversible Reactions and Equilibria
8 marks · HardThis question is about an industrial process involving a reversible reaction in a sealed vessel:
At dynamic equilibrium, the concentrations of all four gases remain constant. The process operator wants to maximise the production of C.
State two characteristics of a reaction mixture at dynamic equilibrium. Explain why both these characteristics occur.
The operator increases the concentration of A. Predict and explain how the position of equilibrium will change, and what effect this has on the yield of C.
If the concentration of D is continuously removed from the vessel as it forms, explain (i) how the position of equilibrium will change, and (ii) what will happen to the rate of the forward and reverse reactions immediately after removal.
Suggest one reason, based on the above equilibrium, why controlling concentrations is important in industrial chemical processes.
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(a) 1. The rate of the forward reaction equals the rate of the reverse reaction (dynamic); 2. The concentrations of reactants and products remain constant (not necessarily equal concentrations). Both occur because the opposing reactions proceed at the same rate, resulting in no net change. · must show Both characteristics stated and explained.
(b) The equilibrium will shift to the right (towards products), producing more C (and D), to oppose the increase in A; thus, the yield of C increases. · must show Correct shift prediction and clear explanation.
(c) (i) Removing D shifts the equilibrium to the right, making more C and D, to replace the D removed. (ii) The rate of the forward reaction increases temporarily as the system responds, while the reverse rate drops until equilibrium is re-established. · must show Shift described, rate change explained.
(d) Controlling concentrations allows industry to drive the equilibrium towards the desired product, increasing yield and process efficiency, or to keep costs down by minimising waste. · must show Link between concentration control and industrial benefit.
Also accept: Alternative explanations for points in a, b, c, d accepted if chemically correct and show clear understanding.
Common mistakes: Confusing constant concentration with equal concentration; Assuming reaction stops at equilibrium; Stating rate becomes zero at equilibrium; Forgetting to link yield with shifting position
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Marking points
B1 (a) · 1 mark · States one valid characteristic of dynamic equilibrium (e.g. forward and reverse reactions occur at the same rate).
B2 (a) · 1 mark · States and/or explains a second valid characteristic (e.g. concentrations remain constant), with explanation.
B3 (b) · 1 mark · Predicts the direction in which the equilibrium shifts when A increases.
B4 (b) · 1 mark · Explains the effect on the yield of C, linked to equilibrium shift.
B5 (c) · 1 mark · States how removing D affects position of equilibrium (shifts right, produces more C and D).
B6 (c) · 1 mark · Describes the temporary effect on rates (forward increases, reverse decreases).
B7 (d) · 1 mark · States a reason why controlling concentrations matters in industry (e.g. increasing yield, efficiency).
B8 (d) · 1 mark · There is clear linkage to the provided equilibrium (explanation fits context).
Worked solution
a) At dynamic equilibrium: (1) The rates of the forward and reverse reactions are equal; (2) The concentrations of reactants and products remain constant. This happens because, in a closed system, as reactants change into products and vice versa at the same rate, the overall amounts do not change.
b) Increasing A causes the equilibrium to shift right (towards C and D), to remove the added A. As more C is produced, the yield of C increases.
c) (i) Removing D causes the equilibrium to shift right to replace D, producing more C and D. (ii) The forward rate increases momentarily because there is less D to react in the reverse reaction, while the reverse rate decreases. Over time, equilibrium is re-established.
d) By controlling concentrations, industries can shift equilibrium to favour product formation, increasing yield and maximising efficiency, or reducing undesired by-products.