A general reversible reaction reaches dynamic equilibrium in a sealed container at constant temperature: P(g) + Q(g) R(g) + S(g) Which statement about the equilibrium mixture is correct?

Answer: C. Step 1: Recall what "dynamic equilibrium" means For a reversible reaction in a closed system, dynamic equilibrium is reached when: 1. The rate of the forward reaction equals the rate of the reverse reaction. 2. The concentrations of reacta…

IGCSE Chemistry 0620: Characteristics of Dynamic Equilibrium

Step 1: Recall what “dynamic equilibrium” means

For a reversible reaction in a closed system, dynamic equilibrium is reached when:

The rate of the forward reaction equals the rate of the reverse reaction.
The concentrations of reactants and products are no longer changing.

Crucially, both reactions are still happening, they have not stopped, they are just occurring at the same rate as each other, so there is no further overall (net) change.

Step 2: Check each option

A: If all of P and Q had been converted into R and S, the reaction would have gone to completion, not reached an equilibrium, and the reverse reaction (R and S turning back into P and Q) could not then be occurring. This does not match a reversible reaction at equilibrium.
B: The reactions have not stopped. Both the forward and reverse reactions are still occurring continuously. It is only the net change that is zero, because the two rates cancel out.
C: This matches the definition exactly, at dynamic equilibrium, the forward and reverse reactions occur at equal rates.
D: Being at equilibrium does not require the concentrations of P, Q, R and S to be equal to each other. It only requires that these concentrations have stopped changing. They can be constant at very different values from one another.

Step 3: Select the correct statement

Only option C correctly describes a reversible reaction at dynamic equilibrium.

Final answer

The rate of the forward reaction is equal to the rate of the reverse reaction, $\boxed{\text{option C}}$ .