Q.Consider the reaction where $K_P = 0.5$ at a particular temperature

[!FORMULA] $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$

if the three gases are mixed in a container so that the partial pressure of each gas is initially 1 atm, then which one of the following is true

By definition, at equilibrium the reaction quotient Q equals the equilibrium constant K (Q = K), not less than it — so the statement claiming Q < K at equilibrium is false, making it the incorrect statement among the choices.

Check each statement:

1. A catalyst lowers the activation energy for both the forward and reverse reactions by the same amount, so it speeds up both equally without shifting the equilibrium position — this statement is CORRECT.
2. The reaction quotient Q is calculated the same way as K, using the concentrations present at any given moment. When the system reaches equilibrium, by definition Q becomes equal to K (Q = K). Saying Q is 'always less than' K at equilibrium is FALSE — this is the incorrect statement. …

Statement (b) is incorrect because at equilibrium Q always equals K, never remains less than it.

The reaction quotient Q has the same mathematical form as the equilibrium constant K, but Q can be evaluated at ANY point during a reaction using the concentrations present at that instant, whereas K uses the equilibrium concentrations only. Comparing Q to K tells us the direction a reaction will shift: if Q < K the forward reaction proceeds (to increase Q), if Q > K the reverse reaction proceeds, and the reaction reaches equilibrium exactly when Q = K. So the claim that Q is 'always less than' K at equilibrium is wrong — it should be Q = K at equilibrium.

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