Under which condition does a reaction behave as pseudo-first-order?

• A. When temperature is doubled.
• B. When solvent is pure.
• C. When one reactant is in large excess, the reaction appears first-order in the limiting reactant; rate = k′[A].
• D. When the reaction is catalytic.

Answer: (C)

A reaction behaves pseudo-first-order when the concentration of one reactant stays effectively constant during the reaction. If a bimolecular reaction is rate = k[A][B] and one reactant is in large excess, say [B] ≈ [B]0, then the rate becomes rate ≈ k[B]0[A] = k′[A]. This makes the reaction appear first-order with respect to the limiting reactant, even though the overall process is second-order. The other scenarios don’t produce this appearance: changing temperature alters the rate constant but not the constancy of a reactant’s concentration; using pure solvent doesn’t impose a large-excess condition; catalysis changes the mechanism and rate law in a different way, not simply yielding a pseudo-first-order form.