Which statement correctly distinguishes true first-order from pseudo-first-order?

Which statement correctly distinguishes true first-order from pseudo-first-order?

• A. True first-order depends on [A] and [B], pseudo-first-order depends only on [A]
• B. True first-order depends on [A] only; pseudo-first-order occurs when another reactant is effectively constant
• C. True first-order is when rate = k[A]^2, pseudo-first-order when rate = k'[A][B]
• D. True first-order occurs in a bimolecular step; pseudo-first-order occurs in a unimolecular step

Answer: (B)

The key idea is how the rate law depends on reactant concentrations. A true first-order reaction has a rate that depends on the concentration of one reactant only: rate = k[A]. Pseudo-first-order behavior happens when another reactant is in such large excess that its concentration stays effectively constant during the reaction, so the rate looks like rate = (k[B]) [A] = k' [A], where k' = k[B] is a constant. So the statement that true first-order depends on [A] only, while pseudo-first-order occurs when another reactant is effectively constant, is the correct distinction. The other choices mix up the dependence on concentrations (for true first-order it isn’t [A] and [B] together), misstate the rate form (true first-order is not rate ∝ [A]^2), or misattribute the distinction to mechanistic steps (bimolecular vs unimolecular) instead of how the rate law depends on concentrations.

The key idea is how the rate law depends on reactant concentrations. A true first-order reaction has a rate that depends on the concentration of one reactant only: rate = k[A]. Pseudo-first-order behavior happens when another reactant is in such large excess that its concentration stays effectively constant during the reaction, so the rate looks like rate = (k[B]) [A] = k' [A], where k' = k[B] is a constant.

So the statement that true first-order depends on [A] only, while pseudo-first-order occurs when another reactant is effectively constant, is the correct distinction. The other choices mix up the dependence on concentrations (for true first-order it isn’t [A] and [B] together), misstate the rate form (true first-order is not rate ∝ [A]^2), or misattribute the distinction to mechanistic steps (bimolecular vs unimolecular) instead of how the rate law depends on concentrations.