Define a pseudo-first-order reaction and give the relation between k' and k.

Define a pseudo-first-order reaction and give the relation between k' and k.

• A. A reaction where one reactant is in large excess; rate = k' [A], with k' = k [B]0.
• B. A reaction where temperature is constant; rate = k [A] B; k' = k/B0
• C. A reaction with multiple steps; rate = k' [A]^2; k' = k [B]0
• D. A reaction where concentration of A is large; rate = k' [B], with k' = k [A]0

Answer: (A)

Pseudo-first-order kinetics happen when one reactant is in large excess, so its concentration stays essentially constant during the reaction. For a bimolecular step with rate law rate = k[A][B], if [B] can be treated as the constant [B]0, then the rate becomes rate ≈ k[B]0 [A]. Here the constant k[B]0 is an apparent, or pseudo, first-order rate constant, often written as k'. So the rate law looks like rate = k' [A], with k' = k[B]0. That’s why this description is the correct way to define a pseudo-first-order reaction. The other formulations don’t capture the same idea. If the rate law is still given as a product of two concentrations in the rate expression, it’s not a pseudo-first-order simplification. If the rate is proportional to [A]^2 or to [B] alone with no constant concentration assumed, that describes different kinetics (second order in A, or dependence on B) and doesn’t reflect the one reactant held effectively constant. The key point is treating one reactant as effectively constant so the overall kinetics reduce to first order in the other reactant, with k' incorporating the constant concentration of the excess reactant.

Pseudo-first-order kinetics happen when one reactant is in large excess, so its concentration stays essentially constant during the reaction. For a bimolecular step with rate law rate = k[A][B], if [B] can be treated as the constant [B]0, then the rate becomes rate ≈ k[B]0 [A]. Here the constant k[B]0 is an apparent, or pseudo, first-order rate constant, often written as k'. So the rate law looks like rate = k' [A], with k' = k[B]0. That’s why this description is the correct way to define a pseudo-first-order reaction.

The other formulations don’t capture the same idea. If the rate law is still given as a product of two concentrations in the rate expression, it’s not a pseudo-first-order simplification. If the rate is proportional to [A]^2 or to [B] alone with no constant concentration assumed, that describes different kinetics (second order in A, or dependence on B) and doesn’t reflect the one reactant held effectively constant. The key point is treating one reactant as effectively constant so the overall kinetics reduce to first order in the other reactant, with k' incorporating the constant concentration of the excess reactant.