Pseudofirstorder

Pseudofirstorder, often referred to as pseudo-first-order kinetics, describes a situation in chemical kinetics where a reaction that is second-order overall behaves as first-order with respect to one reactant because the other reactant is present in large excess and remains effectively constant during the reaction. If the general rate law is rate = k[A]^m[B]^n and [B] is in large excess, then [B] ≈ [B]0 and rate ≈ k'[A]^m with k' = k[B]^n. When m = 1, the system is described as pseudo-first-order: rate ≈ k'[A].

Under pseudo-first-order conditions, the concentration of the limiting reactant A decays approximately according to [A](t) = [A]0 exp(-k't) if m = 1. The integrated form ln([A]0/[A](t)) = k't is used to determine k' from experimental data. The true overall rate constant k and the exponents may be recovered only by more complete modeling if [B] changes significantly during the reaction.

Applications include reactions where one reagent is used in large excess, such as hydrolysis or solvolysis with water, or enzyme-catalyzed processes when substrate is maintained at low concentration relative to the enzyme. Pseudo-first-order analysis simplifies data interpretation because a single exponential decay is expected for the limiting species.

Limitations of the approach rely on the assumption that the excess reactant remains effectively constant; as the reaction proceeds and the excess partner is consumed, the apparent k' may change and the simple first-order form may no longer apply. It is distinct from genuine first-order kinetics, where the rate law is inherently first-order in the reactive species without assuming a constant partner.

See also: pseudo-second-order kinetics.

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