What is a pseudo-first-order reaction?

A pseudo-first-order reaction looks first order because one reactant is present in large, effectively constant excess. For a rate law rate = k[A]^α[B]^β, if [B] ≫ [A] and doesn’t change much, the observed rate becomes rate = k_obs[A]^α with k_obs = k[B]^β. This trick simplifies analysis and is common in solution kinetics and biochemistry (e.g., hydrolyses run with water or buffer in vast excess). Actionable tip: to determine α cleanly, hold [B] and other conditions constant, vary [A], and fit a straight line to ln[A] vs t; then repeat with different [B] to back-out β from how k_obs changes.

The concept reflects IUPAC’s treatment of observed rate coefficients and partial orders: the “order” you deduce depends on which concentrations actually change during your experiment. Document the fixed-excess component and report both the observed k and the underlying overall rate law to avoid ambiguity.