The entropy of activation, Δ"S"‡, gives the extent to which transition state (including any solvent molecules involved in or perturbed by the reaction) is more disordered compared to the starting materials. It offers a concrete interpretation of the pre-exponential factor "A" in the Arrhenius equation; for a unimolecular, single-step process, the rough equivalence "A" = ("k"B"T"/"h") exp(1 + Δ"S"‡/"R") (or "A" = ("k"B"T"/"h") exp(2 + Δ"S"‡/"R") for bimolecular gas-phase reactions) holds. For a unimolecular process, a negative value indicates a more ordered, rigid transition state than the ground state, while a positive value reflects a transition state with looser bonds and/or greater conformational freedom. It is important to note that, for reasons of dimensionality, reactions that are bimolecular or higher have Δ"S"‡ values that depend on the standard state chosen (standard concentration, in particular). For most recent publications, 1 mol L–1 or 1 molar is chosen. Since this choice is a human construct, based on our definitions of units for molar quantity and volume, the magnitude and sign of Δ"S"‡ for a single reaction is meaningless by itself; only comparisons of the value with that of a reference reaction of "known" (or assumed) mechanism, made at the same standard state, is valid.