Pressurefavor

Pressurefavor is a term used in chemical thermodynamics to describe how sensitive a chemical equilibrium is to changes in pressure, particularly for gaseous reactions. In this context, pressurefavor is quantified by the derivative of the natural logarithm of the equilibrium constant with respect to pressure at constant temperature: PF = d(ln K)/dP|T.

Under standard thermodynamic relations for gas-phase equilibria, PF = -ΔV°/(R T), where ΔV° is the standard molar volume change of the reaction, R the gas constant, and T the absolute temperature. This means the sign and magnitude of PF indicate how responsive the equilibrium is to pressure: a negative ΔV° gives PF > 0, so increasing pressure favors the products; a positive ΔV° gives PF < 0, favoring the reactants. The larger the magnitude of ΔV°, the more the equilibrium shifts with pressure.

Application and scope: PF is most meaningful for gas-phase reactions under near-ideal conditions. In liquids or solids, or at high pressures where non-ideal behavior is significant, the simple relation may break down and the concept becomes less precise.

History and usage: The term pressurefavor is not part of standard textbooks and is used primarily in discussions emphasizing how pressure affects equilibrium constants. It provides a compact way to summarize the pressure dependence of K in a single parameter.

Example: For a reaction with ΔV° = -20 cm^3/mol at 298 K, PF = -(-20e-6 m^3/mol)/(8.314 J/mol K × 298 K) ≈ 8×10^-9 Pa^-1. A 10 MPa increase would raise K by about 0.08 (8%).