7.1.6—Expressions for equilibrium constants

For gaseous reactions, Kp is written like Kc but uses equilibrium partial pressures. Coefficients in the balanced equation become powers.

Omit solids and liquids, and use a consistent pressure convention. The numerical value of Kp depends on the chosen pressure units unless the standard-state convention is fixed.

For N₂(g)+3H₂(g)⇌2NH₃(g), Kp = p(NH₃)²/(p(N₂)p(H₂)³).

Do not insert total pressure for every species. Each p value is that gas’s equilibrium partial pressure, and the reaction equation controls the powers.