For a gas-phase equilibrium, the constant can be written using concentrations or:

1. A gas can be described equally well by how concentrated it is or by the pressure it exerts. 2. For gas-phase solutions, the equilibrium constant may be expressed in terms of either the molar concentrations or the partial pressures of the reactants and products. 3. The concentration form is written Kc and the pressure form Kp. 4. Rates and bond energies belong to other parts of chemistry entirely. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 13 'Fundamental Equilibrium Concepts', sections 13.1-13.4_