In Section 11.5, we defined the vapor pressure of a liquid in terms of an equilibrium. (a) Write the equation representing the equilibrium between liquid water and water vapor and the corresponding expression for Kp.

Key Concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of particles to escape from the liquid phase into the vapor phase. The higher the vapor pressure, the more volatile the substance, indicating a greater number of molecules transitioning into the vapor state.

Equilibrium Constant (Kp)

The equilibrium constant, Kp, is a numerical value that expresses the ratio of the partial pressures of the products to the reactants at equilibrium for a given reaction. For the equilibrium between liquid water and water vapor, Kp can be expressed as the ratio of the vapor pressure of water to the pressure of the liquid phase, reflecting the balance between the two states.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time. In the context of vapor pressure, this means that the rate at which water molecules evaporate into vapor equals the rate at which vapor molecules condense back into liquid, establishing a stable vapor pressure.