Equilibrium - Equilibrium in Physical Processes

Equilibrium in physical processes refers to a state where two different phases of a substance coexist, and the rate of transfer of molecules from one phase to another is equal to the rate of the reverse process.

Dynamic Nature: Physical equilibrium is dynamic, meaning the forward and backward processes continue to occur at the same rate even though no macroscopic change is visible.

Solid-Liquid Equilibrium: This occurs at the melting point. For a pure substance at $1 \text{ atm}$, the temperature is the normal melting point. Example: $H_2O(s) \rightleftharpoons H_2O(l)$ at $273 \text{ K}$.

Liquid-Vapor Equilibrium: In a closed container, the pressure exerted by vapors in equilibrium with the liquid at a constant temperature is the equilibrium vapor pressure. Example: $H_2O(l) \rightleftharpoons H_2O(g)$.

Solid-Vapor Equilibrium: This involves sublimation, where a solid directly converts to vapor. Example: $I_2(s) \rightleftharpoons I_2(g)$. The intensity of color (for iodine) remains constant once equilibrium is reached.

Equilibrium involving Dissolution of Solids in Liquids: A saturated solution represents an equilibrium between the undissolved solute and the solute in the solution: $Sugar(s) \rightleftharpoons Sugar(aq)$.

Equilibrium involving Dissolution of Gases in Liquids: Governed by Henry's Law, which states that the solubility of a gas in a liquid at a given temperature is directly proportional to the pressure of the gas above the liquid.

General Characteristics: Equilibrium is possible only in a closed system at constant temperature. Measurable properties like pressure, concentration, and density remain constant at equilibrium.