Equilibrium Constants (Kc and Kp) - SS2 Chemistry Lesson Note
Equilibrium constants, denoted as Kc and Kp, are quantitative measures that describe the composition of a system at equilibrium. Kc represents the equilibrium constant in terms of concentrations (usually in moles per litre), while Kp represents the equilibrium constant in terms of partial pressures (usually in atmospheres). The equilibrium constant expresses the ratio of the concentrations or partial pressures of products to reactants, each raised to the power of their stoichiometric coefficients. For a generic chemical equation:
aA + bB ⇌ cC + dD,
the equilibrium constant is defined as:
Kc = ([C]c x [D]d) / ([A]a x [B]b), and
Kp = (PCc x PDd) / (PAa x PBb).
The value of the equilibrium constant, K, is specific to a particular reaction at a given temperature. It remains constant as long as the temperature is maintained.
The magnitude of the equilibrium constant indicates the extent of the reaction. If K > 1, the equilibrium lies towards the products, while if K < 1, the equilibrium lies towards the reactants. If K ≈ 1, the reactants and products are present in roughly equal amounts at equilibrium. The numerical value of the equilibrium constant does not depend on the initial concentrations or pressures of the reactants and products. It is solely determined by the stoichiometry of the balanced equation.
The equilibrium constant can be used to calculate the concentrations or pressures of reactants and products at equilibrium if the initial conditions are known. Equilibrium constants can be manipulated using mathematical operations. For example, if a reaction is multiplied by a factor, the equilibrium constant is raised to the power of that factor. It's important to note that equilibrium constants are temperature-dependent. As the temperature changes, the equilibrium constant may also change. This is reflected in the Arrhenius equation for K, which involves the temperature and the reaction's enthalpy (ΔH) and entropy (ΔS) values.
Understanding equilibrium constants allows for quantitative analysis of chemical systems at equilibrium. They provide insights into the relative concentrations or partial pressures of reactants and products and can be used to predict the direction and extent of a reaction under given conditions.