The Equilibrium Law

For a reversible reaction at equilibrium at a fixed temperature, the concentrations of reactants and products are related by the equilibrium law. For the general reactionaA + bB ⇌ cC + dDthe equilibrium constant, K_c, isK_c = [C]^c[D]^d / [A]^a[B]^bSquare brackets mean the equilibrium concentration in mol dm⁻³. Products go on the top, reactants on the bottom, and each concentration is raised to the power of its balancing number in the equation.K_c has a fixed value for a given reaction at a given temperature. It is only changed by changing the temperature.Units. Substitute mol dm⁻³ for each concentration and cancel. For H₂(g) + I₂(g) ⇌ 2HI(g):K_c = [HI]² / [H₂][I₂]The units are (mol dm⁻³)² / (mol dm⁻³)², which cancel, so K_c has no units here. For N₂O₄(g) ⇌ 2NO₂(g), K_c = [NO₂]² / [N₂O₄], and the units are mol dm⁻³.Size of K_c. A large K_c (much greater than 1) means the equilibrium lies well to the right: the mixture is mostly products. A small K_c (much less than 1) means it lies to the left, with mostly reactants.The expression depends on how the equation is written. For the reverse reaction, the expression is inverted, so its K_c is 1/K_c.