OBJECTIVES (1.) To gain experience with the important method of visible spectrophotometry; and (2.) use this technique to determine the equilibrium constant for the formation of the complex FeSSA(aq) from Fe³(aq) and H₂SSA (aq). THEORY All chemical reactions move spontaneously towards an equilibrium state when the reactants are mixed in arbitrary concentrations. The equilibrium state for a given temperature and pressure is reached when the concentrations of reactants and products are no longer changing. However, when the system approaches and then attains the equilibrium state, there are two opposite processes taking place: the reactant molecules are reacting to form products, and the product molecules are reacting in the reverse direction to form reactant molecules. At the final equilibrium state, the forward and reverse reactions are taking place at the same rate, so that there is no net change in the concentrations of the products and reactants. Mathematically, when the system reaches equilibrium, the product of the molarities of the products raised to their stoichiometric coefficients divided by the product of the molarities of the reactants raised to their stoichiometric coefficients is equal to a constant, called the equilibrium constant, Keq, SPECTROPHOTOMETRY As a general example, consider the reaction: aA(aq) + bB(aq) cC(aq) +dD(aq) e- = where A (aq) and B(aq) are reactants, C(aq) and D(aq) are product species, and a,b,c, and d are their respective stoichiometric coefficients in the balanced equation. Away from equilibrium, the reaction quotient Q is defined as: [C] [D] [A] [B] (1) where the brackets represent the molarity of a species. As the system approaches equilibrium, Q→Keq, and at equilibrium Q=Keq, [C] [D] [A] [B] (2) where the subscripts eq signify the final equilibrium molarities of each species. Keq has a fixed value for any given temperature and pressure, but is independent of the molarity of the species present at equilibrium.