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Chemistry 12 1 Chemical Equilibrium II Figure 1: The net change in reactant and product concentrations is zero at equilibrium for the reaction: Figure 2: Dynamic Equilibrium: The rates of the forward and the reverse reactions are equal. Even though the net changes in concentrations are zero, the rates are not zero. The Equilibrium Expression This is a mathematical expression that relates the concentrations of reactants and products as a function of the equilibrium. This was first proposed as the Law of Mass Action by the Norwegian chemists Cato Guldberg (1836-1902) and Peter Waage (1833-1900). Consider the following general equilibrium equation: aA + bB cC + dD When a reaction is at equilibrium, the rates of these two reactions are identical, thus no ____ (macroscopic) change is observed. However, individual components are actively being transformed at the microscopic level. Guldberg and Waage showed that the rate of the reaction in either direction is proportional to what they called the “active masses” of the various components: Rate of forward reaction = ________________ Rate of reverse reaction = ________________ Chemistry 12 2 At equilibrium: Rearranging the above equation: Where ____ is known as the ___________ ___________ Note that the equilibrium expression can be expressed by concentrations in terms of _________ for aqueous solutions or _________________ for gases (although for the purposes of Chemistry 12, we will not be using partial pressures) Some rules to follow when writing equilibrium expressions: “____________” concentrations in equilibrium expressions: Substances whose concentrations undergo no significant change in a chemical reaction do not appear in equilibrium constant expressions. Two general cases: i) when the substance is also the _______ – eg. dissociation of acetic acid (_______ acts as a solvent): CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq) ii) when the substance is a _______ or a ________ phase Eg 1. in solubility equilibria: CaF(s) Ca2+(aq) + 2F–(aq) Eg 2. in heterogeneous reactions: Fe3O4(s) + 4H2(g) 4H2O(g) + 3Fe(s) Chemistry 12 3 The Significance of Keq Equilibrium constants can be very large or very small. The magnitude of Keq provides important information about the composition of an equilibrium mixture. Consider the reaction of carbon monoxide and chlorine gas at 100C – a toxic gas called phosgene (COCl2) is formed and used for manufacturing certain polymers and insecticides: CO(g) + Cl2(g) COCl2(g) Keq = [COCl2] = 1.49 x 108 [CO][Cl2] In order for Keq to be so large, the numerator of the equilibrium expression (product concentrations) must be much _______ than the denominator (reactant concentrations). Thus, an equilibrium mixture of all three gases is primarily pure _____. We say that this equilibrium ____ ____________________. Likewise, a very ______ Keq value signifies an equilibrium mixture containing mostly ________ (equilibrium lies to the _____ or ________ side). (diagram taken from Chemistry: The Central Science, 9th edition pg. 582)