Download Fundamentals of Chemical Equilibrium: Dynamic Processes and Equilibrium Constants and more Study notes Chemistry in PDF only on Docsity! Fundamental Aspects of Chemical Equilibrium 1. Chemical equilibrium is dynamic: forward and reverse reactions are always occurring, but at the same rate. 2. No macroscopic evidence of change, i.e. there is no net change in the amounts of reactants and products at equilibrium. 3. Equilibrium reached by spontaneous processes which minimize the energy of a particular chemical system. 4. A given equilibrium condition is the same regardless of the direction of approach. Chemical equilibria are numerically represented by an equilibrium constant, K. The equilibrium constant is: - temperature dependent - constant (at constant temperature), regardless of equilibrium amounts of reactants and products. Important! When a system approaches equilibrium, the [ ]'s or P's change in such a way as to conform to the value of K at that temperature. Thus, P(PCl5) + P(PCl3) + P(Cl2) = 0.84 atm Manipulating Equilibrium Expressions - For a given equilibrium system, the K for the reverse reaction equals the inverse of the K for the forward reaction. - If a balanced chemical equation is multiplied by a factor, then the equilibrium constant is raised to a power equal to that factor. - If chemical equations are added together or subtracted from each other, the equilibrium constants are multiplied or divided. Equilibrium Expression: Stoichiometric Coefficients Other Than "1" Suppose gaseous H2 and F2 are introduced into an evacuated bulb such that their initial partial pressures are both 1.00 atm. A reaction then occurs in which HF gas is produced. The reaction proceeds until equilibrium is established, and the chemical equation is H2(g) + F2(g) 2HF(g) Set up the expression for the equilibrium constant, Kp. (Note: the initial partial pressure of HF is 0!) K and Extent of Reaction (or Equilibrium Position) Consider aA + bB cC + dD K = [C]c [D]d [A]a[B]b What does the magnitude of K tell us? If K >> 1, then [C]c[D]d > [A]a [B]b, therefore products are favored, and equilibrium is said to "lie to the right." e.g. H2(g) + F2(g) 2 HF(g) K = 1.15 x 102 The reaction mixture is predominantly HF (product) If K << 1, then [C]c[D]d < [A]a [B]b, therefore reactants are favored, and equilibrium is said to "lie to the left." e.g. 2 NOCl(g) 2 NO(g) + Cl2(g) K = 1.6 x 10-5 The reaction mixture is predominantly NOCl (reactant) Q and the Direction of Change - follows the law of mass action, except pressures and concentrations are non-equilibrium values. - Q is not a constant. It changes with time as a reaction system approaches equilibrium. - When Q = K, the reaction system has reached equilibrium. - the comparison of Q to K tells you whether the reaction system will proceed towards products or reactants to reach equilibrium. 1. When Q < K, too few products compared to the equilibrium condition. Thus the net direction of change will be towards products to reach equilibrium. 2. When Q > K, too much products compared to the equilibrium condition. Thus the net direction of change will be towards reactants to reach equilibrium.
