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Chapter 6 Chemical Equilibrium Dr. E. Binamira-Soriaga Department of Chemistry Texas A&M University The Equilibrium Constant For the general reversible reaction: a A + b B c C + d D reactants products The equilibrium constant K at a given temperature is written as: K = C c D d A a B b equilibrium concentrations Homogeneous vs Heterogeneous Equilibria Homogeneous: single-phase reaction 2H 2 S(g) + 3O 2 (g) 2H 2 O(g) + 2SO 2 (g) Heterogeneous: multi-phase reaction 2HgO(s) 2Hg(l) + O 2 (g) K = O 2 K = H 2 O 2 S O 2 2 H 2 S 2 O 2 3 Properties of K Dimensionless quantity Reaction-specific Constant for a given temperature Independent of initial concentrations Measured experimentally The Magnitude of K A measure of the extent of reaction Dependent on nature of reaction and temperature Consider: N 2 (g) + 3H 2 (g) 2NH 3 (g) K >> 1: [Products] >> [Reactants] K << 1: [Products] << [Reactants] K = 1: [Products] ~ [Reactants] K = N H 3 2 N 2 H 2 3 = 3.6 x 10 8 The value of K depends on the form of the balanced equation for the reaction If the reaction is reversed, the new K is the reciprocal of the original K : Variations in K K = HCl 2 Br 2 HBr 2 Cl 2 = 1 x 10 4 K = HBr 2 Cl 2 HCl 2 Br 2 = 2.5 x 10-5 2HCl (g) + Br 2(g) 2HBr (g) + Cl 2(g) K = 4 2HBr (g) + Cl 2(g) 2HCl (g) + Br 2(g) If the reaction is multiplied by a factor n, the new K is obtained by raising the original K to the power n: Variations in K N 2(g) + 3H 2(g) 2NH 3(g) K = 3.5 x 10 8 1/2N 2(g) + 3/2H 2(g) NH 3(g) K = (K) 1/2 = (3.5 x 10 8 ) 1/2 = 1.9 x 10 4 Variations in K N 2 + 2O 2 2NO 2 K p1 = 6.9 x 10 19 2NO N 2 + O 2 K p2 = 2.3 x 10 30 2NO + O 2 2NO 2 K p3 = K p1 x K p2 K p3 = 1.6 x 10 12 If reactions are summed up, the overall K is the product of individual K s: Example Given: H 2 O H + + OH- K w = [H + ][OH- ] = 1.0 x 10-14 at 25 o C NH 3 (aq) + H 2 O NH 4 + + OH- K NH3 = 1.8 x 10 5 Find K for the reaction: NH 4 + NH 3 (aq) + H + The Reaction Quotient For the general reaction: a A + b B c C + d D A mass action expression or reaction quotient Q can be set up at any time during a reaction: Q = C c D d A a B b not necessarily equilibrium concentrations Q = K: System at equilibrium ; no further net reaction Q < K: Forward reaction proceeds until equilibrium is established Q > K: Reverse reaction proceeds until equilibrium is established Progress of a Reaction: Q vs K K = C c D d A a B b Factors That Affect Chemical Equilibrium A system at equilibrium will remain in equilibrium until it is disturbed Disturbances (stresses applied) include changes in concentration, pressure, volume, temperature, and the addition of catalysts The system will strive to reattain a new equilibrium state Le Chateliers Principle If a stress is applied to a system at equilibrium (Q K), it will proceed in the direction that will reduce the stress until a... View Full Document

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