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CHMB16H3 (16)
Chapter 6

CHMB16 Chapter 6

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University of Toronto Scarborough
Kagan Kerman

Chapter 6: Chemical Equilibrium The Equilibrium Constant • For the reaction aA + bB ⇌ cC + dD, the equilibrium constant K = [C] [D] /[A] [B]a b • A reaction is favored whenever K > 1. • When you evaluate an equilibrium constant, o Concentrations of solutes should be expressed as moles per liter. o Concentrations of gases should be expressed in bars. o Concentrations of pure solids, pure liquids and solvents are omitted because they are unity. • If the direction of a reaction is reversed, the new value of K is simply the reciprocal of the original value of K; K = 1/K . 1 1 • If two reactions are added, the new K is the product of the two individual values; K = 3 K 1 2 • If n reactions are added, the overall equilibrium constant is the product of n individual equilibrium constants. Equilibrium and Thermodynamics • Enthalpy Change (∆H): the heat absorbed or released when the reaction takes place under constant applied pressure. o Standard Enthalpy Change (∆Hᵒ): the heat absorbed when all reactants and products are in their standard states. o ∆H is positive, heat is absorbed (endothermic). o ∆H is negative, heat is liberated (exothermic). • Entropy (S): a measure of the substance’s disorder. o ∆S is positive, products are more disordered than reactants. o ∆S is negative, products are less disordered than reactants. o In general, a gas is more disordered than a liquid, which is more disordered than a solid. o ∆Sᵒ is the change in entropy when all species are in their standard states. • Gibbs Free Energy (∆G): ∆G = ∆H - T∆S o A reaction is favored if ∆G is negative (spontaneous) or when K > 1. o When ∆H is negative and ∆S is positive, the reaction is clearly favored. o When ∆H is positive and ∆S is negative, the reaction is clearly disfavored. -∆Gᵒ/RT o Free Energy and Equilibrium: K = e , where R is the gas constant (8.314 J/K mol) and T is temperature. o The more negative the ∆Gᵒ, the larger the equilibrium constant. • Le Chatelier’s Principle: the direction in which the system proceeds back to equilibrium is such that the change is partially offset. • Reaction Quotient (Q): the same form as the equilibrium constant but it is evaluated with whatever concentrations happen to exist, even though the solution is not at equilibrium. o If Q > K, the reaction must go to the left to decrease the numerator and increase the denominator until Q = K.  If a reaction is at equilibrium and products are added, the reaction goes to the left.  If a reaction is at equilibrium and reactants are added, the reaction goes to the right. o K = e -∆Hᵒ/x e ∆Sᵒ/R  The equilibrium constant of an endothermic reaction (∆Hᵒ = +) increases if the temperature is raised.  The equilibrium constant of an exothermic reaction (∆Hᵒ = -) decreases if the temperature is raised. o Heat can be treated as if it were a reactant in an endothermic reaction and a product in an exothermic reaction. Solubility Product • Solubility Product: the equilibrium constant for the reaction in which a solid salt dissolves to give its constituent ions in solution; solid is omitted from the equilibrium constant because it is in its standard state. • For the reaction Hg C2 (2) ⇌ Hg 2+
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