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 .
• 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.
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
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: 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+