CHEM 1312 Chapter 12: Chemical Equilibrium

equilibrium

dynamic state where forward and reverse reaction rates are equal

dynamic equilibrium

both forward and reverse reactions continue at the molecular level, even though concentrations appear constant

law of mass action

products over reactants, coefficients become exponents (basis for writing K expressions)

equilibrium constant (K_c)

Large K (»1)

equilibrium favors products

Small K («1)

equilibrium favors reactants

K vs. rate law exponents

K’s exponents come directly from balances equation coefficients; rate law exponents must be found experimentally

K_p

equilibrium constant expressed using partial pressures instead of concentrations

K_p/K_c relationship

Δn

moles of gas (products) - moles of gas (reactants), from balanced equation coefficients

heterogeneous equilibrium

equilibrium involving species in more than one phase

pure solids/liquids in K expressions

omitted entirely; their “concentration” (density) is constant and gets absorbed into K

effect of adding/removing solid/liquid

no effect on equilibrium position of K

reaction quotient (Q)

same expression as K, but using concentrations at any given moment (not necessarily equilibrium)

Q<K

reaction shifts forward (toward products)

Q>K

reaction shifts reverse (toward reactants)

Q=K

system is at equilibrium, no shift

ICE table

Initial, Change, Equilibrium - organizes stoichiometry-based equilibrium calculations

Change row (ICE)

uses stoichiometric coefficients as the ratio for how much each species changes (-ax, +cx, etc.)

approximation (small x)

when K is very small, [A]₀ - x ≈ [A]₀, simplifying the algebra

5% rule

validity check for the approximation

Le Châtelier's principle

a system at equilibrium shifts to partially counteract an applied disturbance

effect of adding reactant

shifts forward

effect of adding product

shifts reverse

effect of decreasing volume (increasing pressure)

shifts toward fewer moles of gas

effect of increasing volume (decreasing pressure)

shifts toward more moles of gas

effect of increasing T (exothermic reaction)

shifts reverse; K increases

Effects of increasing T (endothermic reaction)

shifts forward; K decreases

Only disturbance that changes K

temperature

reversing a reaction

K becomes 1/K (reciprocal)

multiplying all coefficients by n

K becomes Kⁿ

Adding two reactions together

multiply their K values