Equilibrium Constant (K)
Relates products and reactants at equilibrium
General Expression
For aA + bB ⇌ cC + dD:
Kc = ([C]^c [D]^d) / ([A]^a [B]^b)
Use activities/activities approximated by molar concentrations for dilute solutions. For gases, use partial pressures for Kp.
Relationship Kp and Kc
Kp = Kc (RT)^Δn
Δn = moles of gaseous products − moles of gaseous reactants
Example
Reaction: N2 + 3 H2 ⇌ 2 NH3; at equilibrium: [N2]=0.10 M, [H2]=0.30 M, [NH3]=0.20 M.
Kc = (0.20^2) / (0.10^1 × 0.30^3) = 0.04 / (0.10 × 0.027) ≈ 14.8
Answer: Kc ≈ 14.8
Common Mistakes
Including solids/liquids
Pure solids and liquids are omitted (activity = 1).
Wrong exponents
Exponents come from balanced equation stoichiometric coefficients.
FAQ
How does temperature affect K?
Use van't Hoff equation; endothermic reactions increase K with temperature.
Can K be less than 1?
Yes; small K indicates reactants favored at equilibrium.