Chemical Equilibrium Cheat Sheet
K expressions, Kp/Kc, the reaction quotient, ICE tables, and Le Chatelier's principle.
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Chemical Equilibrium
K expressions, Kp/Kc, the reaction quotient, ICE tables, and Le Chatelier's principle.
Equilibrium constant
CORE RULEKc = [products]^ν / [reactants]^ν (Kp uses partial pressures). EXCLUDE pure solids and liquids. N₂ + 3H₂ ⇌ 2NH₃: Kc = [NH₃]²/([N₂][H₂]³). For CaCO₃(s) ⇌ CaO(s) + CO₂(g): Kp = P(CO₂).
Kp and Kc
CORE RULEKp = Kc(RT)^Δn, R = 0.0821, T in K, Δn = (mol gas products) − (mol gas reactants), counting gases only. Δn = 0 → Kp = Kc.
Manipulating K
CORE RULEReverse a reaction → K_rev = 1/K. Add reactions → K_net = K₁·K₂·… Scale all coefficients by x → K_scaled = K^x. Convert moles to molarity (M = mol/V) before substituting.
Q vs K direction
CORE RULEQ has K's form with current concentrations. Q < K → shifts right (toward products); Q > K → shifts left; Q = K → at equilibrium.
ICE table
METHODInitial / Change (±νx) / Equilibrium rows, then solve K for x. Three paths: perfect square (take √), x-small approximation (valid when K < 10⁻³ or [init] ≫ x; check x < 5%), or the quadratic formula (keep the positive root). Always back-substitute to confirm K.
Decomposition from total P
METHODGiven only total pressure + product ratio (e.g. NH₄HS(s) ⇌ NH₃ + H₂S, 1:1): write each partial pressure in one unknown, sum to the total, solve, then substitute into Kp.
Le Chatelier's principle
REFERENCE| Stress | Shift |
|---|---|
| add reactant / remove product | right |
| remove reactant / add product | left |
| V↓ (P↑), Δn ≠ 0 | toward fewer moles of gas |
| V↑ (P↓), Δn ≠ 0 | toward more moles of gas |
| T↑, endothermic | right (K increases) |
| T↑, exothermic | left (K decreases) |
| catalyst / inert gas at const V | no shift |
Temperature is special
CORE RULEOnly temperature changes the value of K. Every other stress leaves K fixed and the system shifts to restore Q = K.
K magnitude → extent
CORE RULEK ≫ 1 (> 10³): products dominate (essentially complete). K ≪ 1 (< 10⁻³): reactants dominate. 10⁻³ ≤ K ≤ 10³: comparable amounts of both. Rule-of-thumb thresholds for extent prediction.