Chemical Equilibrium Study Guide
Chemical equilibrium is the single most reused idea in chemistry: the same K-and-Q logic governs weak acids, buffers, solubility, and electrochemistry. The mechanics are few — write K correctly, compute Q to find direction, use ICE tables to find concentrations, apply Le Chatelier for qualitative shifts — but each has classic traps. This guide drills the full toolkit, including Kp/Kc conversion and the thermodynamic link ΔG° = −RT ln K that explains why K means what it does.
Key formulas
Kc = [C]^c[D]^d / [A]^a[B]^b for aA + bB <-> cC + dD; omit pure solids and pure liquids
Q has the same expression as K but uses current concentrations; Q < K shifts toward products, Q > K shifts toward reactants
Kp = Kc(RT)^Δn, where Δn = (moles of gaseous products) − (moles of gaseous reactants), T in Kelvin
ΔG° = −RT ln K, linking the standard free energy to the equilibrium constant
Ksp = [cation]^x[anion]^y for a sparingly soluble salt; precipitation occurs when Qsp > Ksp
ICE table: Initial, Change (−x for consumed, +x for formed), Equilibrium concentrations
Topic checklist
Dynamic equilibrium
At equilibrium, forward and reverse rates are equal — concentrations stop changing, but reactions continue microscopically.
Kc is constant at a given temperature; changing concentration, pressure, or volume shifts position but not K itself.
Only temperature changes alter the value of K.
Writing the equilibrium constant
Write Kc = [products]^coefficients / [reactants]^coefficients; omit pure solids and pure liquids (their activities are 1).
Aqueous species and gases are included with molar concentrations in brackets.
A large K means products favored; a small K means reactants favored at equilibrium.
Reaction quotient Q
Q has the same form as K but uses current (not necessarily equilibrium) concentrations.
Q < K: reaction shifts toward products; Q > K: shifts toward reactants; Q = K: at equilibrium.
Q is your compass; K is your destination.
ICE tables
ICE = Initial, Change, Equilibrium: set up a table with one row per species.
Use −x for reactants consumed and +x for products formed (reverse signs if the shift goes backward).
Solve for x with algebra or the small-x approximation, then verify x is less than 5% of the smallest initial concentration before trusting the approximation.
Le Chatelier's principle
Stress a system (concentration, pressure, temperature) and it shifts to partially undo the stress.
Adding product or removing reactant shifts toward reactants; adding reactant shifts toward products.
For a gas reaction, increased pressure favors the side with fewer gas moles; inert gas addition at constant volume does nothing.
Temperature, catalysts, and solubility
Temperature is special: heating favors the endothermic direction, shifting K itself.
Catalysts speed up both directions equally — they never change K or the equilibrium position.
Solubility equilibrium uses Ksp; precipitation begins when Qsp > Ksp.
Common mistakes
Mistake: Including pure solids or liquids in the K expression
Fix:
Cross out pure solids and liquids when writing K. For CaCO3(s) <-> CaO(s) + CO2(g), Kc = [CO2] only — the solids are omitted entirely.
Mistake: Confusing Q and K, or predicting direction without computing Q
Fix:
Compute Q from the given concentrations, compare to the known K, and only then decide the shift. Never assume the direction — calculate it.
Mistake: Sign errors in the ICE table change row
Fix:
Write the change row as −x for species being consumed and +x for species being formed, then check the signs against the actual shift direction before solving.
Mistake: Using Kc when the problem gives or asks for Kp (or vice versa) without converting
Fix:
Kc and Kp differ by (RT)^Δn; they are equal only when Δn = 0. Use Kp = Kc(RT)^Δn with R = 0.08206 L·atm/(mol·K) and T in Kelvin.
Exam tips
Build speed on equilibrium-practice; ICE-table algebra is where exams separate scores.
Before any shift question, write 'Q vs K' at the top of your work — it forces the calculation-first habit.
Pair with ph-practice: weak-acid Ka problems are equilibrium problems with a different constant.
For Kp/Kc conversions, write Δn = (gas moles products) − (gas moles reactants) explicitly every time.
Temperature-effect questions are really ΔH questions in disguise; note whether the reaction is exo- or endothermic first.