Chemical equilibrium constants
Kc and Kpreaction quotient QKspsolubility productlaw of mass actionICE table
Kc, Kp, the reaction quotient Q, Ksp and the link to Gibbs free energy — every way of asking which side a reaction favours.
Equilibrium Constant Kc (A + B ⇌ C + D)
Computes the equilibrium constant or reaction quotient for a one-to-one reaction from the four species concentrations, the law of mass action in its simplest form.
Reaction Quotient Q (aA + bB ⇌ cC)
Reaction quotient Q from any concentrations, raised to stoichiometric powers — the number you compare with K to predict which way a mixture shifts.
Kp from Kc (Kp = Kc(RT)^Δn)
Converts a gas-phase equilibrium constant between pressure and concentration bases using the change in moles of gas, with R = 0.08206 L·atm/(mol·K).
Solubility Product of a 1:1 Salt
Links the solubility product of an AB salt such as AgCl or BaSO4 to its molar solubility, since each formula unit releases one cation and one anion.
Solubility Product of an AB₂ Salt
Links the solubility product of an AB2 or A2B salt such as CaF2 or Mg(OH)2 to its molar solubility, with the factor 4 from the doubled ion.
Gibbs Free Energy and the Equilibrium Constant
Converts between a reaction's standard free energy change and its equilibrium constant, the bridge joining thermodynamics to equilibrium tables.
How they fit together
One expression underlies all of them: products over reactants, each raised to its balanced coefficient. Kc uses molar concentrations, Kp uses partial pressures and the two differ by (RT)^Δn where Δn counts moles of gas, Ksp is the same expression for a dissolving salt with the solid left out, and ΔG° = −RT ln K says the constant is just the free-energy difference in another costume. Note the standard state: these constants are only dimensionless because every concentration is divided by c° = 1 mol/L before it is raised to its power — for a reaction with equal moles on both sides that factor cancels and nobody notices, but the moment the stoichiometric powers differ it does not.
Q and K are the same arithmetic asked at different moments, and that is the decision rule. K describes a system that has already settled; Q is the same ratio evaluated with whatever concentrations you have right now. Compare them: Q < K means the reaction still runs forward, Q > K means it runs back, Q = K means nothing further happens. For a salt, Q > Ksp is the condition for precipitation. The two habitual mistakes are including pure solids and pure liquids in the expression — their activity is 1, so a solid reactant never appears — and assuming K is a property of the reaction. It is a property of the reaction at a given temperature, and it is the one thing a catalyst can never change.