The equilibrium constant (K) is a numerical value that expresses the ratio of product concentrations to reactant concentrations at chemical equilibrium for a reversible reaction at a given temperature.
Key points:
- Definition: K tells us how far a reaction proceeds toward products or reactants at equilibrium.
- Large K (>1): Products are favored; more products than reactants at equilibrium.
- Small K (<1): Reactants are favored; more reactants than products at equilibrium.
- Expression: K is calculated using the concentrations (or partial pressures) of reactants and products, each raised to the power of their stoichiometric coefficients.
- Temperature dependence: K changes with temperature but not with concentration, pressure, or catalysts.
- Dimensionless form: Sometimes expressed without units if concentrations are divided by a standard reference (usually 1 M or 1 atm).
Example: For a reaction A + B ⇌ C + D,
- K = [C][D] / [A][B] at equilibrium.
In simple terms, K tells you the “balance point” of a reversible reaction: how much is products vs. reactants when the system is at equilibrium.