Kc (equilibrium)
Kc is the equilibrium constant expressed in terms of molar concentrations (mol/L) of reactants and products at equilibrium. Each concentration is raised to the power of its stoichiometric coefficient, and pure solids and pure liquids are excluded from the expression because their concentrations are constant. Kc is temperature-dependent and is the most commonly used form of the equilibrium constant in solution-phase and heterogeneous equilibria.
Key Formula
Kc = (product of [product]^coeff) / (product of [reactant]^coeff)
LaTeX: K_c = \frac{\prod [\text{products}]^{\text{coeff}}}{\prod [\text{reactants}]^{\text{coeff}}}
| Symbol | Meaning | Unit |
|---|---|---|
| [X] | Molar concentration of species X at equilibrium | mol/L |
| coeff | Stoichiometric coefficient of species X | dimensionless |
Worked Example
Problem
Write the Kc expression and calculate Kc for: H₂(g) + I₂(g) ⇌ 2HI(g). At equilibrium: [H₂] = 0.200 mol/L, [I₂] = 0.200 mol/L, [HI] = 1.600 mol/L.
Solution
Step 1 – Write Kc: Kc = [HI]² ÷ ([H₂][I₂]). Step 2 – Substitute: Kc = (1.600)² ÷ (0.200 × 0.200). Step 3 – Numerator: 1.600² = 2.560. Step 4 – Denominator: 0.200 × 0.200 = 0.0400. Step 5 – Kc = 2.560 ÷ 0.0400 = 64.0.
Answer
Kc = 64.0 at this temperature
Rules for Writing Kc Expressions
| Species Type | Included in Kc? | Reason | Example |
|---|---|---|---|
| Aqueous solutes | Yes | Concentration varies | [Na⁺], [OH⁻] |
| Gases | Yes | Concentration varies | [CO₂(g)] |
| Pure solids | No | Constant concentration | CaCO₃(s) |
| Pure liquids | No | Constant concentration | H₂O(l) as solvent |
| Dilute solvent water | No | Concentration ≈ 55.5 mol/L, constant | H₂O in aqueous rx |
Interactive Tools
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Related Terms
Equilibrium Constant
The equilibrium constant (K) is a dimensionless number that expresses the ratio of the concentrations (or partial pressures) of products to reactants, each raised to the power of their stoichiometric coefficients, for a reversible reaction at equilibrium at a given temperature. A large K (K >> 1) indicates the equilibrium favours products, while a small K (K << 1) indicates reactants predominate. K changes with temperature but is independent of initial concentrations, catalysts, or pressure (for Kc).
Kp (equilibrium)
Kp is the equilibrium constant expressed in terms of the partial pressures of gaseous reactants and products, each raised to the power of their stoichiometric coefficients. It is used exclusively for reactions involving gases and is related to Kc through the ideal gas equation, with the conversion factor depending on the change in moles of gas in the reaction. Kp is particularly useful in industrial gas-phase reactions such as the Haber process and the Contact process.
Chemical Equilibrium
Chemical equilibrium is the state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products over time. The system appears static but is actually dynamic — molecules continuously react in both directions at matching rates. The equilibrium state is quantified by an equilibrium constant (K), whose value depends only on temperature for a given reaction.
The subscript "c" in Kc stands for "concentration" (from Latin "concentratio"), distinguishing it from Kp (pressure-based). The notation became standard in the mid-20th century as IUPAC unified thermodynamic symbols.