Equilibrium Constant Calculator

Compute precise equilibrium constants (Kc and Kp) from balanced chemical reactions. Derive standard Gibbs free energy (ΔG° = –RT ln K°), reaction quotient (Q), and visualize species concentration profiles.

Chemical Species & Equilibrium Data
Critical: Pure solids (s) and liquids (l) have activity = 1 and must be omitted from inputs. Only enter gases (g) and dissolved species (aq).
For gas‑phase species, append (g) to the name (e.g., N₂(g), H₂(g)). Δn and Kp are computed only from (g) species.
Standard: 298.15 K
R = 0.082057 L·atm·mol⁻¹·K⁻¹ or 0.08314 L·bar·mol⁻¹·K⁻¹
ΔG° uses dimensionless K°
For gas‑phase reactions: K° = Kp (numeric). For condensed only: K° = Kc.
? Haber Process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
⚗️ Iodine: H₂(g) + I₂(g) ⇌ 2HI(g)
? Acetic acid: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)
? Methane steam: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g)
? Water autoionization: H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)
? Calcium carbonate decomposition: CaCO₃(s) ⇌ CaO(s) + CO₂(g)
Privacy first: all calculations performed locally, no data transfer.

Chemical Equilibrium & the Equilibrium Constant

The equilibrium constant (K) quantifies the ratio of product activities to reactant activities at equilibrium. For a general reaction aA + bB ⇌ cC + dD, Kc = ([C]ᶜ[D]ᵈ)/([A]ᵃ[B]ᵇ) using molar concentrations. For gas-phase reactions, Kp employs partial pressures, related by Kp = Kc(RT)Δn. This calculator provides both, along with the standard Gibbs free energy change (ΔG° = -RT ln K°), where K° is the dimensionless thermodynamic equilibrium constant (for gases, K° = Kp numerically when standard pressure is 1 atm/bar).

ΔG° = – R T ln K°   →   K° = e–ΔG°/(RT)

The magnitude of K indicates reaction favorability: K >> 1 favors products; K << 1 favors reactants. The tool also calculates the reaction quotient Q using the same expression but with non-equilibrium concentrations, allowing prediction of net reaction direction (Q < K → forward; Q > K → reverse).

Derivation & Rigorous Implementation

Our algorithm computes Δn (sum of gaseous product coefficients minus sum of gaseous reactant coefficients) by detecting the (g) tag in species names – only these contribute to Δn. Kc is evaluated from user‑supplied equilibrium concentrations and stoichiometric coefficients, excluding pure solids and liquids (activity = 1). Kp is derived using Kp = Kc (R·T)Δn with consistent units (L·atm·mol⁻¹·K⁻¹ or bar). The standard Gibbs free energy is evaluated using ΔG° = –R·T·ln(K°) where K° = Kp (numeric value) if any gas species is present, otherwise K° = Kc. This follows IUPAC recommendations for dimensionless equilibrium constants.

Scientific and Industrial Relevance

  • Haber-Bosch Process: Optimizing Kp for ammonia synthesis at high pressure.
  • Biochemical Reactions: Enzyme kinetics and metabolic control analysis rely on equilibrium constants.
  • Environmental Chemistry: Acid rain, ocean carbonate equilibrium, and pollutant partitioning.
  • Pharmaceutical Design: Drug–receptor binding affinities derived from ΔG°.
Case Study: Ammonia Synthesis (Haber Process)

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). At 400 °C (673 K), experimental Kp is ~1.6×10⁻⁴ (atm⁻²). Our calculator replicates this: using equilibrium partial pressures yields Kp, and ΔG° = –RT ln Kp ≈ +73 kJ·mol⁻¹. The positive ΔG° at standard state explains why high pressure is industrially employed to shift equilibrium (Le Chatelier). Students can explore how temperature affects ΔG° via the van 't Hoff equation.

Peer-reviewed methodology – Based on IUPAC Gold Book, Thermodynamics Research Center (TRC) standards, and "Physical Chemistry" by P. Atkins & J. de Paula. Version 2.4 (corrected exclusion of solids/liquids from Kc).Last updated June 2026

Frequently Asked Questions

Kp uses partial pressures, Kc uses concentrations. The relationship Kp = Kc(RT)^(Δn) accounts for the difference in units and gas expansion. For reactions with Δn = 0, Kp = Kc.

Yes. Pure solids and liquids do not appear in K expression – simply omit them from the species list. Only gas-phase species (with (g) tag) affect Δn and Kp. Aqueous species are included in Kc.

ΔG° is derived directly from the equilibrium constant you provide, consistent with ΔG° = -RT ln K° (R = 8.314 J·mol⁻¹·K⁻¹). For gas-phase reactions, K° is taken as the numerical value of Kp (assuming standard pressure = 1 atm/bar). For purely condensed systems, Kc is used (dimensionless). This matches standard thermodynamic conventions.

The reaction quotient Q uses the same expression but with instantaneous concentrations. Comparing Q to K tells you the direction the reaction will shift to reach equilibrium.
References: Atkins, P.W. “Physical Chemistry” (11th ed.); IUPAC. Compendium of Chemical Terminology; NIST Chemistry WebBook. For additional tools: Gibbs Free Energy Calculator, van ’t Hoff Calculator.