# Equilibrium Constant Calculator

Calculate equilibrium constants Kc and Kp from concentrations or pressures. Convert between Kc and Kp. Find ΔG° from K.

## What this calculates

Calculate equilibrium constants (Kc or Kp) from equilibrium concentrations or partial pressures, convert between Kc and Kp, and determine the standard Gibbs free energy change (ΔG°). Supports reactions with up to 2 products and 2 reactants.

## Inputs

- **Calculation Type** — options: Kc from Concentrations, Kp from Pressures, Kc ↔ Kp Conversion — Select what to calculate.
- **Product 1 (conc. or pressure)** — min 0 — Equilibrium concentration (mol/L) or partial pressure (atm) of product 1.
- **Product 1 Coefficient** — min 1, max 10 — Stoichiometric coefficient of product 1.
- **Product 2 (conc. or pressure)** — min 0 — Equilibrium value of product 2. Leave 0 if not applicable.
- **Product 2 Coefficient** — min 1, max 10 — Stoichiometric coefficient of product 2.
- **Reactant 1 (conc. or pressure)** — min 0 — Equilibrium concentration (mol/L) or partial pressure (atm) of reactant 1.
- **Reactant 1 Coefficient** — min 1, max 10 — Stoichiometric coefficient of reactant 1.
- **Reactant 2 (conc. or pressure)** — min 0 — Equilibrium value of reactant 2. Leave 0 if not applicable.
- **Reactant 2 Coefficient** — min 1, max 10 — Stoichiometric coefficient of reactant 2.
- **Temperature** (K) — min 0.01 — Temperature in Kelvin. Used for Kc↔Kp conversion and ΔG° calculation.
- **Δn (Moles gas change)** — Change in moles of gas: (sum of gaseous product coefficients) - (sum of gaseous reactant coefficients). Used for Kc↔Kp conversion.
- **Known K Value (for conversion)** — min 0 — The known equilibrium constant to convert. Used only in Kc↔Kp conversion mode.
- **Known K Type** — options: Known value is Kc, Known value is Kp — Specify whether the known K is Kc or Kp.

## Outputs

- **K Value** — The calculated equilibrium constant.
- **ln(K)** — Natural logarithm of the equilibrium constant.
- **ΔG°** (kJ/mol) — Standard Gibbs free energy change.
- **Details** — formatted as text — Calculation details and interpretation.

## Details

The equilibrium constant quantifies the position of a chemical equilibrium. For a generic reaction aA + bB ⇌ cC + dD, the equilibrium constant Kc is defined as [C]^c[D]^d / [A]^a[B]^b, where brackets denote molar concentrations at equilibrium.

Key Relationships

- Kc uses molar concentrations (mol/L). Kp uses partial pressures (atm) for gas-phase reactions.

- Conversion: Kp = Kc(RT)^Δn, where R = 0.08206 L·atm/(mol·K) and Δn is the change in moles of gas (products minus reactants).

- Gibbs Free Energy: ΔG° = -RT·ln(K), where R = 8.314 J/(mol·K). A large K (>> 1) means products are favored and ΔG° is negative (spontaneous). A small K (<< 1) means reactants are favored.

Interpreting K Values

K >> 1: Equilibrium lies far to the right (product-favored). K << 1: Equilibrium lies far to the left (reactant-favored). K ≈ 1: Significant amounts of both reactants and products are present at equilibrium. Temperature changes shift K according to the van't Hoff equation.

## Frequently Asked Questions

**Q: What is the difference between Kc and Kp?**

A: Kc is the equilibrium constant expressed in terms of molar concentrations (mol/L), while Kp uses partial pressures (atm). They are related by Kp = Kc(RT)^Δn. For reactions where Δn = 0 (no change in moles of gas), Kp = Kc.

**Q: What does the value of K tell us about a reaction?**

A: A large K (much greater than 1) means the reaction strongly favors products at equilibrium. A small K (much less than 1) means reactants are favored. K = 1 means roughly equal amounts of products and reactants are present. K does not tell us how fast equilibrium is reached.

**Q: How does temperature affect the equilibrium constant?**

A: For exothermic reactions, increasing temperature decreases K (shifts equilibrium toward reactants). For endothermic reactions, increasing temperature increases K (shifts toward products). This is described quantitatively by the van't Hoff equation.

**Q: Why do pure solids and liquids not appear in the K expression?**

A: Pure solids and liquids have constant concentrations (activities = 1) regardless of amount, so they do not affect the position of equilibrium. Only species whose concentrations can change (gases and dissolved species) appear in the equilibrium expression.

**Q: What is the reaction quotient Q?**

A: Q has the same form as K but uses current (non-equilibrium) concentrations. If Q < K, the reaction proceeds forward to reach equilibrium. If Q > K, it proceeds in reverse. If Q = K, the system is at equilibrium.

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Source: https://vastcalc.com/calculators/chemistry/equilibrium-constant
Category: Chemistry
Last updated: 2026-04-21