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Equilibrium Constant Calculator

Chemistry

Calculate the equilibrium constant Kc from equilibrium concentrations of reactants and products for a reversible reaction, plus ΔG° at 25°C.

Reviewed by the thecalcu.com team · Last updated July 30, 2026

0.5 mol/L
mol/L
2
0.2 mol/L
mol/L
1

Equilibrium Constant (Kc)

1.25
log Kc
0.097
ΔG° at 25°C (kJ/mol)
-0.553

This calculator computes your Equilibrium Constant (Kc), log Kc, ΔG° at 25°C (kJ/mol) from the values you enter.

Inputs
Product Concentration [P] (mol/L)Stoichiometric Coefficient of ProductReactant Concentration [R] (mol/L)Stoichiometric Coefficient of Reactant
Outputs
Equilibrium Constant (Kc)log KcΔG° at 25°C (kJ/mol)

What is a Equilibrium Constant?

The Equilibrium Constant Calculator computes Kc, the dimensionless equilibrium constant, from the equilibrium concentrations and stoichiometric coefficients of one product and one reactant species in a reversible chemical reaction. It also calculates log Kc (for use in thermodynamic relationships) and the standard Gibbs free energy change ΔG° at 25°C, connecting chemical equilibrium directly to thermodynamics.

Kc is the central quantity in chemical equilibrium analysis. For any reversible reaction at a fixed temperature, the ratio of product concentrations to reactant concentrations (each raised to their stoichiometric coefficients) reaches a constant value at equilibrium, this is Kc. A Kc much greater than 1 means products predominate at equilibrium; a Kc much less than 1 means reactants predominate. Temperature is the only variable that changes Kc, adding more reactant, removing product, or changing pressure shifts the position of equilibrium but not the value of Kc itself.

The relationship between Kc and thermodynamics is direct: ΔG° = −RT ln(Kc). A reaction with Kc >> 1 has a large negative ΔG°, meaning products are thermodynamically much more stable than reactants. This tool displays ΔG° at 25°C (T = 298.15 K) using R = 8.314 J/(mol·K). For ΔG° calculations at other temperatures or using enthalpy and entropy directly, use the Gibbs Free Energy Calculator.

For reactions not yet at equilibrium, the reaction quotient Q has the same form as Kc but uses current (non-equilibrium) concentrations. Comparing Q to Kc predicts the direction of spontaneous reaction.

Why Use an Equilibrium Constant Calculator?

Kc calculations involve exponentiation of concentrations, [P]^n, which is straightforward when n = 1 but error-prone for fractional or large integer coefficients. This calculator raises each concentration to the correct power and takes the ratio immediately, removing the most common arithmetic errors in equilibrium calculations.

The ΔG° output is particularly valuable for students and researchers connecting equilibrium constants to thermodynamics. Rather than separately computing ln(Kc) and multiplying by −RT, this calculator delivers ΔG° in kJ/mol alongside Kc in one step.

For JEE Advanced and NEET students, equilibrium constant problems appear every year and typically require computing Kc from a table of equilibrium concentrations, interpreting whether the reaction favours products or reactants, and sometimes relating Kc to ΔG° or to the reaction quotient Q.

Who Should Use This Calculator?

Class 11–12 and undergraduate chemistry students covering chemical equilibrium in NCERT (Chapter: Equilibrium) or advanced physical chemistry. Equilibrium constant numericals are among the most frequently tested topics in JEE Main, JEE Advanced, and NEET.

Physical chemistry and thermodynamics students needing to link Kc to ΔG° quickly during problem sets or exam preparation.

Process engineers and industrial chemists at chemical and fertiliser manufacturing plants who need to evaluate Kc at operating conditions to assess equilibrium conversion and set reactor conditions.

Environmental chemists calculating the equilibrium distribution of pollutants, heavy metals, or dissolved gases between phases in natural water systems.

Biochemists and pharmaceutical scientists using equilibrium constants (acid dissociation Ka, ligand-protein binding Kd) to characterise molecular interactions, where the same Kc formula applies.

What Insights Does the Equilibrium Constant Calculator Give You?

Equilibrium Constant (Kc) is the core output. The magnitude of Kc tells you immediately whether the equilibrium position favours products (Kc >> 1), reactants (Kc << 1), or lies in between (Kc ≈ 1). Reactions with Kc > 10⁶ go essentially to completion; reactions with Kc < 10⁻⁶ essentially do not proceed in the forward direction under equilibrium conditions alone.

log Kc is Kc expressed on a logarithmic scale. Because Kc spans many orders of magnitude (from 10⁻⁵⁰ to 10⁵⁰ for different reactions), log Kc provides a more interpretable number: 0 = balanced, positive = product-favoured, negative = reactant-favoured. Log Kc appears directly in the Nernst equation in electrochemistry and in the van't Hoff equation for temperature dependence.

ΔG° at 25°C (kJ/mol) is the standard Gibbs free energy change for the reaction, calculated from ΔG° = −RT ln(Kc) at 298.15 K. A negative value confirms the reaction is thermodynamically spontaneous in the forward direction under standard conditions; a positive value means the reaction is non-spontaneous. This links the equilibrium calculation to the broader Gibbs Free Energy Calculator framework.

How to use this Equilibrium Constant calculator

  1. Write the balanced equation for your reversible reaction and identify the equilibrium concentrations (in mol/L) of all species from your data or ICE table.
  2. Enter the equilibrium concentration of the product species in the Product Concentration [P] (mol/L) field. For multi-product reactions, calculate the numerator manually: [C]^c × [D]^d, and enter the result as a single equivalent concentration (with coefficient 1) if using this calculator for the full product term.
  3. Enter the stoichiometric coefficient of the product in the Stoichiometric Coefficient of Product field.
  4. Enter the equilibrium concentration of the reactant in the Reactant Concentration [R] (mol/L) field. Similarly, if there are multiple reactants, combine them manually.
  5. Enter the stoichiometric coefficient of the reactant in Stoichiometric Coefficient of Reactant.
  6. Read Kc, note whether it is greater or less than 1, and by how many orders of magnitude. Read ΔG° (kJ/mol) to confirm the thermodynamic spontaneity direction.
Show formula & methodology ↓Show less ↑

Formula & Methodology

Kc expression (single product, single reactant):

Kc = [P]^nP / [R]^nR

General Kc expression:

Kc = [C]^c × [D]^d / ([A]^a × [B]^b)      for aA + bB ⇌ cC + dD

Derived outputs:

log Kc = log₁₀(Kc) ΔG° (kJ/mol) = −R × T × ln(Kc) / 1000               where R = 8.314 J/(mol·K), T = 298.15 K

Worked example, hydrogen iodide equilibrium:

Reaction: H₂(g) + I₂(g) ⇌ 2 HI(g) at 445°C

Equilibrium concentrations measured: [H₂] = 0.107 mol/L, [I₂] = 0.107 mol/L, [HI] = 0.786 mol/L.

Kc = [HI]² / ([H₂]¹ × [I₂]¹)    = (0.786)² / (0.107 × 0.107)    = 0.618 / 0.01145    = 53.97  log Kc = log(53.97) = 1.732  ΔG° at 25°C = −(8.314)(298.15) ln(53.97) / 1000              = −2478.8 × 3.988 / 1000              = −9.88 kJ/mol

Kc = 54 is greater than 1, confirming that HI is favoured at equilibrium at 445°C. The negative ΔG° at 25°C indicates that HI is also thermodynamically favoured at room temperature, though the specific Kc value at 25°C would differ from the one measured at 445°C.

Frequently Asked Questions

What is the equilibrium constant (Kc)?
The equilibrium constant Kc is a dimensionless number that quantifies the ratio of product concentrations to reactant concentrations at equilibrium for a reversible reaction at a given temperature. Each concentration is raised to the power of its stoichiometric coefficient from the balanced equation. A large Kc (much greater than 1) means the equilibrium lies toward the products; a small Kc (much less than 1) means it lies toward the reactants. Kc is temperature-dependent, it changes when temperature changes but is unaffected by concentration changes, pressure changes, or catalysts.
What is the formula for Kc?
For a reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is Kc = [C]^c × [D]^d / ([A]^a × [B]^b), where square brackets denote molar concentrations (mol/L) at equilibrium and the exponents are the stoichiometric coefficients from the balanced equation. Pure solids and pure liquids (including water in dilute aqueous solutions) are omitted from the Kc expression, their concentrations are constant and absorbed into the Kc value.
What is the difference between Kc and Kp?
Kc uses molar concentrations (mol/L) in its expression; Kp uses partial pressures (usually in atm or Pa) of gaseous reactants and products. For reactions involving gases, both are defined and are related by Kp = Kc × (RT)^Δn, where R is the gas constant (0.0821 L·atm/mol·K), T is the absolute temperature in Kelvin, and Δn is the change in moles of gas (moles of gaseous products minus moles of gaseous reactants). For reactions with no change in moles of gas (Δn = 0), Kc and Kp are numerically equal.
What does the sign of ΔG° tell you about equilibrium?
The standard Gibbs free energy change ΔG° is related to Kc by ΔG° = −RT ln(Kc). A negative ΔG° means Kc > 1, the reaction is thermodynamically favoured in the forward direction and products are favoured at equilibrium. A positive ΔG° means Kc < 1, the reaction is thermodynamically unfavoured in the forward direction and reactants predominate at equilibrium. A ΔG° of zero means Kc = 1, neither reactants nor products are favoured. This relationship connects thermodynamics and equilibrium, linking the [Gibbs Free Energy Calculator](/gibbs-free-energy-calculator/) to Kc.
What is the reaction quotient (Q) and how does it differ from Kc?
The reaction quotient Q has the same mathematical form as Kc, products over reactants with concentration exponents, but uses current concentrations instead of equilibrium concentrations. Comparing Q to Kc predicts which direction the reaction will proceed: if Q < Kc, the reaction proceeds forward (toward products); if Q > Kc, it proceeds in reverse (toward reactants); if Q = Kc, the system is already at equilibrium. The Reaction Quotient Calculator helps compute Q and compare it to Kc.
How does temperature affect Kc?
Kc is temperature-dependent, it is a constant only at a fixed temperature. For an exothermic reaction (negative ΔH°), increasing temperature shifts the equilibrium toward reactants and decreases Kc (Le Chatelier's principle). For an endothermic reaction (positive ΔH°), increasing temperature shifts equilibrium toward products and increases Kc. The quantitative relationship is given by the van't Hoff equation: ln(K₂/K₁) = −ΔH°/R × (1/T₂ − 1/T₁), which relates Kc values at two temperatures T₁ and T₂.
How do I use the Equilibrium Constant Calculator?
Enter the equilibrium concentration of products in the 'Product Concentration' field (in mol/L) and the product's stoichiometric coefficient in its coefficient field. Enter the equilibrium concentration of reactants in the 'Reactant Concentration' field and its coefficient. The calculator returns Kc using the formula Kc = [P]^nP / [R]^nR, along with log Kc and the standard Gibbs free energy change ΔG° at 25°C.
Can the Equilibrium Constant Calculator handle multi-species reactions?
The current calculator handles one product species and one reactant species, it covers the most common single-step equilibrium problems taught in NCERT, JEE, and NEET chemistry. For reactions with multiple products or reactants, apply the formula manually: Kc = ([P1]^p1 × [P2]^p2) / ([R1]^r1 × [R2]^r2). Multiply together each species' concentration raised to its coefficient on each side, then divide products by reactants.
Is the equilibrium constant relevant to industrial chemistry in India?
Yes, equilibrium constants govern the design of major industrial chemical processes. The Haber process for ammonia synthesis (N₂ + 3H₂ ⇌ 2NH₃), critical for fertiliser production, operates at conditions selected to balance Kc (temperature, pressure, catalyst) for maximum yield. India is a major fertiliser producer, and plants at IFFCO, NFL, and Rashtriya Chemicals operate these equilibrium-governed processes at scale. Understanding Kc enables process engineers to optimise temperature and pressure conditions to shift equilibrium toward the desired product.
What does log Kc tell you that Kc does not?
Kc values can span many orders of magnitude, from 10⁻³⁰ for reactions strongly favouring reactants to 10⁴⁰ for reactions that essentially go to completion. Log Kc compresses this range into a linear scale: log Kc = 0 means Kc = 1 (balanced equilibrium); log Kc = 10 means Kc = 10¹⁰ (strongly product-favoured). Log Kc is also the form that appears directly in the ΔG° equation (ΔG° = −2.303 RT × log Kc) and in electrochemistry (related to standard cell potential via the Nernst equation).
What is the Kc value for water formation and why is it so large?
For the reaction 2 H₂(g) + O₂(g) ⇌ 2 H₂O(g) at 25°C, Kc is approximately 3 × 10⁸⁰, an astronomically large value indicating that the equilibrium lies overwhelmingly toward water at room temperature. This is reflected in the very negative ΔG° (≈ −457 kJ/mol). In practice, this reaction does not proceed measurably at 25°C without a spark or catalyst because the activation energy is very high, Kc tells you only about thermodynamic favourability, not reaction rate.