What does the equilibrium constant Keq tell you?

The equilibrium constant (Keq) expresses the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficient. A large Keq (much greater than 1) means the equilibrium strongly favours products. A small Keq (much less than 1) means the equilibrium strongly favours reactants. A Keq close to 1 means products and reactants are present in comparable amounts at equilibrium. Keq depends only on temperature, not on initial concentrations or the path taken to reach equilibrium.

What is the difference between Keq, Kc, and Kp?

Keq is a general term for any equilibrium constant. Kc (concentration equilibrium constant) uses molar concentrations in mol/L. Kp (pressure equilibrium constant) uses partial pressures in atm or Pa, and applies specifically to gas-phase reactions. For reactions involving only liquids and solids dissolved in a solvent, Kc is used. For acid-base reactions, Ka (acid dissociation constant) and Kb (base dissociation constant) are specific forms of Kc. For sparingly soluble salts, Ksp (solubility product) applies. This calculator computes Kc from molar concentrations.

Why are pure solids and liquids excluded from the Keq expression?

Pure solids and pure liquids have constant concentrations (their activity is defined as 1 by convention), so they do not appear in the equilibrium constant expression. Only dissolved species and gases are included. For example, in the dissolution of calcium carbonate, CaCO3(s) does not appear in the Kc expression because its concentration does not change during the reaction. Water (as solvent) is also typically excluded from Kc and Ka expressions for the same reason.

Equilibrium Constant (Keq) Calculator

Calculate the equilibrium constant Kc for any reversible chemical reaction. Enter the equilibrium molar concentrations and stoichiometric coefficients for up to two reactants and two products. The calculator builds the Kc expression and evaluates it instantly.

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Standard formula Law of Mass Action. General chemistry (IUPAC convention). No NZ-specific rates apply.

1. Reactants (Left Side)

Enter equilibrium concentration and stoichiometric coefficient for each reactant. Leave a reactant's concentration blank or 0 to exclude it.

Reactant A e.g. N2

Coefficient (a)

[A] mol/L

Reactant B optional

Coefficient (b)

[B] mol/L

Temperature for reference only

°C

2. Products (Right Side)

Enter equilibrium concentration and stoichiometric coefficient for each product. Leave a product's concentration blank or 0 to exclude it.

Product C e.g. NH3

Coefficient (c)

[C] mol/L

Product D optional

Coefficient (d)

[D] mol/L

Reaction type for context

Kc Expression

Keq = [C]^c / ([A]^a [B]^b)

Equilibrium Constant Result

Keq Value
-
Equilibrium constant

log₁₀(Keq)

Logarithmic scale

Reaction Favours

At this temperature

Numerator (Products)

Product C: [C]-

Coefficient c-

[C]^c-

Product D: [D]-

Coefficient d-

[D]^d-

Numerator total-

Denominator (Reactants)

Reactant A: [A]-

Coefficient a-

[A]^a-

Reactant B: [B]-

Coefficient b-

[B]^b-

Denominator total-

Interpretation: Enter concentrations above to calculate Keq.

What Is the Equilibrium Constant?

When a reversible chemical reaction reaches equilibrium, the concentrations of reactants and products no longer change because the forward and reverse reactions proceed at equal rates. The equilibrium constant (Keq, also written Kc when using concentrations) is a dimensionless number that quantifies the relative amounts of products and reactants present at equilibrium.

For the general reaction:

aA + bB ⇌ cC + dD

The expression for Kc is:

Kc = [C]^c [D]^d / ([A]^a [B]^b)

Where square brackets denote molar concentration (mol/L) at equilibrium, and the letters a, b, c, d are the stoichiometric coefficients from the balanced equation. This relationship is known as the Law of Mass Action.

How to Use This Calculator

Write your balanced chemical equation and identify the stoichiometric coefficients for each species.
Determine the equilibrium concentration of each species (in mol/L). These are the concentrations after the system has reached equilibrium, not the initial concentrations.
Enter the coefficient and concentration for reactant A and, if you have a second reactant, reactant B.
Enter the coefficient and concentration for product C and, if you have a second product, product D. Leave a field empty or at 0 to exclude that species.
Read the Kc value from the results panel.

Interpreting Keq

Keq value	What it means	Equilibrium position
Much greater than 1 (e.g. 10⁶)	Reaction goes nearly to completion	Strongly favours products
Greater than 1	More products than reactants at equilibrium	Favours products
Equal to 1	Equal amounts of products and reactants	Neither side favoured
Less than 1	More reactants than products at equilibrium	Favours reactants
Much less than 1 (e.g. 10^-6)	Reaction barely proceeds in forward direction	Strongly favours reactants

Worked Example: Haber Process (Default Inputs)

The synthesis of ammonia in the Haber process involves the equilibrium:

N2(g) + 3H2(g) ⇌ 2NH3(g)

Stoichiometric coefficients: a = 1 (N2), b = 3 (H2), c = 2 (NH3). Suppose the equilibrium concentrations are [N2] = 0.50 mol/L, [H2] = 0.60 mol/L, [NH3] = 0.15 mol/L.

The Kc expression is:

Kc = [NH3]² / ([N2]¹ [H2]³)

Substituting:

Kc = (0.15)² / (0.50 x (0.60)³)
Kc = 0.0225 / (0.50 x 0.216)
Kc = 0.0225 / 0.108
Kc = 0.2083

A Kc of approximately 0.208 at 25 degrees Celsius means the equilibrium slightly favours the reactant side under these conditions. In industrial practice, the Haber process is run at 400-500 degrees Celsius and high pressure to optimise yield and rate.

Important Notes

Pure solids and liquids are excluded. Only dissolved species (aqueous, aq) and gases (g) appear in the Kc expression. Pure solids and the solvent (usually water) have activity equal to 1 and are omitted.

Concentrations must be at equilibrium. Kc is only valid when all concentrations entered are equilibrium values, not initial or instantaneous values. If you have initial concentrations and want to find equilibrium concentrations, you need to use an ICE (Initial, Change, Equilibrium) table first.

Temperature dependence. Keq is constant only at a fixed temperature. Raising the temperature shifts the equilibrium in the endothermic direction (Le Chatelier's principle), changing the value of Keq. The relationship between Keq and temperature is described by the van't Hoff equation.

Types of Equilibrium Constants

Symbol	Name	Used for
Kc	Concentration equilibrium constant	Reactions in solution or gas phase (using mol/L)
Kp	Pressure equilibrium constant	Gas phase reactions (using partial pressures)
Ka	Acid dissociation constant	Weak acid ionisation in water
Kb	Base dissociation constant	Weak base ionisation in water
Ksp	Solubility product	Dissolution of sparingly soluble salts
Kw	Water dissociation constant	Self-ionisation of water (1.0 x 10^-14 at 25 degrees C)

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Method and sources: Law of Mass Action (Guldberg and Waage, 1864). IUPAC recommendations for equilibrium constant expressions (IUPAC Green Book, 3rd ed.). Atkins, P. and de Paula, J., Physical Chemistry (10th ed., Oxford University Press).

This calculator computes Kc from molar equilibrium concentrations using the Law of Mass Action. It does not account for activity coefficients (relevant at high ionic strength) or for reactions involving pure solids or liquids (which must be excluded from the expression). Results are mathematically exact given the inputs provided; accuracy depends on the accuracy of the equilibrium concentrations you supply.