Equilibrium Constant (Keq) Calculator
This calculator works out the equilibrium constant (Keq, expressed as Kc) for any reversible chemical reaction using the Law of Mass Action, which describes how far a reaction proceeds before the forward and reverse rates balance out. You enter the equilibrium molar concentration (in mol/L) and stoichiometric coefficient for up to two reactants and two products from your balanced equation, plus a reaction temperature for reference and a reaction type (Kc, Ka, Kb, Ksp or Kp) for context. Leave a second reactant or product blank or at zero if your equation only has one of each. The calculator builds the Kc expression live and returns the Keq value, its log base 10, and a verdict on whether the equilibrium favours products, reactants, or neither, alongside a full breakdown of the numerator and denominator terms. Use it to check homework, verify lab results, or explore how shifting concentrations changes the equilibrium position. Remember that all concentrations you enter must be equilibrium values, not initial ones; if you only have starting concentrations, work through an ICE table first. Because Keq depends on temperature, the same reaction gives a different value at a different temperature. The maths is exact for the numbers you supply, but the calculator does not correct for activity coefficients at high ionic strength, and assumes pure solids and liquids have already been excluded from your inputs.
1. Reactants (Left Side)
Enter equilibrium concentration and stoichiometric coefficient for each reactant. Leave a reactant's concentration blank or 0 to exclude it.
2. Products (Right Side)
Enter equilibrium concentration and stoichiometric coefficient for each product. Leave a product's concentration blank or 0 to exclude it.
Numerator (Products)
Denominator (Reactants)
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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- Charles's Law Calculator: relate temperature and volume for an ideal gas.
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.