Last updated: July 3, 2026
Equilibrium Constant Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The equilibrium constant calculator computes Kc for aA + bB ⇌ cC + dD as ([C]^c × [D]^d) / ([A]^a × [B]^b). Enter equilibrium concentrations and stoichiometric coefficients for up to two reactants and two products. A large Kc favors products, while a small Kc favors reactants.
The equilibrium constant Kc equals product concentrations raised to their coefficients divided by reactant concentrations raised to their coefficients, using equilibrium concentrations only.
Key Takeaways
- Kc equals product concentration terms divided by reactant concentration terms.
- Stoichiometric coefficients become exponents in the Kc expression.
- Use equilibrium concentrations, not initial concentrations.
- Set omitted species coefficients to zero so their terms equal 1.
- Kc changes with temperature and indicates whether products or reactants are favored.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Kc = ([C]^c × [D]^d) / ([A]^a × [B]^b)
Where:
- Kc=Concentration equilibrium constant(dimensionless or concentration powers)
- [A], [B]=Equilibrium concentrations of reactants(mol/L)
- [C], [D]=Equilibrium concentrations of products(mol/L)
- a, b=Stoichiometric coefficients of reactants(dimensionless)
- c, d=Stoichiometric coefficients of products(dimensionless)
Worked Examples
A + B ⇌ C + D
All coefficients are 1, products are twice as concentrated as reactants.
- 1Write Kc = ([C]^1 × [D]^1) / ([A]^1 × [B]^1).
- 2Substitute concentrations: Kc = (1.0 × 1.0) / (0.5 × 0.5).
- 3Calculate denominator 0.25, so Kc = 1 / 0.25 = 4.
N₂ + 3H₂ ⇌ 2NH₃
Ammonia formation uses one reactant slot for N₂, one for H₂, and one product slot for NH₃.
- 1Write Kc = [NH₃]^2 / ([N₂]^1 × [H₂]^3).
- 2Substitute values: Kc = 0.4^2 / (0.2^1 × 0.2^3).
- 3Compute 0.16 / (0.2 × 0.008) = 0.16 / 0.0016 = 100.
2A ⇌ B
A single-reactant, single-product reaction omits B and D slots with zero coefficients.
- 1Set omitted species coefficients to zero so their terms equal 1.
- 2Write Kc = [B]^1 / [A]^2 = 8 / 2^2.
- 3Calculate 8 / 4 = 2.
Introduction
The equilibrium constant calculator evaluates Kc for a balanced reversible reaction using equilibrium concentrations. For aA + bB ⇌ cC + dD, the concentration form is Kc = (C]^c × [D]^d) / ([A]^a × [B]^b). It is a compact way to compare product and reactant amounts after the reaction has reached dynamic equilibrium. If you need concentrations first, use the [molarity calculator or concentration calculator. The notation follows equilibrium conventions discussed by IUPAC and general chemistry texts.
What does Kc mean?
Kc is the equilibrium constant written in terms of molar concentrations. A large Kc means products are favored at equilibrium; a small Kc means reactants are favored. A value near 1 means neither side is strongly favored. The number is calculated only after the reaction has reached equilibrium, not from initial concentrations.
Kc > 1: products are more abundant relative to stoichiometry.
Kc < 1: reactants are more abundant relative to stoichiometry.
Kc changes with temperature but not with the starting mixture.
Pure solids and pure liquids are omitted because their activities are effectively constant.
Equilibrium constant formula
For a balanced reaction aA + bB ⇌ cC + dD, each concentration is raised to its stoichiometric coefficient. Products appear in the numerator and reactants in the denominator. This calculator models up to two reactants and two products; set a concentration and coefficient to zero to omit a species.
A zero coefficient is treated as x^0 = 1, so omitted species do not affect Kc.
How to calculate Kc step by step
First balance the chemical equation. Second, write the product concentrations over the reactant concentrations. Third, raise each concentration to the coefficient from the balanced equation. Finally, multiply the terms on each side and divide products by reactants. For related acid-base equilibria, compare this with the pKa calculator and pH calculator.
Use equilibrium concentrations, not initial concentrations.
Keep all concentration units consistent, usually mol/L.
Enter zero coefficients for omitted slots.
Use tolerance-based rounding when comparing calculated values.
Kc versus Qc
Kc has the same mathematical form as the reaction quotient Qc, but Kc uses equilibrium concentrations. Qc can be calculated at any moment to predict direction: if Qc < Kc the reaction tends to form products, and if Qc > Kc it tends to form reactants. Electrochemical versions of reaction quotients are used in the Nernst equation.
If you are not at equilibrium yet, call the same expression Qc rather than Kc.
Units, activities, and approximations
Introductory calculations often plug molarities directly into the expression. More rigorous thermodynamics uses activities, making the thermodynamic equilibrium constant dimensionless. Concentration-based Kc values are still extremely useful for dilute solutions and teaching-lab calculations, but high ionic strength or nonideal solutions may require activity coefficients. See the NIST Chemistry WebBook for reference thermochemical data.
Do not include pure solids or pure liquids in the equilibrium expression; gases normally use Kp instead of Kc unless converted.
Common Kc expression patterns
The table summarizes how coefficients change the expression. Powers matter: tripling a coefficient does not multiply a concentration term by three; it raises that term to the third power.
| Reaction pattern | Kc expression | Interpretation |
|---|---|---|
| A + B ⇌ C + D | [C][D]/([A][B]) | All first powers |
| N₂ + 3H₂ ⇌ 2NH₃ | [NH₃]²/([N₂][H₂]³) | Hydrogen has a cubic term |
| 2A ⇌ B | [B]/[A]² | Reactant A is squared |
| A ⇌ 2C + D | [C]²[D]/[A] | Product coefficient becomes an exponent |
Common mistakes to avoid
The most frequent errors are using unbalanced coefficients, entering initial concentrations, or forgetting exponents. Another common mistake is including species that should be omitted, such as a pure solid catalyst or solvent water in a dilute aqueous reaction. If the reaction involves titration data before equilibrium, the titration calculator can help establish concentrations first.
Quick Reference Card
Equilibrium Constant Kc — Quick Reference
Quick reference • Equilibrium Constant Calculator
Kc = ([C]^c × [D]^d) / ([A]^a × [B]^b)Valid range: Kc is positive; values can be far below 1 or far above 1 depending on temperature and reaction.
Common Values
⚠ Watch Out
- •Use a balanced chemical equation before entering coefficients.
- •Do not use initial concentrations unless they already equal equilibrium concentrations.
- •Do not include pure solids, pure liquids, or solvents in dilute aqueous expressions.
- •A zero concentration with a positive coefficient makes Kc undefined; verify data.
- •Kc values are temperature-specific.
Pro Tips
- →Write the symbolic Kc expression before substituting numbers.
- →Check powers carefully for coefficients greater than one.
- →Use mol/L consistently for all solution concentrations.
- →Compare Qc with Kc to predict reaction direction before equilibrium.
- →Round final Kc values according to the precision of concentration measurements.
FAQs
What is the equilibrium constant Kc?
Kc is the product of equilibrium product concentrations raised to their coefficients divided by the product of equilibrium reactant concentrations raised to their coefficients.
Do I use initial or equilibrium concentrations?
Use equilibrium concentrations only. Initial concentrations may be used to build an ICE table, but the Kc expression itself is evaluated after equilibrium is reached.
How do I omit a species in this calculator?
Set that species coefficient to 0. The term is treated as x^0 = 1, so it is skipped in the numerator or denominator.
Can Kc have units?
Thermodynamic equilibrium constants are dimensionless when written with activities. Classroom concentration constants may appear to carry composite units depending on the reaction order, but they are usually reported as Kc values.
Why are coefficients used as exponents?
The exponents come from the balanced chemical equation and the law of mass action. For example, 3H₂ contributes [H₂]^3 to the denominator.
What does a large Kc mean?
A large Kc means products are favored at equilibrium. A very small Kc means reactants are favored, while Kc near 1 means both sides are comparable.