K can be huge or tiny
An equilibrium constant much greater than 1 means products are favored at equilibrium, while a very small value means reactants dominate.
Equilibrium Constant Calculator
Inputs
Used only when comparing Q with a known K.
Enter equilibrium concentrations for dissolved species and gases. Pure solids and pure liquids are omitted from Kc and Qc expressions.
Balanced reaction table
For Kc, enter molar concentrations for included species. Phases (s) and (l) are omitted automatically.
| Species | Side | Coeff. | Phase | Value | Row |
|---|
Results
Primary result
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Expression used
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Included and omitted species
Interpretation
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Formula summary
| Quantity | Formula | Use |
|---|---|---|
| Equilibrium constant in concentration form | Kc = [products]^coeff / [reactants]^coeff | Use equilibrium molar concentrations for included species. |
| Equilibrium constant in pressure form | Kp = (Pproducts)^coeff / (Preactants)^coeff | Use gas partial pressures only. |
| Reaction quotient | Q = same expression as K, using current values | Compare with K to predict net direction. |
| Kc to Kp conversion | Kp = Kc(RT)^Δn | For gas-phase stoichiometry, with Δn = gas products − gas reactants. |
This page uses the standard textbook approximation that activities can be represented by molarity for solutes or partial pressure for gases. Pure solids and pure liquids are treated as constant and omitted from the equilibrium expression.
Worked examples
| Case | Balanced reaction | Inputs | Result |
|---|---|---|---|
| Ammonia synthesis at equilibrium | N2(g) + 3H2(g) ⇌ 2NH3(g) | [N2] = 0.200 M, [H2] = 0.300 M, [NH3] = 0.500 M | Kc ≈ 46.3 |
| Reaction direction check | N2(g) + 3H2(g) ⇌ 2NH3(g) | Current values: [N2] = 0.400 M, [H2] = 0.800 M, [NH3] = 0.200 M, known Kc = 46.3 | Qc ≈ 0.195, so the reaction tends to move toward products. |
| Heterogeneous equilibrium | CaCO3(s) ⇌ CaO(s) + CO2(g) | CO2 partial pressure = 0.250 atm | Kp = 0.250; both solids are omitted. |
The calculator assumes your reaction is already balanced. Stoichiometric coefficients become exponents in the equilibrium expression.
How this calculator handles phases and direction
For Kc or Qc, the tool includes aqueous species and gases, while pure solids and pure liquids are omitted. For Kp or Qp, only gases appear in the expression because the inputs are gas partial pressures. The page always shows which species were included and which were left out.
When you choose a quotient comparison, the page evaluates the same expression as K but uses the current values you entered. If Q < K, the system tends to move toward products; if Q > K, it tends to move toward reactants; and if Q = K, the system is already at equilibrium within numerical tolerance.
Balanced equation required
Coefficients become exponents
Pure solids/liquids omitted
Q < K: forward shift
Q > K: reverse shift
5 Fun Facts about Equilibrium Constants
Solids often disappear from the formula
Pure solids and pure liquids are usually omitted from equilibrium expressions, which is why decomposition reactions can reduce to a single gas term.
Q predicts the direction of travel
The reaction quotient uses the same mathematical form as K. Comparing Q with K tells you which side the system tends to move toward.
Kc and Kp differ only by gas stoichiometry
The conversion between them depends on temperature and Δn(gas), so reactions with equal moles of gaseous products and reactants have Kp = Kc.
Catalysts do not change K
A catalyst can help a reaction reach equilibrium faster, but it does not change the equilibrium constant because it speeds both directions together.
FAQ
Can I use this for concentrations in mol/L and partial pressures in atm?
Yes. Kc mode expects concentration-style values for included species, and Kp mode expects gas partial pressures. The Kc ↔ Kp conversion uses R = 0.082057 L·atm·mol⁻¹·K⁻¹.
Why are solids and liquids removed from the expression?
In the standard equilibrium treatment, the activity of a pure solid or pure liquid is effectively constant, so it is absorbed into the constant and does not appear explicitly in the expression.
Does this solve ICE tables or equilibrium concentrations from a starting mixture?
No. This page evaluates the equilibrium expression once you supply either equilibrium values, current values for Q, or a known constant plus temperature for Kc ↔ Kp. It does not solve nonlinear equilibrium-composition problems.
Is this suitable for graded homework or lab method verification?
It is useful for checking expressions and arithmetic, but it does not replace your course conventions, activity-based thermodynamics, or any validated laboratory method.
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Assumptions and limits
This page assumes the reaction is already balanced and uses the common chemistry approximation of activities by molarity or partial pressure. It does not infer stoichiometry, handle activity coefficients, ionic-strength corrections, fugacity, solvent standard states, or solve full equilibrium-composition problems. For assessed, research, or regulated work, verify the exact convention your source uses.