Equilibrium Constant Calculator
Enter equilibrium concentrations and stoichiometric coefficients for up to four species to calculate Kc, with a sensitivity chart of how Kc responds to concentration.
⇌ What is the Equilibrium Constant?
The equilibrium constant (Kc) is a number that quantifies the balance between reactants and products in a reversible chemical reaction once it reaches equilibrium, the state where the forward and reverse reaction rates are equal and concentrations no longer change over time. It is calculated from the law of mass action as the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient in the balanced equation.
Chemists use Kc constantly: predicting whether a reaction will favor products or reactants before running it, calculating equilibrium concentrations from initial conditions with an ICE table, understanding how catalysts, temperature, and pressure shift reactions, and designing industrial processes like the Haber process for ammonia synthesis, where the equilibrium constant directly determines achievable yield at a given temperature and pressure.
A common point of confusion is mixing up Kc with the reaction quotient Q. They use the identical formula, but Kc is only valid using concentrations measured precisely at equilibrium, while Q can be calculated at any point during a reaction using whatever concentrations currently exist. Comparing a calculated Q to the known Kc tells you which direction a reaction will shift to reach equilibrium, a technique used constantly in equilibrium problems.
This calculator handles the general two-reactant, two-product case, aA + bB rightleftharpoons cC + dD, and lets any coefficient be set to 0 to drop a species that is not part of your specific reaction, so it works equally well for simple one-reactant, one-product equilibria and for full four-species systems.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Haber process (N2 + 3H2 ⇌ 2NH3)
At equilibrium: [N2] = 0.5 M, [H2] = 1.5 M, [NH3] = 0.2 M.
Example 2 — Simple isomerization (A ⇌ B)
At equilibrium: [A] = 0.3 M, [B] = 0.7 M, a simple 1:1 reaction with only one reactant and one product.
Example 3 — Nitrogen dioxide dimerization (2NO2 ⇌ N2O4)
At equilibrium: [NO2] = 0.02 M, [N2O4] = 0.0035 M.
❓ Frequently Asked Questions
🔗 Related Calculators
What is an equilibrium constant?
An equilibrium constant (Kc) is a number that describes the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient, at chemical equilibrium. It quantifies how far a reversible reaction proceeds toward products before reaching a stable balance between the forward and reverse reactions.
What is the formula for Kc?
For a reaction aA + bB rightleftharpoons cC + dD, Kc = ([C]^c times [D]^d) divided by ([A]^a times [B]^b), using the concentrations of each species at equilibrium and their respective stoichiometric coefficients from the balanced equation.
What does a large or small Kc value mean?
A Kc value much greater than 1 means the equilibrium mixture is mostly products, the forward reaction is favored. A Kc value much less than 1 means the equilibrium mixture is mostly unreacted reactants, the reverse reaction is favored. A Kc near 1 means the equilibrium mixture contains comparable amounts of both reactants and products.
Do I need all four species (A, B, C, D) to use this calculator?
No. Set the coefficient of any species that does not appear in your specific reaction to 0, and it will be removed from the Kc expression automatically, since any concentration raised to the power of 0 equals 1. This lets the same calculator handle reactions with just one reactant and one product, or with two of each.
What is the difference between Kc and the reaction quotient Q?
Kc and Q use exactly the same mathematical formula, the ratio of products to reactants raised to their stoichiometric powers, but Kc uses concentrations measured specifically at equilibrium, while Q can be calculated at any point in the reaction, using whatever concentrations exist at that moment. Comparing Q to Kc tells you which direction the reaction will shift to reach equilibrium.
Does Kc have units?
Strictly, no, the IUPAC definition of an equilibrium constant uses dimensionless activities rather than concentrations, making Kc formally unitless. In practice, most introductory chemistry courses compute Kc directly from molar concentrations and report it with implied concentration units, which is a useful teaching simplification rather than a strict thermodynamic definition.
How does temperature affect Kc?
Kc changes with temperature because temperature shifts the position of equilibrium itself, according to Le Chatelier's principle. For an exothermic reaction, Kc decreases as temperature rises, for an endothermic reaction, Kc increases as temperature rises. A Kc value is only meaningful when the temperature it was measured at is also specified.
What is the relationship between Kc and Kp?
Kp is the equilibrium constant expressed in terms of partial pressures of gaseous species instead of molar concentrations. The two are related by Kp = Kc times (RT) raised to the power of delta n, where delta n is the change in moles of gas between products and reactants, R is the gas constant, and T is the absolute temperature. Use the Kp Calculator to convert between the two directly.
Why does the chart on this page vary concentration [C] specifically?
The chart holds [A], [B], and [D] fixed at your entered values and varies [C] alone, showing how Kc responds as that one concentration changes while the reaction's other species stay constant. If coefficient c is 1, the curve is a straight line, if c is 2 or higher, the curve bends upward, reflecting the nonlinear power relationship in the mass action expression.
Can Kc be calculated from initial concentrations instead of equilibrium concentrations?
No, not directly, Kc is defined using equilibrium concentrations only. If you start with initial concentrations and a known change, you first need to build an ICE (Initial, Change, Equilibrium) table to work out the equilibrium concentrations, then enter those equilibrium values into this calculator to find Kc.