Equilibrium Constant Calculator
Calculate Kc from equilibrium concentrations.
Default 1
Leave blank if none
Default 1
Default 1
Leave blank if none
Default 1
Results update as you type.
How it works
The equilibrium constant Kc describes the balance between products and reactants when a reversible reaction reaches equilibrium, and this calculator computes it from equilibrium concentrations. You enter the molar concentrations of up to two reactants and two products along with their coefficients from the balanced equation, leaving the second reactant or product blank if the reaction has only one of each. It returns Kc and the separate product and reactant terms. The expression multiplies each product concentration raised to the power of its coefficient in the numerator, and each reactant concentration raised to its coefficient in the denominator. A large Kc, much greater than one, means products dominate at equilibrium — the reaction strongly favors the forward direction. A small Kc, much less than one, means reactants dominate and little product forms. A Kc near one means comparable amounts of both. Kc is constant for a given reaction at a fixed temperature but changes if the temperature changes. It is central to predicting the extent of reactions, comparing to the reaction quotient to judge which way a reaction will shift, and understanding chemical equilibrium. Pure solids and liquids are omitted from the expression. Enter the equilibrium concentrations and coefficients to find Kc.
Kc = ([C]^c × [D]^d) ÷ ([A]^a × [B]^b)
The equilibrium constant Kc is the product of the product concentrations, each raised to its coefficient, divided by the product of the reactant concentrations, each raised to its coefficient. It expresses the ratio of products to reactants at equilibrium and is constant at a given temperature.
Source:Chemical equilibrium
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter [a] reactant a and [c] product c.
- 2
Adjust the options
Adjust coefficient a, [b] reactant b, coefficient b, coefficient c, [d] product d and coefficient d if they apply — sensible defaults are already set.
- 3
Read your result
Equilibrium constant Kc updates instantly as you type — no button to press.
Frequently asked questions
How do you calculate the equilibrium constant Kc?
Raise each product's equilibrium concentration to the power of its coefficient and multiply them for the numerator, do the same for the reactants for the denominator, then divide. For A + B ⇌ C + D with all coefficients one and given concentrations, Kc is the products' concentrations multiplied divided by the reactants' concentrations multiplied.
What does the value of Kc tell you?
A large Kc (much greater than one) means products are favored and the reaction proceeds far toward completion. A small Kc (much less than one) means reactants are favored and little product forms. A Kc near one indicates significant amounts of both at equilibrium. Kc thus reveals the position of equilibrium and how far a reaction goes.
Does temperature affect Kc?
Yes. The equilibrium constant is constant only at a fixed temperature; changing the temperature changes Kc. For exothermic reactions, raising the temperature lowers Kc, while for endothermic reactions it raises Kc, consistent with Le Chatelier's principle. Pressure and concentration changes shift the position of equilibrium but do not change Kc itself at a given temperature.
Why are pure solids and liquids left out?
Pure solids and pure liquids have effectively constant concentration (activity of one), so including them would not change the ratio. They are therefore omitted from the Kc expression, which contains only gases and dissolved species whose concentrations vary. This is why heterogeneous equilibria have simpler expressions that ignore any solid or liquid phases present.
What is the difference between Kc and the reaction quotient Q?
Kc uses equilibrium concentrations and is fixed at a given temperature, while the reaction quotient Q uses the concentrations at any moment. Comparing Q to Kc predicts the direction a reaction will shift: if Q is less than Kc it moves forward toward products, if greater it moves backward toward reactants, and if equal the system is at equilibrium.