Last updated: June 2026
Use this Kc calculator to organise equilibrium calculations involving equilibrium concentrations. This page also includes a reaction quotient calculator section because Qc uses the same style of expression as Kc, except it is used before equilibrium is reached.
What is Kc?
Kc is the equilibrium constant expressed in terms of concentration. It tells us the ratio of products to reactants at equilibrium for a reversible reaction at a fixed temperature.
A large Kc value suggests that the equilibrium mixture contains more products than reactants. A small Kc value suggests that the equilibrium mixture contains more reactants than products.
Kc expression
For a general reversible reaction:
aA + bB ⇌ cC + dD
the Kc expression is:
Kc = ([C]c[D]d) / ([A]a[B]b)
The concentrations of products are placed on top, while the concentrations of reactants are placed below. Each concentration is raised to the power of its coefficient in the balanced equation.
What should be included in Kc?
Kc expressions usually include aqueous species and gases whose concentrations can change.
Pure solids and pure liquids are not included in the Kc expression because their concentrations are effectively constant during the reaction.
How to calculate Kc
To calculate Kc, first write the balanced equation. Then write the Kc expression using the concentrations of the species included in the equilibrium. Finally, substitute the equilibrium concentrations into the expression.
Remember that Kc must be calculated using equilibrium concentrations, not initial concentrations.
Reaction Quotient Calculator
The reaction quotient, Qc, uses the same concentration expression as Kc, but the concentrations do not have to be equilibrium concentrations.
For:
aA + bB ⇌ cC + dD
the reaction quotient is:
Qc = ([C]c[D]d) / ([A]a[B]b)
The expression looks the same as Kc, but the meaning is different. Kc is the value at equilibrium. Qc is the value at a particular moment before equilibrium has necessarily been reached.
Comparing Qc and Kc
Once Qc is calculated, compare it with Kc.
If Qc < Kc, the reaction shifts forward to form more products.
If Qc > Kc, the reaction shifts backward to form more reactants.
If Qc = Kc, the system is at equilibrium.
A simple way to remember this is that Qc is the current product-to-reactant ratio, while Kc is the equilibrium product-to-reactant ratio.
Worked examples
Example 1: Writing a Kc expression
For the reaction:
H2(g) + I2(g) ⇌ 2HI(g)
the Kc expression is:
Kc = [HI]2 / ([H2][I2])
The concentration of HI is squared because its coefficient is 2 in the balanced equation.
Example 2: Calculating Kc
For the reaction:
H2(g) + I2(g) ⇌ 2HI(g)
suppose the equilibrium concentrations are:
[H2] = 0.200 mol dm−3, [I2] = 0.100 mol dm−3, and [HI] = 0.800 mol dm−3.
Kc = (0.800)2 / (0.200 × 0.100)
Kc = 0.640 / 0.0200 = 32.0
Example 3: Using Qc to predict direction
For the same reaction, suppose Kc = 32.0. At one moment, the concentrations are:
[H2] = 0.300 mol dm−3, [I2] = 0.300 mol dm−3, and [HI] = 0.600 mol dm−3.
Qc = (0.600)2 / (0.300 × 0.300)
Qc = 0.360 / 0.0900 = 4.00
Since Qc < Kc, the reaction shifts forward to form more HI.
Common exam traps
Frequently Asked Questions
What does a Kc calculator calculate?
A Kc calculator helps calculate the equilibrium constant from equilibrium concentrations using the balanced equation.
What is the reaction quotient?
The reaction quotient, Qc, uses the same expression as Kc, but it is calculated using concentrations at any moment before equilibrium has necessarily been reached.
Why is Reaction Quotient Calculator inside the Kc Calculator?
Qc and Kc use the same concentration expression. The difference is whether the system is already at equilibrium, so it is clearer to teach them together.
How do I know which direction the reaction shifts?
If Qc is smaller than Kc, the reaction shifts forward. If Qc is larger than Kc, the reaction shifts backward. If Qc equals Kc, the system is at equilibrium.
Do catalysts change Kc?
No. A catalyst helps the system reach equilibrium faster, but it does not change the value of Kc.