Generated on: June 17, 2026 at 05:24 PM

Equilibrium Constant Calculator

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Concentration of A0.5 M
Stoichiometric Coefficient a1
Concentration of B1 M
Stoichiometric Coefficient b3
Concentration of C0.8 M
Stoichiometric Coefficient c2
Concentration of D1 M
Stoichiometric Coefficient d0

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Chemistry

Equilibrium Constant Calculator

Calculate the equilibrium constant Kc from equilibrium concentrations for the reaction aA + bB ⇌ cC + dD.

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At equilibrium, reactants are favored (Kc < 1). Very little product forms under these conditions.

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Understanding the equilibrium constant

When a reversible reaction reaches equilibrium, the concentrations of reactants and products stop changing — not because the reaction stops, but because the forward and reverse rates become equal. The equilibrium constant Kc captures this balance as a single number. A large Kc means the equilibrium lies toward products; a small Kc means it lies toward reactants.

The equilibrium constant expression

For the general reaction:

aA+bB⇌cC+dDa\text{A} + b\text{B} \rightleftharpoons c\text{C} + d\text{D}

the equilibrium constant in terms of molar concentrations is:

Kc=[C]c[D]d[A]a[B]bK_c = \frac{[\text{C}]^c [\text{D}]^d}{[\text{A}]^a [\text{B}]^b}

Square brackets denote equilibrium molar concentrations (mol/L). The exponents are the stoichiometric coefficients from the balanced equation. Pure solids and pure liquids are omitted because their concentrations are constant and absorbed into the value of Kc.

Interpreting Kc

Kc valueWhat it means
Kc >> 1Products strongly favored at equilibrium
Kc ≈ 1Reactants and products present in comparable amounts
Kc << 1Reactants strongly favored; little product forms

Worked example: the Haber process

The synthesis of ammonia is N₂ + 3H₂ ⇌ 2NH₃. At a particular temperature, equilibrium concentrations are measured as:

[N2]=0.50 M,[H2]=1.00 M,[NH3]=0.80 M[\text{N}_2] = 0.50\ \text{M}, \quad [\text{H}_2] = 1.00\ \text{M}, \quad [\text{NH}_3] = 0.80\ \text{M}

Substituting into the expression:

Kc=[NH3]2[N2]1[H2]3=(0.80)2(0.50)(1.00)3=0.640.50=1.28\begin{aligned} K_c &= \frac{[\text{NH}_3]^2}{[\text{N}_2]^1 [\text{H}_2]^3} \\[6pt] &= \frac{(0.80)^2}{(0.50)(1.00)^3} \\[6pt] &= \frac{0.64}{0.50} \\[6pt] &= 1.28 \end{aligned}

Because Kc > 1, products are favored under these conditions.

Excluding a species: setting a coefficient to zero

Any species with a stoichiometric coefficient of 0 contributes a factor of [X]⁰ = 1, which drops out of the expression. This makes it straightforward to handle reactions with fewer than two reactants or products — set the unused coefficient to 0 and the concentration value does not matter.

Kc versus Kp

For gas-phase reactions, chemists sometimes express the equilibrium constant in terms of partial pressures:

Kp=Kc(RT)ΔnK_p = K_c (RT)^{\Delta n}

where R = 0.08206 L·atm/(mol·K), T is temperature in kelvin, and Δn is the change in the total moles of gas (moles of gaseous products minus moles of gaseous reactants). When Δn = 0, Kp and Kc are numerically equal.

The reaction quotient Q

The reaction quotient Q has the same algebraic form as Kc, but it uses concentrations measured at any point in time — not necessarily at equilibrium. Comparing Q to Kc predicts the direction of reaction:

  • Q < Kc: reaction proceeds forward (toward products)
  • Q > Kc: reaction proceeds in reverse (toward reactants)
  • Q = Kc: system is at equilibrium

Effect of temperature

Kc is a function of temperature only. For an exothermic reaction, increasing temperature shifts equilibrium toward reactants and decreases Kc. For an endothermic reaction, increasing temperature shifts equilibrium toward products and increases Kc. Catalysts and concentration changes do not alter Kc; they only affect how quickly equilibrium is reached.

Frequently Asked Questions (FAQ)

What is the formula for the equilibrium constant?

For the general reaction aA + bB ⇌ cC + dD, the concentration-based equilibrium constant is Kc = ([C]^c · [D]^d) / ([A]^a · [B]^b), where the square brackets denote molar concentrations at equilibrium and the exponents are the stoichiometric coefficients from the balanced equation. Pure solids and pure liquids are excluded from the expression because their concentrations are constant and absorbed into Kc.

What does the value of Kc tell you?

Kc quantifies the ratio of product concentrations to reactant concentrations at equilibrium. Kc >> 1 means the reaction lies far to the right and products predominate; Kc << 1 means the reaction lies far to the left and reactants predominate; Kc ≈ 1 means both sides are present in comparable amounts. Kc depends on temperature but not on initial concentrations or on whether a catalyst is used.

What is the difference between Kc and Kp?

Kc uses molar concentrations (mol/L) while Kp uses partial pressures (atm or Pa) and applies to gas-phase reactions. They are related by Kp = Kc · (RT)^Δn, where R is the gas constant, T is temperature in kelvin, and Δn is the change in the total number of moles of gas (moles of gaseous products minus moles of gaseous reactants). When Δn = 0, Kp and Kc are numerically equal.

How does the reaction quotient Q differ from Kc?

The reaction quotient Q has the same algebraic form as Kc but uses concentrations measured at any point in time, not necessarily at equilibrium. Comparing Q to Kc tells you which direction the reaction will proceed: if Q < Kc, the reaction shifts right (toward products) to reach equilibrium; if Q > Kc, it shifts left (toward reactants); if Q = Kc, the system is already at equilibrium.

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