5.0000e+0
0.699
-3.99
kJ/mol
5.0000e-1
1.0000e-1
5.0000e+0
0.699
-3.99
kJ/mol
5.0000e-1
1.0000e-1
The Equilibrium Constant Calculator determines the equilibrium constant (Keq) for a reversible chemical reaction based on the concentrations of products and reactants at equilibrium. The equilibrium constant is a fundamental quantity in chemistry that describes the ratio of product concentrations to reactant concentrations, each raised to their respective stoichiometric coefficients. A large Keq indicates that products are favored at equilibrium, while a small Keq means reactants are favored. This calculator also computes the standard Gibbs free energy change (ΔG°) from Keq using the thermodynamic relationship ΔG° = −RT ln K. Understanding equilibrium constants is essential for predicting reaction outcomes, designing chemical processes, and analyzing acid-base, solubility, and redox equilibria in both academic and industrial settings.
The equilibrium constant expression is derived from the law of mass action:
$$K_{eq} = \frac{[C]^c [D]^d}{[A]^a [B]^b}$$
where [A], [B] are reactant concentrations and [C], [D] are product concentrations at equilibrium, with a, b, c, d as stoichiometric coefficients. The calculator evaluates the numerator as the product of all product concentrations raised to their coefficients, and the denominator as the product of all reactant concentrations raised to their coefficients.
The relationship between Keq and Gibbs free energy is:
$$\Delta G^\circ = -RT \ln K_{eq}$$
where R = 8.314 J/(mol·K) and T = 298 K (standard conditions). A negative ΔG° indicates a thermodynamically favorable reaction. The logarithmic form log(Keq) helps compare orders of magnitude across different equilibria. Concentrations set to zero are excluded from the calculation, allowing flexible use for reactions with fewer species.
If Keq > 1, the reaction favors product formation at equilibrium. If Keq < 1, reactants are favored. A very large Keq (e.g., 1010) indicates the reaction goes nearly to completion. The log(Keq) value provides a convenient scale: positive values mean products dominate, negative values mean reactants dominate. The ΔG° value at 298 K tells you the thermodynamic spontaneity under standard conditions — negative ΔG° means spontaneous in the forward direction.
Inputs
Results
With [B] = 0.8 M and [A] = 0.2 M: Keq = 0.8/0.2 = 4.0. Since Keq > 1, products are favored. ΔG° = −8.314 × 298 × ln(4)/1000 = −3.44 kJ/mol (spontaneous).
Inputs
Results
Keq = (0.3² × 0.5)/(0.4 × 0.6³) = (0.09 × 0.5)/(0.4 × 0.216) = 0.045/0.0864 = 0.5208. Since Keq < 1, reactants are favored at equilibrium.
The equilibrium constant (Keq) is a dimensionless number that describes the ratio of product concentrations to reactant concentrations at chemical equilibrium, each raised to their stoichiometric coefficients. It is a fixed value at a given temperature for a specific reaction.
No. Keq is independent of initial concentrations. It only changes with temperature. Altering concentrations shifts the equilibrium position (Le Chatelier's principle) but does not change the K value.
Kc uses molar concentrations (mol/L) while Kp uses partial pressures (atm or Pa). They are related by Kp = Kc(RT)Δn, where Δn is the change in moles of gas.
A very large Keq (e.g., > 103) means the reaction strongly favors products at equilibrium. The forward reaction goes nearly to completion under standard conditions.
The relationship is ΔG° = −RT ln Keq. A large K corresponds to a negative ΔG° (spontaneous), while a small K corresponds to a positive ΔG° (non-spontaneous under standard conditions).
No. Since Keq is a ratio of concentrations (which are always positive), Keq is always a positive number. It can be very small (close to zero) but never negative.
When Keq = 1, the concentrations of products and reactants are comparable at equilibrium. ΔG° = 0, meaning neither direction is thermodynamically preferred under standard conditions.
For exothermic reactions, increasing temperature decreases Keq. For endothermic reactions, increasing temperature increases Keq. This follows from the van 't Hoff equation.
No. Pure solids and pure liquids have activity = 1 and are excluded from the equilibrium expression. Only aqueous species and gases are included.
Formally, Keq is dimensionless because it is defined using activities (ratios to standard states). In practice, Kc may carry concentration units depending on Δn, but the thermodynamic K is unitless.
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