βοΈ Equilibrium Constant Calculator
Kc, Kp, and reaction direction from Q.
How to Use This Calculator
For the reaction aA + bB β cC + dD, enter the stoichiometric coefficients and equilibrium values. The calculator returns K and interprets it.
The Formulas
Kp = Kc(RT)^Ξn | Ξn = (c+d) - (a+b) gases
Exponents come from the balanced equation coefficients. R = 0.08206 L atm/(mol K) when converting, with T in kelvin.
Worked Example
N2 + 3H2 β 2NH3 with [N2]=0.5, [H2]=0.3, [NH3]=0.8: Kc = 0.8^2 / (0.5 x 0.3^3) = 0.64 / 0.0135 = 47.4. Products favored.
What K Actually Means
K is the ratio of products to reactants when the reaction stops changing. K = 1e10 means essentially all product; K = 1e-10 means essentially all reactant. Near K = 1, significant amounts of both exist. But K says nothing about speed: a reaction with huge K can still take centuries if the activation energy is high, like diamond turning to graphite.
Le Chatelier's Principle
When Q differs from K, the reaction shifts to close the gap. Add reactant and the system makes more product; remove product and it makes more too. The Haber process for ammonia exploits this by condensing ammonia out of the gas mixture, continuously pulling the reaction forward. Temperature changes shift K itself: heating favors the endothermic direction.
Kp vs Kc
For gas reactions, Kp uses partial pressures and Kc uses concentrations, related by Kp = Kc(RT)^Ξn where Ξn is the change in gas moles. When Ξn is zero, Kp equals Kc. Heterogeneous equilibria omit solids and pure liquids since their concentrations are constant; they do not appear in the expression at all.
Estimating K from ΞG
Standard free energy and K are linked by ΞGΒ° = -RT ln K. A ΞGΒ° of -10 kJ/mol at room temperature means K is about 56; -30 kJ/mol means K near 180,000. This relationship lets you predict equilibrium position from thermodynamic tables without measuring concentrations directly.
Frequently Asked Questions
What is the difference between Kc and Kp?
Kc uses molar concentrations, Kp uses partial pressures. They are related by Kp = Kc(RT)^Ξn, where Ξn is the change in moles of gas. For reactions with no gas mole change, Kc equals Kp.
What does a large K mean?
K much greater than 1 means products dominate at equilibrium: the reaction essentially goes to completion. K much less than 1 means reactants dominate. K near 1 means significant amounts of both.
How do I predict which way a reaction shifts?
Compare the reaction quotient Q to K. If Q < K, the reaction shifts right toward products. If Q > K, it shifts left. If Q = K, the system is at equilibrium.
Do solids and liquids appear in K?
No. Pure solids and liquids have constant concentration, so they are omitted from the expression. Only gases and aqueous species appear. Water is omitted when it is the solvent.
How does temperature change K?
For exothermic reactions, raising temperature decreases K (shifts left). For endothermic, raising temperature increases K (shifts right). This is the van't Hoff equation in action.