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Equilibrium Calculator

Enter product and reactant concentrations with their coefficients to calculate the reaction quotient Q. Optionally provide Keq to determine reaction direction.

Format: concentration:coefficient separated by commas. Example: 0.5:2, 0.3:1

Chemical equilibrium

A reversible reaction reaches equilibrium when the forward and reverse rates match and concentrations stop changing. The equilibrium constant Keq captures the ratio of product to reactant activities at that point, with each species raised to its stoichiometric coefficient. For the generic reaction aA + bB reversibly forming cC + dD, Keq equals [C]^c[D]^d divided by [A]^a[B]^b. Pure solids and pure liquids drop out of the expression because their activities are defined as 1.

This calculator handles the three problem types you encounter with that expression: computing Keq from measured equilibrium concentrations, computing Q from current concentrations to predict reaction direction, and solving for equilibrium concentrations when you know Keq and starting amounts. The third case is where the ICE-table algebra lives — Initial, Change, Equilibrium rows scaled by coefficients, plugged into the Keq expression, then solved for the change variable x.

What the calculator does

The widget runs three modes. To find Q, enter current concentrations and it computes the mass-action ratio and compares it to Keq — Q below Keq means net forward reaction, Q above means net reverse. To find Keq, enter equilibrium concentrations and it reads off the constant. To find equilibrium concentrations, enter Keq plus initial concentrations and it builds the ICE table, sets up the polynomial in x, and solves it. The small-x approximation kicks in when Keq is small enough to justify it; otherwise the full quadratic (or higher) is solved.

Worked examples

Calculating Keq. For N2 + 3H2 reversibly forming 2NH3 with [N2] = 0.50 M, [H2] = 0.30 M, [NH3] = 0.20 M:

  • Keq = (0.20)^2 / (0.50 × (0.30)^3) = 0.040 / 0.0135 = 2.96

Comparing Q to Keq. Same reaction, Keq = 2.96. Current state [N2] = 1.0, [H2] = 1.0, [NH3] = 0.50:

  • Q = 0.25 / 1.0 = 0.25 < Keq, so the system shifts forward toward more NH3.

ICE table for weak-acid dissociation. HA reversibly dissociating to H+ and A-, Ka = 1.8 × 10^-5, [HA]_0 = 0.10 M.

HAH+A-
I0.1000
C-x+x+x
E0.10 - xxx
  • Ka = x^2 / (0.10 - x) = 1.8 × 10^-5
  • Small-x approximation valid (Ka × 100 << initial concentration): x^2 ≈ 1.8 × 10^-6
  • x = [H+] = 1.34 × 10^-3 M, pH = 2.87

Kp from Kc. For 2SO3 reversibly forming 2SO2 + O2 at 1000 K, Kc = 4.08 × 10^-3:

  • Δn = (2 + 1) - 2 = 1
  • Kp = Kc(RT)^Δn = 4.08 × 10^-3 × 82.06 = 0.335

Le Chatelier and the direction of shift

Stress the system and it shifts to partially undo the stress. Add reactant or remove product → forward shift. Compress a gas-phase reaction → shift toward the side with fewer moles of gas. Heat an exothermic reaction → shift backward (treating heat as a “product”). Catalysts change neither Keq nor the equilibrium position; they only shorten the time to reach it.

Where the math shows up

Acid–base chemistry (Ka, Kb, Kw), solubility (Ksp), gas-phase industrial reactions (Haber, contact process), enzyme–substrate binding, and the carbonate equilibria that drive ocean acidification all use the same Keq machinery. The expression and the ICE-table method don’t change; only the species and the value of K do.

Frequently Asked Questions

What is the equilibrium constant (Keq)?
Keq is the value of the mass-action expression at equilibrium: products over reactants, each concentration raised to its stoichiometric coefficient. For aA + bB reversibly forming cC + dD, Keq equals [C]^c[D]^d divided by [A]^a[B]^b. Pure solids and pure liquids drop out because their activities are 1, so only aqueous and gaseous species appear in the ratio.
What is the difference between Keq and Q?
Q uses the same algebraic form as Keq but with whatever concentrations you have right now, not equilibrium values. Comparing the two tells you which way the reaction will move. Q below Keq means there are too few products, so the net reaction proceeds forward. Q above Keq means too many products, so it shifts in reverse. When Q equals Keq, the system is at equilibrium and net change stops.
What is an ICE table?
ICE stands for Initial, Change, Equilibrium. It is a bookkeeping grid: the top row holds starting concentrations, the middle row tracks the change in terms of a single unknown x scaled by stoichiometric coefficients, and the bottom row sums the two. You then plug the bottom row into the Keq expression and solve for x. It is the standard way to convert a Keq value plus initial conditions into actual equilibrium concentrations.
Are pure solids and liquids included in the Keq expression?
No. The convention defines activity as 1 for any pure condensed phase, so a solid like CaCO3 or a pure liquid like water in a non-aqueous reaction simply does not appear. Only species whose concentration or partial pressure can vary, meaning aqueous solutes and gases, get written into the ratio. This is why Ksp expressions for ionic solids contain only the dissolved ions.
How does temperature affect Keq?
Keq is temperature-dependent because the underlying free energy term is. For an exothermic reaction, raising temperature pushes equilibrium toward reactants and Keq decreases; for an endothermic reaction the opposite holds. The van't Hoff equation makes this quantitative by relating the change in ln(Keq) to the enthalpy of reaction divided by RT squared, integrated over the temperature change.