This equilibrium constant calculator computes Kc for aA + bB ⇌ cC + dD from equilibrium concentrations, converts to Kp, and compares the reaction quotient Q against K to call the direction of change.
Reaction: aA + bB ⇌ cC + dD — enter coefficients and equilibrium concentrations (mol/L). Set a coefficient to 0 to drop a species.
| Kc | – |
| Kp = Kc(RT)^Δn | – |
| Reading | – |
| Q vs K | – |
K: The Score at Which Forward and Reverse Rates Shake Hands
The rate side is our Arrhenius Equation Calculator. Part of our Chemistry Calculators collection.
Worked Example (Verified)
- N₂O₄ ⇌ 2 NO₂ (defaults: a 1, c 2, [N₂O₄] 0.01, [NO₂] 0.02): Kc = 0.02²/0.01 = 0.040 · Δn = +1 → Kp = 0.04 × (0.08206 × 298) = 0.978 at 298 K
- K’s size is the summary: K > 10³ ≈ products win, K < 10⁻³ ≈ reactants win, between them a genuine mixture — the defaults’ 0.04 leans reactant-favored with a substantial NO₂ minority
- Pure solids and liquids never appear in K expressions (their “concentration” is constant and folded into K) — only gases and aqueous species earn exponents here
Equilibrium Discipline
- Exponents are the stoichiometric coefficients — products over reactants, each raised to its coefficient. The single most-graded formula in general chemistry.
- Q is the same expression at ANY moment; comparing Q with K predicts direction: Q < K → right, Q > K → left, equality → equilibrium.
- K changes only with temperature (van ‘t Hoff for how) — catalysts and concentration changes move rates and positions, never K itself.
- Kp = Kc(RT)^Δn counts gas moles only (Δn from gas-phase coefficients); when Δn = 0, Kp = Kc exactly.
Related Tools
- Arrhenius Equation Calculator — the rate side
- Solubility Product Calculator — K for dissolving
- Chemistry Calculators — the full collection
Frequently Asked Questions
How do you calculate the equilibrium constant?
Kc = ([C]^c [D]^d) / ([A]^a [B]^b) at equilibrium — products over reactants, each concentration raised to its stoichiometric coefficient. Example: N₂O₄ ⇌ 2NO₂ with [NO₂] 0.02 and [N₂O₄] 0.01 → Kc = 0.02²/0.01 = 0.040. Only gases and dissolved species appear.
How are Kp and Kc related?
Kp = Kc(RT)^Δn, with R = 0.08206 L·atm/(mol·K), T in kelvin, and Δn = gas moles of products − gas moles of reactants. The example above: Δn = 1, so Kp = 0.04 × 24.45 = 0.978 at 298 K. This calculator reports both.
What does Q tell you?
Q is K’s formula evaluated at current (non-equilibrium) concentrations. Q < K: too little product — the reaction runs forward. Q > K: overshot — it runs in reverse. Q = K: equilibrium. Enter a Q beside your K here for the call.
Does a catalyst change K?
No. A catalyst accelerates forward and reverse reactions equally (it lowers both activation barriers), so equilibrium arrives sooner at the same position. K depends on the reaction and temperature only.
Why aren’t solids in the K expression?
A pure solid’s “concentration” (its density ÷ molar mass) is constant regardless of how much sits there, so it’s absorbed into K’s value — activities of pure solids and liquids are defined as 1. Adding more solid CaCO₃ doesn’t shift CaCO₃ ⇌ CaO + CO₂; only CO₂ pressure does.
Is this equilibrium constant calculator free?
Yes — free, instant, no sign-up.