Reaction Quotient (Q) vs Keq Calculator

The reaction quotient (Q) has the same form as the equilibrium constant (Keq), but uses current (non-equilibrium) concentrations instead of equilibrium concentrations.

For aA + bB ⇌ cC + dD:

Q = [C]^c × [D]^d / ([A]^a × [B]^b)

Comparing Q to Keq:

• Q < Keq → Reaction proceeds forward (toward products) to reach equilibrium
• Q > Keq → Reaction proceeds reverse (toward reactants) to reach equilibrium
• Q = Keq → System is already at equilibrium, no net reaction

Intuition: Think of Q as the “current score” and Keq as the “target score.” If Q is too low, the reaction needs to make more products to reach Keq. If Q is too high, the reaction needs to reverse to consume excess products.

Example (Haber process: N₂ + 3H₂ ⇌ 2NH₃, Kc ≈ 3.5 × 10⁸ at 25°C): If [N₂] = 1.0 M, [H₂] = 2.0 M, [NH₃] = 0.5 M: Q = (0.5)² / (1.0 × (2.0)³) = 0.25 / 8 = 0.03125 Q = 0.031, which sits about ten orders of magnitude below Kc = 3.5 × 10⁸, so the reaction runs hard toward more NH₃.

That gap is worth pausing on. A Q sitting ten orders of magnitude below K does not mean the reaction is fast, only that it has a long way left to travel. Mix nitrogen and hydrogen in a flask on your bench and nothing measurable happens for years, because the nitrogen triple bond will not break at room temperature. Q tells you the destination and the direction. It has nothing to say about the journey.

How far from equilibrium, in energy terms

Combining ΔG = ΔG° + RT ln Q with ΔG° = −RT ln K collapses to one line:

ΔG = RT × ln(Q/K)

Negative ΔG means forward, positive means reverse, and zero means you are already there. The calculator reports this alongside the direction, so the answer is not only which way but how hard.

Le Chatelier’s principle in mathematical terms: When a system is disturbed (adding reactant, changing pressure, and so on), Q changes relative to Keq, and the system reacts to restore Q = Keq.

This calculator is also useful for:

• Checking if a precipitate will form (compare Q = ion product to Ksp)
• Predicting acid-base equilibrium shifts
• Determining direction of reaction in industrial processes