Reaction Quotient (Q) vs Keq Calculator
Calculate the reaction quotient Q and compare it to Keq to predict which direction a reaction will proceed.
Uses up to 2 reactants and 2 products.
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, Keq = 977 at 500°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 = 0.031 Q = 0.031 « Keq = 977 → reaction proceeds forward to make more NH₃
Le Chatelier’s principle in mathematical terms: When a system is disturbed (adding reactant, changing pressure, etc.), 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