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
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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