Reaction Quotient Calculator

Calculate the reaction quotient (Q) for any reversible chemical reaction. Compare Q to the equilibrium constant Kc to predict the direction of the reaction.

For comparison with Q

Reaction Quotient Formula: For aA + bB ⇌ cC + dD: Q = [C]^c × [D]^d / [A]^a × [B]^b Comparing Q to Kc: • Q < Kc: Reaction shifts RIGHT (forward) • Q = Kc: System at equilibrium • Q > Kc: Reaction shifts LEFT (reverse) Special Cases: • Q = 0: Pure reactants (no products yet) • Q = ∞: Pure products (no reactants left) • Q = 1: Products and reactants in same ratio as coefficients
Example 1 (Basic Q Calculation): Reaction: N₂ + 3H₂ ⇌ 2NH₃ Coefficients: a=1, b=3, c=2, d=0 [N₂] = 0.5 M, [H₂] = 1.5 M, [NH₃] = 0.2 M Q = (0.2)² / (0.5 × 1.5³) = 0.04 / 1.6875 = 0.0237 Since Q < Kc (typically ~0.05), shift RIGHT Example 2 (At Equilibrium): [H₂] = 1 M, [I₂] = 1 M, [HI] = 2 M Kc = 50 Q = 2² / (1 × 1) = 4 Q < Kc, shift RIGHT Example 3 (Shift Left): 2NO₂ ⇌ N₂O₄ [NO₂] = 0.4 M, [N₂O₄] = 0.05 M Q = 0.05 / (0.4)² = 0.3125 If Kc = 0.2, then Q > Kc → shift LEFT

What is the reaction quotient Q?

The reaction quotient (Q) measures the relative amounts of products and reactants at any point during a reaction. For the general reaction aA + bB ⇌ cC + dD, Q = [C]^c[D]^d / [A]^a[B]^b. Unlike Kc (equilibrium constant), Q can have any value depending on current concentrations.

How does Q compare to Kc?

When Q < Kc: The reaction proceeds forward (right), making more products. When Q > Kc: The reaction proceeds in reverse (left), making more reactants. When Q = Kc: The system is at equilibrium. This comparison predicts the direction a reaction will shift to reach equilibrium.

What does it mean if Q is very large or very small?

A very large Q (>> Kc) means products heavily dominate and the reaction will shift left significantly. A very small Q (<< Kc) means reactants dominate and the reaction will shift right significantly. The larger the deviation from Kc, the stronger the drive to reach equilibrium.

Can Q be used to determine equilibrium concentrations?

Yes! Set Q = Kc and solve for unknown concentrations. For simple reactions, you can set up an ICE table (Initial, Change, Equilibrium) to find equilibrium concentrations. For more complex systems, you may need to solve systems of equations or use approximation methods.

What is the relationship between Q and Gibbs free energy?

ΔG = ΔG° + RT ln(Q), where ΔG° = -RT ln(K). At equilibrium, ΔG = 0 and Q = K. When Q < K, ΔG is negative (spontaneous forward). When Q > K, ΔG is positive (spontaneous reverse). The relationship shows that concentration affects reaction spontaneity.

🧮
Equilibrium Constant Calculator
🧮
Gibbs Free Energy Calculator
🧮
Concentration Calculator
🧮
Molarity Calculator