Reaction Quotient (Q) Calculator

Q: What’s the difference between Q and K?

Q uses current amounts; K is the temperature-dependent equilibrium constant. Comparing Q to K predicts the shift direction.

Q: Should I include pure solids or liquids?

Typically no; their activities are ~1 and omitted.

Q: Do the units of Q matter?

Q is treated as dimensionless with activities. This tool approximates using concentrations for Qc or partial pressures for Qp.

Compute the reaction quotient Q from species amounts and stoichiometric coefficients for either Qc (using concentrations) or Qp (using partial pressures). See steps, a mini bar chart of contributions, and a log-scale Q vs K gauge to predict the shift direction.

Background

For a reaction aA + bB ⇌ cC + dD, the reaction quotient is Q = (a_C^c · a_D^d) / (a_A^a · a_B^b), where a_i denotes activity (approximated by concentration for Qc, or by partial pressure for Qp). Compare with K at the same temperature: if Q < K the reaction shifts forward; if Q > K it shifts backward; if Q ≈ K it is at equilibrium.

Enter reaction & measured amounts

Mode & display:

Qc (use concentrations)

Qp (use partial pressures)

Step-by-step working

Mini chart: species contributions

Log gauge: Q vs K

Round to sensible significant figures

Switch between Qc (mol·L⁻¹) and Qp (atm). Units are applied to all rows automatically.

Species (up to 6):

Name / formula

Amount

Role

Coeff

Use?

Leave a row unchecked to temporarily exclude it from Q. Coefficients must be positive. Pure solids/liquids are typically omitted (activity ≈ 1).

Equilibrium constant (optional):

Providing K enables the Q vs K gauge and shift prediction.

Quick picks:

Chips prefill rows, mode, and (optionally) K.

Result:

No results yet. Enter inputs and click Calculate.

How this calculator works

Choose Qc to use concentrations (mol·L⁻¹) or Qp to use partial pressures (atm).
For each species, set role (reactant/product), the coefficient (its stoichiometric exponent), and its measured amount. Uncheck a row to omit it.
Q is computed as product of (amount)^coeff for products divided by product of (amount)^coeff for reactants.
Optionally enter K to see whether the system tends to shift forward (Q < K) or backward (Q > K).

Example Problems & Step-by-Step Solutions

Example 1 — N₂ + 3H₂ ⇌ 2NH₃ (Qc)

Given [N₂] = 0.20 M, [H₂] = 0.60 M, [NH₃] = 0.10 M.
Q_c = [NH₃]² / ( [N₂] · [H₂]³ )
= (0.10)² / (0.20 · 0.60³) ≈ 0.694.
If K = 0.50 at this temperature, then Q > K → shifts left (toward reactants).

Example 2 — A ⇌ B (Qc)

Reaction: A ⇌ B (1:1).
Given [A] = 0.10 M, [B] = 0.01 M.
Q_c = [B] / [A] = 0.01 / 0.10 = 0.10.
If K = 1.0, then Q < K → reaction proceeds forward (toward B).

Example 3 — 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (Qp)

Given P_SO₂ = 0.30 atm, P_O₂ = 0.20 atm, P_SO₃ = 0.10 atm.
Q_p = (P_SO₃)² / ( (P_SO₂)² · P_O₂ )
= (0.10)² / (0.30)² · 0.20 ≈ 0.556.
If K_p = 2.5, then Q < K → shifts forward (toward SO₃).