Last updated: July 3, 2026
Reaction Quotient Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The reaction quotient calculator computes Qc for aA + bB ⇌ cC + dD as ([C]^c × [D]^d) / ([A]^a × [B]^b) using current concentrations. If K is supplied at the same temperature, Q < K predicts a forward shift, Q > K predicts a reverse shift, and Q = K means equilibrium.
The reaction quotient Q equals product concentrations raised to their coefficients divided by reactant concentrations raised to their coefficients. Compare it with K: Q less than K shifts forward, Q greater than K shifts reverse, and Q equal to K is equilibrium.
Key Takeaways
- Q equals product concentration terms divided by reactant concentration terms.
- Stoichiometric coefficients become exponents in the reaction quotient expression.
- Q uses current concentrations; K uses equilibrium concentrations.
- Q < K predicts a forward shift, while Q > K predicts a reverse shift.
- Set omitted species coefficients to zero so their terms equal 1.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Q = ([C]^c × [D]^d) / ([A]^a × [B]^b)
Where:
- Q=Reaction quotient from current concentrations(dimensionless or concentration powers)
- [A], [B]=Current reactant concentrations(mol/L)
- [C], [D]=Current product concentrations(mol/L)
- a, b=Stoichiometric coefficients of reactants(dimensionless)
- c, d=Stoichiometric coefficients of products(dimensionless)
- K=Equilibrium constant at the same temperature(dimensionless or matching concentration powers)
Worked Examples
A + B ⇌ C + D at equilibrium
All coefficients are 1 and Q is compared with K = 4.
- 1Write Q = ([C]^1 × [D]^1) / ([A]^1 × [B]^1).
- 2Substitute concentrations: Q = (1.0 × 1.0) / (0.5 × 0.5).
- 3Calculate Q = 1 / 0.25 = 4; since Q = K = 4, the mixture is at equilibrium.
N₂ + 3H₂ ⇌ 2NH₃ shifts forward
Ammonia synthesis with current concentrations and K = 200.
- 1Write Q = [NH₃]^2 / ([N₂]^1 × [H₂]^3).
- 2Substitute values: Q = 0.4^2 / (0.2 × 0.2^3).
- 3Compute Q = 0.16 / 0.0016 = 100; because Q < K, the reaction shifts forward (right).
2A ⇌ B shifts reverse
A single-reactant, single-product reaction compared with K = 1.
- 1Set omitted species coefficients to zero so their terms equal 1.
- 2Write Q = [B]^1 / [A]^2 = 8 / 2^2.
- 3Calculate Q = 8 / 4 = 2; because Q > K, the reaction shifts reverse (left).
Introduction
The reaction quotient calculator evaluates Q for a reversible reaction using the concentrations present right now, before or at equilibrium. For aA + bB ⇌ cC + dD, Q = (C]^c × [D]^d) / ([A]^a × [B]^b). The expression is the same form as the [equilibrium constant, but Q uses current concentrations while K uses equilibrium concentrations. If you first need mol/L values, use the molarity calculator. This mass-action comparison follows the equilibrium framework described by IUPAC and standard general chemistry texts.
What is the reaction quotient?
The reaction quotient, usually written Q or Qc for concentration form, measures the current product-to-reactant ratio for a balanced reaction. It can be calculated immediately after mixing, during a reaction, or at equilibrium. A small Q means the current mixture has relatively more reactants than products; a large Q means relatively more products than reactants.
Q has the same algebraic form as Kc.
Q uses current concentrations, not necessarily equilibrium concentrations.
Q is positive when all included concentrations are positive.
Pure solids, pure liquids, and solvents are normally omitted from the expression.
Reaction quotient formula
For a balanced equation aA + bB ⇌ cC + dD, multiply product concentration terms in the numerator and reactant concentration terms in the denominator. Raise each concentration to its stoichiometric coefficient. This calculator supports up to two reactants and two products; set a coefficient to zero to omit that species.
A zero coefficient is treated as x^0 = 1, so the omitted species has no effect on Q.
How Q predicts reaction direction
Compare Q with the equilibrium constant K at the same temperature. If Q < K, the mixture has too few products relative to equilibrium and the reaction proceeds forward, to the right. If Q > K, there are too many products relative to equilibrium and the reaction proceeds reverse, to the left. If Q = K within experimental tolerance, the reaction is at equilibrium.
- Q < K:
forward shift, products form.
- Q > K:
reverse shift, reactants form.
- Q = K:
no net shift; dynamic equilibrium.
Use the same concentration convention for Q and K.
How to calculate Q step by step
First balance the chemical equation. Second, write the mass-action expression with products over reactants. Third, substitute the current molar concentrations and raise each to its coefficient. Finally, divide numerator by denominator and compare with K if a K value is known. For gases, related pressure-based expressions are handled by the Kp calculator.
Balance the equation before entering coefficients.
Use current concentrations, not guessed equilibrium values.
Keep all concentration units consistent, usually mol/L.
Check powers carefully when coefficients are greater than one.
Q versus Kc
Q and Kc are not different formulas; they differ by when the concentrations are measured. Q can change as the reaction progresses. Kc is fixed for a given reaction at a fixed temperature. When Q approaches Kc, the net reaction rate approaches zero even though forward and reverse microscopic processes continue. See the LibreTexts equilibrium chapter for detailed ICE-table examples.
If the mixture is already at equilibrium, Q calculated from those concentrations equals K within measurement uncertainty.
Common Q expression patterns
The table shows how coefficients become exponents in Q expressions. The exponents often dominate the result, especially for coefficients of 2 or 3.
| Reaction pattern | Q expression | Direction clue |
|---|---|---|
| A + B ⇌ C + D | [C][D]/([A][B]) | First-power ratio |
| N₂ + 3H₂ ⇌ 2NH₃ | [NH₃]²/([N₂][H₂]³) | Hydrogen term is cubed |
| 2A ⇌ B | [B]/[A]² | Reactant term is squared |
| A ⇌ 2C + D | [C]²[D]/[A] | Product coefficient changes Q strongly |
Common mistakes to avoid
Common errors include using unbalanced coefficients, comparing Q to a K value from a different temperature, entering equilibrium concentrations when the question asks for an initial Q, and including pure solids or pure liquids. If concentrations come from acid-base work, the pH calculator and buffer pH calculator can help check the chemistry before Q is evaluated.
Quick Reference Card
Reaction Quotient Q — Quick Reference
Quick reference • Reaction Quotient Calculator
Q = ([C]^c × [D]^d) / ([A]^a × [B]^b)Valid range: Q is positive for valid included concentrations; compare only with K at the same temperature.
Common Values
⚠ Watch Out
- •Use a balanced chemical equation before entering coefficients.
- •Compare Q only with a K value measured at the same temperature.
- •Do not include pure solids, pure liquids, or solvents in dilute aqueous expressions.
- •A zero concentration with a positive coefficient makes Q undefined; verify data.
- •Keep concentration units consistent across all species.
Pro Tips
- →Write the symbolic Q expression before substituting numbers.
- →Use mol/L concentrations from a molarity or concentration calculation.
- →Check powers carefully for coefficients greater than one.
- →Use Q < K, Q > K, and Q = K as the direction decision rule.
- →Round final Q values according to the precision of the concentration data.
FAQs
What is the reaction quotient Q?
Q is the product concentration terms divided by reactant concentration terms for the current mixture, with each term raised to its stoichiometric coefficient.
How is Q different from K?
Q uses concentrations at any moment. K uses equilibrium concentrations at a fixed temperature. When Q equals K, the reaction mixture is at equilibrium.
What does Q less than K mean?
Q < K means the mixture has too few products relative to equilibrium, so the net reaction shifts forward, or to the right, to make products.
What does Q greater than K mean?
Q > K means the mixture has too many products relative to equilibrium, so the net reaction shifts reverse, or to the left, to make reactants.
How do I omit a species in this calculator?
Set that species coefficient to 0. The calculator treats x^0 as 1, so the species is skipped in the numerator or denominator.
Can I use Q for gases or solids?
For gases, Q may be written with partial pressures as Qp. Pure solids and pure liquids are normally omitted because their activities are effectively constant.