Reaction Quotient Calculator

Determine reaction direction by comparing Q to equilibrium constant K

Reaction Quotient Calculator

Reaction Quotient (Q): Q = [products]ⁿ / [reactants]ᵐ
Compare Q to K to predict reaction direction

Enter K to compare with Q and determine which direction the reaction will proceed

Notes:

  • • Use molar concentrations (M) for aqueous species
  • • Pure solids and liquids are not included in Q
  • • For gases, can use partial pressures instead (Qp)
  • • Coefficients are the exponents from balanced equation

Understanding Reaction Quotient

The reaction quotient (Q) is calculated the same way as the equilibrium constant (K), but uses the current concentrations rather than equilibrium concentrations. Comparing Q to K tells us which direction a reaction will proceed to reach equilibrium.

The Reaction Quotient

For the general reaction: aA + bB ⇌ cC + dD

Q = [C]c[D]d / [A]a[B]b

Where [A], [B], [C], [D] are the current molar concentrations, and a, b, c, d are stoichiometric coefficients

Predicting Reaction Direction

Q < K

The ratio of products to reactants is too small. The reaction will proceed forward (→) to produce more products until Q = K.

Q = K

The system is at equilibrium. No net change will occur, though forward and reverse reactions continue at equal rates.

Q > K

The ratio of products to reactants is too large. The reaction will proceed reverse (←) to produce more reactants until Q = K.

Example: N₂O₄ ⇌ 2NO₂

Given: K = 4.7 × 10-3 at 25°C

Scenario 1: [N₂O₄] = 0.0500 M, [NO₂] = 0.0100 M

Q = [NO₂]² / [N₂O₄]

Q = (0.0100)² / (0.0500)

Q = 0.0001 / 0.0500 = 2.0 × 10-3

Q < K → Reaction goes forward

Scenario 2: [N₂O₄] = 0.0200 M, [NO₂] = 0.0300 M

Q = (0.0300)² / (0.0200)

Q = 0.0009 / 0.0200 = 4.5 × 10-2

Q > K → Reaction goes reverse

Important Considerations

Species TypeIncluded in Q?Notes
Aqueous ions/moleculesYesUse molarity (M)
GasesYesUse M or partial pressure
Pure solidsNoActivity = 1
Pure liquidsNoActivity = 1
Solvent (if dilute)NoNearly constant

Applications

  • ⚖️
    Predicting Shifts: Determine which way a reaction will proceed under current conditions
  • 🏭
    Process Control: Optimize industrial reactions by monitoring Q relative to K
  • 🧪
    Lab Planning: Design experiments to drive reactions toward desired products
  • 🌊
    Environmental Chemistry: Predict precipitation and dissolution in natural waters

⚖️Quick Reference

Q < K:

Forward reaction (→)

Q = K:

At equilibrium

Q > K:

Reverse reaction (←)

🔗Related Calculators

📐Related Formulas

🎯Where It's Used

  • 🏭

    Industry

    Process optimization

  • 🧪

    Research

    Reaction prediction