Reaction Quotient Calculator

Calculate reaction quotient Q and compare to Kc to predict reaction direction. If Q < K reaction goes forward; if Q > K reaction goes reverse; if Q = K system is at equilibrium.

⚖️ Equilibrium📐 Q = [products]^n / [reactants]^n🧪 Chemistry
Product concentration raised to powers
Reactant concentration raised to powers
Equilibrium constant K
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Reaction Quotient CalculatorSee formulaQ = [products]^n / [reactants]^ndimensionless

Step-by-Step Examples

Example 1
Q vs K Comparison

Reaction with K=2.96. Current: [NH3]^2=0.010, [N2][H2]^3=0.0200. Q=?

  • Q = 0.010/0.0200 = 0.500
  • Q (0.500) < K (2.96)
  • Reaction proceeds forward (makes more NH3)
✓ Q < K: forward reaction proceeds
Example 2
At Equilibrium

K = 53.9. Q = [HI]^2/([H2][I2]) = 53.9.

  • Q = 53.9 = K
  • System is at equilibrium
  • No net reaction occurs
✓ Q = K: at equilibrium, no net change
Example 3
Reverse Reaction

K = 0.0040. Q = 0.025.

  • Q (0.025) > K (0.0040)
  • Reaction proceeds in reverse
  • PCl3+Cl2 recombine to form PCl5
✓ Q > K: reverse reaction proceeds

Real-World Applications

⚖️
Reaction Engineering
Q vs K tells engineers whether more product can form or if reaction has already passed equilibrium.
🌍
Environmental Chemistry
Q for CO2 dissolution predicts direction of ocean acidification reactions.
🧪
Lab Chemistry
Q predicts whether adding more reactant will shift equilibrium to make more product.
💊
Biochemistry
Cellular reaction quotient (often written Gamma) vs K tells cells if a reaction is thermodynamically favorable.

Common Mistakes to Avoid

⚠️
Q uses current concentrations, K uses equilibrium

Q can be calculated at any moment. K is only the value when the system reaches equilibrium.

⚠️
Same expression as K

Q and K have identical mathematical form. The difference is Q uses any concentrations; K uses equilibrium concentrations.

⚠️
Q predicts direction, not speed

Q vs K tells direction of net reaction. Kinetics (rate laws) determines how fast equilibrium is reached.

Frequently Asked Questions

How is Q different from K? ▾
Q uses current (non-equilibrium) concentrations. K uses equilibrium concentrations only. Same formula, different input values.
What does Q/K tell us? ▾
Q/K < 1: must make more products to reach equilibrium. Q/K > 1: must make more reactants. Q/K = 1: at equilibrium.
Can Q change over time? ▾
Yes. As reaction proceeds, concentrations change and Q moves toward K. At equilibrium, Q = K.
What is dG in terms of Q and K? ▾
dG = RT ln(Q/K) = dG-deg + RT ln Q. When Q = K: dG = 0 (equilibrium). When Q < K: dG < 0 (forward spontaneous).
When is Q useful in industry? ▾
Chemical plants continuously monitor Q to determine if conditions are optimal for product formation, and adjust T, P, or concentrations accordingly.

Related Chemistry Calculators

⚖️ Equilibrium Constant⚗️ Kp to Kc⚖️ Le Chatelier⚡ Gibbs Free Energy💧 Ksp Calculator

Formula Explorer connections

Interpretation: This formula describes how reactants, products, ions or phases distribute when opposing processes reach equilibrium. Assumption: Use equilibrium rather than initial concentrations, correct stoichiometric exponents, and the specified temperature; activities may replace concentrations in nonideal systems.

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