How to use this calculator
- Enter the reaction and a phase for each species.
- Enter the current activity of each included species.
- Enter the equilibrium constant K.
- Calculate Q and read the predicted reaction direction.
Formula and variables
Q has the K expression; Q<K forward, Q≈K equilibrium, Q>K reverse
| Symbol | Meaning | Unit |
|---|---|---|
| Q | reaction quotient | activity-based |
| nu | stoichiometric coefficient | power |
Worked example
For N2(g) + 3 H2(g) ⇌ 2 NH3(g), the expression is [NH3]^2 / ([N2][H2]^3).
Interpretation
The comparison uses a documented relative tolerance instead of testing Decimal values for accidental exact equality.
Common mistakes
- Declare a phase for every equilibrium species.
- Activities and partial-pressure or concentration approximations must be positive.
Frequently asked questions
What does Q less than K mean?
The reaction can proceed forward to form more products under the stated conditions.
What does Q greater than K mean?
The reaction can proceed in reverse to form more reactants under the stated conditions.
Is Q calculated with the same expression as K?
Yes. Q uses current activities. K uses equilibrium activities.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.