Chemistry
Equilibrium Constant Concentration Calculator
Calculate Kc, current Qc, the Q/K direction ratio, and a standard Gibbs-energy reference from concentration data.
Equilibrium product activity term-
Equilibrium reactant activity term-
Concentration equilibrium constant Kc-
Current product reaction term-
Current reactant reaction term-
Current reaction quotient Qc-
Qc / Kc direction ratio-
Standard ΔG° from Kc (kJ/mol)-
Decision view
Reaction quotient balance against the Kc equilibrium boundary
Reaction quotient balance against the Kc equilibrium boundaryCurrent composition sits on a logarithmic Q/K direction rail while reactant and product activity terms remain visible on opposite pans.
Exact scenario comparisonCurrent [C] for reaction quotient (mol/L) changes while all other entered assumptions remain constant.
| Current [C] for reaction quotient (mol/L) | Equilibrium product activity term | Equilibrium reactant activity term | Concentration equilibrium constant Kc | Current product reaction term | Current reactant reaction term | Current reaction quotient Qc | Qc / Kc direction ratio | Standard ΔG° from Kc (kJ/mol) |
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How to use Equilibrium Constant Concentration Calculator
- Use one balanced reaction direction.
- Enter equilibrium concentrations for Kc.
- Enter current concentrations for Qc at the same temperature.
Calculator guide
Understanding Equilibrium Constant Concentration Calculator
The equilibrium constant is built from equilibrium concentrations raised to balanced coefficients. A reaction quotient uses the same expression at a non-equilibrium composition.
Detailed calculation process
Detailed equilibrium-quotient calculation
The default reaction is A+B⇌C+D with equilibrium concentrations 0.4, 0.6, 1.1, and 0.9 M.
General formula: K_c=[C]^c[D]^d/([A]^a[B]^b)Q_c=[C]_q^c[D]_q^d/([A]_q^a[B]_q^b)R_Q=Q_c/K_cΔG°=-RT ln K_c The balanced coefficients become exponents. K uses equilibrium composition; Q uses the current state.
What each symbol means
Worked substitution with the default inputs
The default Kc is 4.125; Qc is below Kc, so the current composition favors net forward change.
Worked situations
Practical examples
- For default 1:1:1:1 coefficients, Kc=(1.1×0.9)/(0.4×0.6)=4.125.
- The current Qc is 0.357, so Q/K≈0.0866 and net forward change is favored.
Better inputs
Useful tips
- Omit pure solids and liquids from the expression.
- Use activities for rigorous concentrated systems.
- Keep K and Q tied to the same written reaction direction.
Before relying on the result
Limitations and common mistakes
- Concentrations substitute for activities.
- Temperature dependence, ionic strength, coupled reactions, and uncertainty are omitted.
- The Q/K ratio indicates direction, not rate or final time.
Reference
Key terms
- Kc
- Equilibrium reaction quotient expressed with concentration terms.
- Qc
- The same quotient evaluated at the current composition.
- Reaction direction
- Net composition change favored as Q approaches K.
Important note
Use activities, validated equilibrium measurements, temperature-specific constants, and full speciation for rigorous chemical equilibrium work.
Frequently asked questions
Does a large K mean a fast reaction?
No. K describes equilibrium position, not kinetics.
Why exclude solids?
The activity of a pure solid is conventionally one.
What happens if the equation is reversed?
The new equilibrium constant is 1/K for the reversed reaction.