Example Data Table
| Reaction | Product Term | Reactant Term | Kc | Interpretation |
|---|---|---|---|---|
| H2 + I2 ⇌ 2HI | [HI]^2 = 1.20^2 | [H2][I2] = 0.50 × 0.40 | 7.2 | Products favored |
| N2 + 3H2 ⇌ 2NH3 | [NH3]^2 = 0.80^2 | [N2][H2]^3 = 0.25 × 0.50^3 | 20.48 | Products favored |
| 2SO2 + O2 ⇌ 2SO3 | [SO3]^2 = 1.50^2 | [SO2]^2[O2] = 0.80^2 × 0.70 | 5.0223 | Products favored |
Formula Used
General reaction: aA + bB ⇌ cC + dD
Kc = ([C]^c × [D]^d) / ([A]^a × [B]^b)
This calculator multiplies each product equilibrium concentration raised to its stoichiometric coefficient. It then divides by the equivalent reactant product. Blank rows are ignored, so you can model simpler reactions without changing the structure. Optional thermodynamic output uses ΔG° = -RT ln(Kc), where R = 8.314462618 J·mol⁻¹·K⁻¹.
Enter only species that appear in the equilibrium concentration expression. Pure solids and pure liquids are omitted because their activities are treated as constant. The signed log contribution values in the graph are useful because their total equals log10(Kc). That makes the graph a fast diagnostic tool for spotting which concentrations and coefficients dominate the final equilibrium constant.
How to Use This Calculator
- Enter the equilibrium temperature in kelvin.
- Fill the product species names, coefficients, and equilibrium concentrations.
- Fill the reactant species names, coefficients, and equilibrium concentrations.
- Leave any unused row blank so it stays out of the calculation.
- Click Calculate Kc to display the result above the form.
- Review the summary table, species contribution table, and graph.
- Use the CSV or PDF buttons to export the calculated report.
About This Kc Tool
The equilibrium constant Kc is one of the fastest ways to judge whether a reversible reaction prefers products or reactants at a given temperature. Because the expression uses concentrations raised to stoichiometric powers, even a modest concentration shift can noticeably change the final ratio. That is why a structured input layout helps when you want to inspect several species at once.
This page supports up to three product species and three reactant species. That keeps the form flexible for many textbook and laboratory problems while staying simple enough for fast checks. The summary section reports Kc and several related values. The reciprocal is useful when you want to think in the reverse direction, while ln(Kc) and log10(Kc) are convenient for thermodynamics and quick comparisons.
The contribution table turns each species into a powered term and a signed logarithmic contribution. Products contribute positively to the final log total, while reactants contribute negatively. When you view the graph, you can quickly see which side of the expression has the stronger influence. That helps when validating manual calculations or preparing worked examples for classes, homework, and lab notes.
FAQs
1. What does a large Kc mean?
A large Kc means equilibrium lies toward products at that temperature. It does not guarantee complete conversion, but products are favored over reactants when equilibrium is reached.
2. Can I include solids or pure liquids?
No. Pure solids and pure liquids are omitted because their activities are treated as constant. Only species that appear as concentration terms belong in the Kc expression.
3. Why are coefficients used as exponents?
Stoichiometric coefficients become exponents because the equilibrium expression comes from the balanced reaction equation. Changing a coefficient changes the power applied to that species concentration.
4. What is the difference between Kc and Qc?
Kc uses equilibrium concentrations only. Qc uses any current concentrations. Comparing Qc with Kc tells you whether a reaction will shift toward products or reactants.
5. Does Kc always have units?
Units can arise from the concentration exponents. Many advanced treatments use activities, which makes the equilibrium constant effectively dimensionless for thermodynamic work.
6. Why does temperature matter?
Temperature changes the equilibrium constant itself. A reaction can favor products at one temperature and reactants more strongly at another temperature.
7. Can concentrations be zero or negative?
No. This calculator requires positive concentration values because logarithms and powered equilibrium terms are not valid here for zero or negative inputs.
8. Why is ΔG° shown with Kc?
ΔG° links equilibrium to thermodynamics through the relation ΔG° = -RT ln(Kc). It helps you interpret whether product formation is favored under standard conditions.