Equilibrium in Chemical Reactions

Chemical reactions often proceed in both forward and reverse directions. Over time, the rates become equal, establishing a dynamic equilibrium. At that point the ratio of product to reactant concentrations settles to a constant value known as the equilibrium constant. Understanding this balance enables chemists to predict how much product will form, how a reaction responds to temperature or pressure changes, and how catalysts shift the outcome.

The Law of Mass Action

The relationship between concentrations and the equilibrium constant was formulated in the nineteenth century by Cato Guldberg and Peter Waage. Their law of mass action states that for a reaction , the equilibrium constant is

$K=\frac{{\left[}^C{\left[}^D}{{\left[}^A{\left[}^B}$

The square brackets indicate molar concentrations and the exponents are the stoichiometric coefficients from the balanced reaction. This calculator accepts up to two reactants and two products, though real reactions can involve many species.

Using the Calculator

Enter the concentrations of reactants and products at equilibrium and specify their coefficients as they appear in the balanced chemical equation. If a species does not participate, you may leave its concentration blank. Upon clicking the Calculate button, the script multiplies the product concentrations raised to their coefficients and divides by the analogous term for reactants. The resulting equilibrium constant reveals the position of equilibrium—values much greater than one favor products, while those much less than one favor reactants.

Interpreting Kₑₗ

A large equilibrium constant means the reaction lies far to the right, producing substantial amounts of product. Conversely, a small constant indicates reactants predominate. Because the value depends on temperature, chemists often report Kₑₗ at a specific temperature. Le Chatelier's principle predicts that changing conditions shifts the equilibrium to counteract the stress, and the magnitude of Kₑₗ helps gauge the direction of that shift.

Example Table

Scenario	Reactant A (M)	Reactant B (M)	Product C (M)	Kₑₗ
Initial Mix	1.0	1.0	0.2	0.04
After Heating	0.5	0.5	0.8	2.56

Limitations

This calculator assumes ideal behavior and constant temperature. In real solutions, ionic strength, pressure, and non-ideal interactions can alter the effective concentrations. Nonetheless, the law of mass action provides a valuable first approximation. When precision matters, experimental measurement or advanced activity corrections are necessary.

Conclusion

The Equilibrium Constant Calculator simplifies the process of evaluating how far a reversible reaction proceeds. By entering a few concentrations and coefficients, you can test how different conditions or stoichiometries affect product yield. Because the computation runs entirely in your browser, it offers a quick and private way to explore chemical equilibria and plan experiments.