Definition of the Law of Chemical Equilibrium

Chemistry Glossary Definition of Law of Chemical Equilibrium

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Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Updated on June 13, 2018

When a chemical reaction is at equilibrium, the concentration of the reactants and the products remains the same over time. In other words, the forward and reverse chemical reaction are the same. Note: this does not mean the concentration of reactants and products is the same. There is a law that relates the concentration of reactants and products to the equilibrium constant.

Law of Chemical Equilibrium Definition

The Law of Chemical Equilibrium is a relation stating that in a reaction mixture at equilibrium, there is a condition (given by the equilibrium constant, Kc) relating the concentrations of the reactants and products. For the reaction:

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Chemical Equilibrium Definition
By Anne Marie Helmenstine, Ph.D.
aA(g) + bB(g) ↔ cC(g) + dD(g)

The equilibrium constant is calculated by the formula:

Kc = [ C ]c·[ D ]d / [ A ]a·[ B ]b

Equilibrium Constant Example

For example, for the chemical reaction:

2HI(g) ⇆ H2 + I2(g)

The equilibrium constant would be calculated by:

Kc = ([H2][I2] )/ [HI]2
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Helmenstine, Anne Marie, Ph.D. "Definition of the Law of Chemical Equilibrium." ThoughtCo, Aug. 27, 2020, thoughtco.com/definition-of-law-of-chemical-equilibrium-604407. Helmenstine, Anne Marie, Ph.D. (2020, August 27). Definition of the Law of Chemical Equilibrium. Retrieved from https://www.thoughtco.com/definition-of-law-of-chemical-equilibrium-604407 Helmenstine, Anne Marie, Ph.D. "Definition of the Law of Chemical Equilibrium." ThoughtCo. https://www.thoughtco.com/definition-of-law-of-chemical-equilibrium-604407 (accessed April 23, 2024).