If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Equilibria > Chemical Equilibria > The Equilibrium Constant > Calculating An Equilibrium Concentrations > Equilibrium

Copyright (c) 2006-2014 MindTouch Inc.

This file and accompanying files are licensed under the MindTouch Master Subscription Agreement (MSA).

At any time, you shall not, directly or indirectly: (i) sublicense, resell, rent, lease, distribute, market, commercialize or otherwise transfer rights or usage to: (a) the Software, (b) any modified version or derivative work of the Software created by you or for you, or (c) MindTouch Open Source (which includes all non-supported versions of MindTouch-developed software), for any purpose including timesharing or service bureau purposes; (ii) remove or alter any copyright, trademark or proprietary notice in the Software; (iii) transfer, use or export the Software in violation of any applicable laws or regulations of any government or governmental agency; (iv) use or run on any of your hardware, or have deployed for use, any production version of MindTouch Open Source; (v) use any of the Support Services, Error corrections, Updates or Upgrades, for the MindTouch Open Source software or for any Server for which Support Services are not then purchased as provided hereunder; or (vi) reverse engineer, decompile or modify any encrypted or encoded portion of the Software.

A complete copy of the MSA is available at http://www.mindtouch.com/msa


Dynamic equilibrium occurs when two opposing processes take place at equal rates and there are no net change in composition. Thus the rate Forward is equal to the rate Reverse. And the amounts of product equals amounts of reactant in both forward and reverse reactions.

Types of Equilibrium

Chemical Equilibrium Physical Equilibrium
Chemical equilibrium occurs when reaction changes chemically such as in decomposition Dynamic equilibrium when rates of forward and reverse reactions are equal
Example of Decomposition: When gaseous phosphorus pentachloride is heated, it decomposes to phosphorus trichloride and chlorine gas: PCl5(g)=> PCl3(g) +Cl2(g) Vapor Pressure- Example: in a closed container, vapor molecules condense to the liquid state at the same rate which liquid molecules vaporize. This is a dynamic equilibrium, because the molecules exchange phases at the same rate and the pressure exerted by the vapor remains constant with time.
  Solubility- Example: In a saturated solution, a solute is added to a solvent and the system reaches a point at which the rate of dissolution is equal to the rate at which the dissolved solute crystallizes.
  Distribution Coefficient- Example: When iodine,I2, is shaken with pure carbon tetrachloride, CCl4(l), the I2 molecules move into the CCl4 layer . As concentration of I2 builds up in the CCl4, the rate of I2 in which in returns to the water layer becomes significant. When the I2 molecules pass the two liquids at equal rates, dynamic equilibrium is reached, the concentration of I2 in each layer is constant.

Equilibrium Constant:


For a general reaction:

reference to undefined name 'math' Exception of type 'MindTouch.Deki.Script.Runtime.DekiScriptUndefinedNameException' was thrown. (click for details)


reference to undefined name 'math' Exception of type 'MindTouch.Deki.Script.Runtime.DekiScriptUndefinedNameException' was thrown. (click for details)

Why does K (constant) have no unit?

Because K is the activities of the reaction, it has no units.
Consider the reaction: CO(g)+ 2H2 (g)<=>CH3OH (g) at T=483K
[CO]=1.03M, [CH3OH]=1.56M
What is the equilibrium constant of H2?


Solution Calculations
First write the equilibrium constant expression in terms of activities K= (CH3OH/ (CO(H2)2))eq =14.5
Assume that reaction conditions are activities that can be replaced by their concentration values,
therefore concentration units are canceled out. K=([CH3OH]/CO([H2])2)eq =14.5
Substitute given concentrations into the equilibrium constant expression K= [CH3OH]/(CO[H2]2)=1.56/(1.03 [H2]2)=14.5
Solve for the unknown concentration, [H2]=activity(H2) x c° =0.322 x 1.00M=0.322 M [H2]2 =1.56/(1.03 x 14.5)=0.104
  [H2]=Square Root of 0.104= 0.322M

Determining the equilibrium constant, K

2O3 (g) <=> 3O2(g)
Product: O2
Reactant: O3
Notice how the coefficients become the powers, so you get [O2]3 for products and [O3]2 for the reactants.
Therefore you have:
K=[O2]3 / [O3]2

An Example of Equilibrium

Methanol is synthesized from a carbon monoxide-hydrogen mixture called sythesis gas in the following reaction:
CO(g) + 2H2 ---> CH3OH (g)
Methanol synthesis is a reversible reaction because at the same time CH3OH (g) is being formed, it decomposes to:
CH3OH(g)-->CO(g) +2H2(g) (Chemical Equilibrium)
When methanol synthesis decomposes to CO(g) and 2H2(g), it is classified as a chemical equilibrium. Because the forward and reverse reactions of methanol synthesis occur at equal rates it is a dynamic equilibrium and therefore can be represented with a double arrow <=>.
Rewritten as: CO(g)+ 2H2 (g)<=>CH3OH (g)

Le Chatlier's principle

Le Chatlier's Principle: When an equilibrium system is affected by changes in temperature , pressure, or concentration of a reacting species, the system responds by attaining a new equilibrium that corresponds to the impact of the changes.

Effect of Adding More of a Reactant to an Equilibrium Mixture

Predict the effect of adding more H2(g) to a constant-volume equilbrium mixture of N2, H2, NH3.

N2(g)+ 3H2(g) <=> 2NH3(g)

Notice that increasing [H2] stimulates the forward rection and a shift in the equilibrium concentration to the right. To learn more about Le Chatlier's Principle, please click the following link : external link: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch16/lechat.html


  1. Petrucci, R., Harwood, W., Herring, F., Madura, J., General Chemistry, 9th ed., Pearson, New Jersey, 2007
  2. Zumdahl, Steven; Zumdahl, Susan; DeCoste, Don, World of Chemistry, McDougal Littell, Boston, 2002
  3. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch16/lechat.html


  • Jackie Xie (UCD)

Last modified
09:26, 2 Oct 2013



(not set)
(not set)

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use