Le Chatelier's principle

The Le Chatelier's principle, also called the principle of least constraint was formulated by Henry Le Chatelier and Ferdinand Braun 1884-1888:

Or more specifically:

The principle is thus very general, so that it does not allow quantitative statements. Nevertheless, it is often used as a qualitative prediction in many areas for the first steps is sufficient. Furthermore, it is very easy to use.


  • The forced temperature increase is dodged with heat consumption.
  • The forced removal of heat is counteracted by heat production.

" Constraints " in this sense changes in temperature, pressure or concentration:

  • Increasing the temperature, the heat supplying reaction is suppressed, and vice versa.
  • If you increase the pressure, the system differs from so that the volumenverkleinernde reaction is promoted and vice versa.
  • If you change the concentration, eg by removing a product from the reaction mixture, the equilibrium system responds by this product is reproduced.

The accuracy of this approach can be confirmed either empirically, that is in the experiment, as well as by calculation of temperature, pressure and concentration dependence of the free enthalpy.

Temperature change

Heat supply or removal of heat cause a shift in equilibrium, that is, setting a new equilibrium with altered concentrations. Heat extraction of heat- supplying (exothermic ) reaction, heat the heat consuming ( endothermic ) reaction. Characterized the change in temperature of the system falls lower than without the equilibrium shift.

A temperature change will always result in a change in the equilibrium concentration. Concentration which thereby increases or decreases, depending on whether the formation of the exothermic or endothermic products:

As an example, the gas mixture can be used from the equilibrium between the brown nitrogen dioxide and the colorless dinitrogen tetroxide:

The enthalpy of the forward reaction is, i.e., is an exothermic reaction, as energy is released. The reverse reaction is endothermic.

We now increase the temperature at constant volume, the reaction in the opposite, ie is run in the endothermic direction, so the equilibrium shifts to the left, the gas mixture is darker. Reduction in temperature causes the exothermic reaction whereby the equilibrium shifts to the right and the gas mixture lightening.

Volume or pressure change

The chemical equilibrium reactions where no gases are involved, is hardly affected by a change in volume caused by the outside. If, however, gaseous substances involved, the balance is only affected when the particle number changes in the gas phase by the equilibrium shift.

A pressure change affects only in a closed system at equilibrium from. Depending on the reaction condition can detect a change in pressure or a change in volume: the system reduces the pressure generated by a volume decrease by proceeds in favor of the side which has the lower number of particles and therefore requires the smaller volume. Thus, the pressure increase is significantly less than if the gases from strong would be able to no response. Accordingly, an increase in volume shifts the equilibrium toward larger particle numbers.

The position of the equilibrium may be affected by a pressure increase from outside:

  • At constant reaction volume by further supply of reactants
  • Upon changing the reaction volume by compression.

The reaction takes place in an open system, the gas produced during the reaction may continuously escape. This new gas is constantly being produced, which in turn escapes. This imbalance means that it can not adjust: the reaction proceeds completely to the product page.

A known reaction is the production of ammonia in the Haber- Bosch process, of nitrogen and hydrogen:

So there are 4 gas molecules on the educt left, 2 gas molecules on the product page on the right. If the pressure is increased, the system is different to the volumenverkleinernde side - so that fewer molecules - for. Thus, by increasing the pressure favor the formation of ammonia.

The same principle can also be applied to the nitrogen dioxide - dinitrogen tetroxide equilibrium.

Material change in quantity

By supply or removal of a reaction partner, the equilibrium is disturbed, thus, the reaction proceeds until equilibrium is again reached, increasing in one direction. By changing the concentration of one of the substances involved in the equilibrium, thereby also changing the concentrations of all other partners. Is an equilibrium reaction to proceed to completion in favor of the product, it suffices to multiply one of the reactants from the reaction mixture or to remove one of the products from the reaction mixture. While the reverse reaction is suppressed thereby, until the original balance is restored.

Since the equilibrium is dependent only on the temperature and optionally the pressure, the reaction is carried out according to the change in concentration so that the initial equilibrium is restored. For an equilibrium reaction:


Can be distinguished in the following cases:

A change in the reaction conditions of temperature and pressure results in a displacement of the equilibrium and hence a change in the equilibrium concentration.

The influence of changes in the amount of substance on the position of equilibrium can be illustrated by the esterification of carboxylic acids and the hydrolysis of carboxylic acid esters.

When a carboxylic acid is dissolved in an alcohol, the system is initially not in equilibrium. The balance has been set ( eg by adding a catalyst or after - very - long wait) so the amount of alcohol due to the large excess has hardly changed; it has the ester and a corresponding amount of water formed and it is a very small amount of carboxylic acid left. Drains in addition to the equilibrium by distillation, for example, the ester, the ester is simulated due to the mass action law. The addition of sulfuric acid as a catalyst, can also affect the balance by binding of the resulting water. One can therefore very well based optimize the yield of the ester in this way on the carboxylic acid.

On the other hand the effect, an addition of water to the reaction mixture, a shift of the equilibrium to the side of the reactants.

This means that you can by adding a component in excess taxes ( alcohol or water ) that the product predominates (ester or acid) in equilibrium.

Other examples are:

  • Precipitation reactions
  • Eisenrhodanid reaction
  • Calcite saturation (Previously: lime and carbonic acid )

Combination of temperature, pressure and amount of substance change

A very good example of the influence of external conditions on a production process, the above-indicated Haber- Bosch process. As described, the pressure increase causing the yield of products. However, since the reaction requires a high activation energy and the reaction is exothermic, the balance of the high temperature on the reactant side, ie, to the higher volumes moved. In the process optimization therefore an optimum combination of pressure and temperature must be found. In the Haber- Bosch process, this reaction is therefore bar pressure and a temperature of 550 ° C carried out at about 300. In addition, the yield by removing the ammonia, thus reducing the amount of substance in the reaction mixture increases.

Effects of light

A photostationary equilibrium is a state which is generated by light irradiation in the visible or ultraviolet range. Examples are photochromic equilibria with certain dyes. Changes depending on the light of the state of equilibrium and hence the color. The reverse reaction can be made either by light or heat. Irradiation for example, colorless spiropyran with UV light, as occurs with the formation of merocyanine, an isomeric compound of spiropyran, an intense blue color is produced. In this case, one speaks of an isomeric equilibrium. Equilibration is generally determined by the wavelength of light. So the orange triphenyl fulgide stained bluish when irradiated with short-wavelength light when irradiated with red light turns back the original color.