The principle of Le Chatelier describes the response of a system in equilibrium to counteract the effects caused by an external agent. It was formulated in 1888 by the French chemist Henry Louis Le Chatelier. It is applied to any chemical reaction that is capable of reaching equilibrium in closed systems.

What is a closed system? It is one where there is the transfer of energy between its borders (for example, a cube), but not of matter. However, to exert a change in the system it is necessary to open it, and then close it again to study how it responds to the disturbance (or change).

Henry Louis Le Chatelier

Once closed, the system will return to equilibrium and its way of achieving this can be predicted thanks to this principle. Is the new equilibrium the same as the old one? It depends on the time to which the system is subjected to external disturbance; if it lasts long enough, the new equilibrium is different.

What does it consist of?

The following chemical equation corresponds to a reaction that has reached equilibrium:

aA + bB <=> cC + dD

In this expression a, b, c and d are the stoichiometric coefficients. Since the system is closed, no reactants (A and B) or products (C and D) enter from outside that disturb the equilibrium.

But what exactly does balance mean? When this is set, the rates of the forward (to the right) and reverse (to the left) reaction equalize. Consequently, the concentrations of all species remain constant over time.

The above can be understood in this way: as soon as a little A and B react to produce C and D, they react with each other at the same time to regenerate the A and B consumed, and so on while the system remains in equilibrium.

However, when a disturbance is applied to the system -whether by the addition of A, heat, D or by reducing the volume-, Le Chatelier's principle predicts how it will behave to counteract the effects caused, although it does not explain the mechanism molecular by allowing it to return to equilibrium.

Thus, depending on the changes made, the sense of a reaction can be favored. For example, if B is the desired compound, a change is exerted such that the equilibrium shifts to its formation.

Factors that modify the chemical balance

To understand Le Chatelier's principle, an excellent approximation is to assume that equilibrium consists of a scale.

Seen from this approach, the reagents are weighed on the left pan (or basket) and the products are weighed on the right pan. From here, the prediction of the response of the system (the balance) becomes easy.

Changes in concentration

aA + bB <=> cC + dD

The double arrow in the equation represents the stem of the balance and the underlined the pans. So if a quantity (grams, milligrams, etc.) of A is added to the system, there will be more weight on the right pan and the balance will tilt to that side.

As a result, the C + D saucer rises; in other words, it gains importance compared to dish A + B. In other words: with the addition of A (as with B) the balance shifts products C and D upwards.

In chemical terms, the equilibrium ends up shifting to the right: towards the production of more C and D.

The opposite occurs if amounts of C and D are added to the system: the left pan becomes heavier, causing the right pan to lift.

Again, this results in a rise in the concentrations of A and B; therefore, an equilibrium shift to the left is generated (the reactants).

Changes in pressure or volume

aA (g) + bB (g) <=> cC (g) + dD (g)

Changes in pressure or volume caused in the system only have notable effects on species in the gaseous state. However, for the higher chemical equation none of these alterations would modify the equilibrium.

Why? Because the number of total moles of gas on both sides of the equation is the same.

The balance will seek to balance the pressure changes, but since both reactions (direct and inverse) produce the same amount of gas, it remains unchanged. For example, for the following chemical equation the balance does respond to these changes:

aA (g) + bB (g) <=> eE (g)

Here, in the face of a decrease in volume (or increase in pressure) in the system, the balance will raise the pan to reduce this effect.

How? Decreasing the pressure, through the formation of E. This is because, as A and B exert more pressure than E, they react to decrease their concentrations and increase that of E.

Likewise, the Le Chatelier principle predicts the effect of increasing volume. When this occurs, the balance then needs to counteract the effect by promoting the formation of more gaseous moles that restore the loss of pressure; this time, shifting the balance to the left, lifting pan A + B.

Temperature changes

Heat can be considered both reactive and product. Therefore, depending on the enthalpy of reaction (ΔHrx), the reaction is exothermic or endothermic. Then the heat is placed on the left or right side of the chemical equation.

aA + bB + heat <=> cC + dD (endothermic reaction)

aA + bB <=> cC + dD + heat (exothermic reaction)

Here, heating or cooling the system generates the same responses as in the case of changes in concentrations.

For example, if the reaction is exothermic, cooling the system favors the displacement of equilibrium to the left; while if it is heated, the reaction continues with a greater tendency to the right (A + B).

Applications

Among its innumerable applications, given that many reactions reach equilibrium, there are the following:

In the process of Haber

N ₂ (g) + 3H ₂ (g) <=> 2NH ₃ (g) (exothermic)

The upper chemical equation corresponds to the formation of ammonia, one of the major compounds produced on an industrial scale.

Here, the ideal conditions for obtaining NH ₃ are those in which the temperature is not very high and, likewise, where there are high levels of pressure (200 to 1000 atm).

In gardening

The purple hydrangeas (top image) establish a balance with the aluminum (Al ³⁺) present in the soils. The presence of this metal, Lewis acid, results in their acidification.

However, in basic soils the flowers of hydrangeas are red, because aluminum is insoluble in these soils and cannot be used by the plant.

A gardener familiar with the Le Chatelier principle could change the color of his hydrangeas by cleverly acidifying the soils.

In the formation of caves