Definition of Le Chatelier's Principle

Disturbances

When we speak of disturbances we refer to changes that can alter the Balance of a chemical reaction. Basically, Le Chatelier explains the influence of three factors specifically that can be disturbed, they are: the temperature, the pressure and the concentration of the species at stake. Next, we will see how the effect produced against the different shocks is studied.

Temperature

Temperature will affect the equilibrium constant of the chemical reaction. In this sense, if there is an increase in temperature, the system will seek to evolve in such a way as to counteract that increase in temperature. If we are in the case where a reaction is exothermic in the direct sense, the release of

caloric energy It will not help to counteract the observed temperature increase, so it will tend to evolve in the opposite direction, with the reverse reaction prevailing over the direct one.

Similarly, suppose an endothermic reaction in the direct sense, where heat can be seen as a reactant, in the face of an increase in temperature. it will tend to consume that heat to counteract the effect of the increase in temperature, so the system evolves towards products, in the direct sense.

Pressure

Pressure and volume have a similar treatment, since, as is known, they are inversely proportional for gases. That is why changes in pressure influence only systems that work with species in state of gaseous aggregation and where there is appreciable variation in the number of moles in state gaseous.

If the pressure of a system increases, it will evolve in such a way as to counteract the pressure increase, that implies a lower production of moles of gas. Similarly, if there is a decrease in pressure, the system will evolve to where there is greater production of moles in the gaseous state in order to restore the pressure drop.

The study can be done in the same way from the volume, since one way to increase the pressure is to decrease the volume and vice versa.

It should be noted that variations in pressure or volume do not affect the equilibrium constant of the chemical reaction.

Concentration

Changes in the concentrations of reactants or products will cause the system to move in search of the production of the species that was diminished or of the consumption of the species that was increased, as long as it is worked at a constant volume.

Adding a Catalyst

The addition of a catalyst, that is, a species that does not participate in the chemical reaction, aims to reduce the Energy activation, therefore, equilibrium is reached more quickly. In other words, it has no influence on the degree of progress of the reaction (it does not modify the equilibrium) but rather has an influence on the speed with which it occurs.

The different disturbances are useful in industrial processes to increase the production of certain compounds. Let's see as an example the Haber Process for the production of ammonia from Nitrogen and Hydrogen:

N_{2 g)}+ 3H_{2 g)} ⟷ NH_{3 (g)}

Let's see then what are the disturbances that can be made to the system in equilibrium to increase the production of ammonia. In the first place, by increasing the pressure (or decreasing the volume) at a constant temperature, the system will evolve towards where there is less production of moles in the gaseous state. If we observe the reaction this occurs in the direct sense, so this disturbance would allow to increase the production of ammonia.

Second, it is an exothermic process (which releases heat); therefore, a decrease in the temperature of the system will allow it to evolve towards where it is generated energy in the form of heat to counteract the disturbance, so that, again, the production of ammonia.

Finally, regarding the concentrations of the species. The addition of either of the two reactants will cause the reaction to move in the forward direction, seeking to form more ammonia. Meanwhile, the removal of ammonia from the system will also allow to increase its production thanks to the fact that the system will seek to evolve to counteract the observed decrease.

Themes in Le Chatelier's Principle