Displacement of balance

In many syntheses, one of the principal goals is to improve the output. When these syntheses are balances, as within the framework of the esterification, one seeks with to move these balances. Here the principal laws are joined together which govern these displacements of balances.

Definitions

That is to say a physicochemical system in a state of balance. If one imposes to him a variation of a parameter Intensif or Extensif, there is in general evolution towards a new state of balance.

If the final state is a state of balance of the same physicochemical system as the initial state (even chemical species, in the same physical status), one speaks about displacement of balance .

If the final state is a state of balance of a different physicochemical system, one speaks about rupture of balance .

General law of moderation: Principle of Chatelier

The principle of Chatelier , or general law of moderation , deduced from experimental observations, was stated by Henry Chatelier in 1884. This principle is applicable within the framework of a displacement of balance , and not of a rupture of balance .

Ennoncé:

" When the external modifications made to a physicochemical system in balance cause an evolution towards a new state of balance, the evolution is opposed to the disturbances which generated it and moderates some the effect. "

This kind of principle also exists in electromagnetism, in the field of the known induction under the name of Lenz's law.

Other formulations:

a thermodynamic system in balance subjected to a disturbance (introduction of a new component, increase in the pressure, variation in the temperature, etc…) tends to be opposed to this disturbance, the displacement of balance tending to restore the initial conditions.
If a chemical system with balance is subjected to a modification of certain parameters resulting in disturbing this balance, the aforementioned system evolves/moves in the direction which thwarts the imposed modification, so as to be readjusted in a new state of balance consequently characterized value of constant thermodynamics.

This principle results from the Loi of action of mass and from the law of Jacobus Henricus van 'T Hoff.

Influence temperature: Experimental law of Van' T Hoff

One places oneself at constant volume or pressure , in closed system.

Statement

" A rise in temperature applied to a system closed in balance and maintained with pressure or constant volume involves a displacement even a rupture of balance in the direction of the reaction which, at constant temperature and pressure or volume is endothermic ".

In practice

For a system closed in balance if the temperature is raised, an endothermic chemical reaction will see its favoured direct reaction and an exothermic chemical reaction will see its favoured opposite reaction. It is about a particularization of the Principe of Chatelier .

Examples

Demonstrations

One applies to the system a temperature variation dT (dT > 0)

Influence pressure: Experimental law of Chatelier

One places oneself at constant temperature, in a closed enclosure.

Statement

" An increase in pressure applied to a system closed in balance and maintained at constant temperature causes a displacement of balance, even a rupture of balance in the direction for which the reaction is accompanied, at temperature and pressure constants, of a reduction in volume ".

In practice

If one raises the pressure, a chemical balance which presents gases in its products or reagents will see the reaction which consumes the most gas (and thus produces some less) favoured. Balance will evolve/move in the direction of the reduction in the gas phases (if they exist). It is still about a particularization of the Principe of Chatelier .

Examples

Category: Thermochemistry

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