Le Chatellier & van t Hof

These are the names of two scientists who investigated (1888) the chemical equilibrium and draw some conclusions.
the following rule:

If from outside the system you change one of the conditions of a chemical equilibrium,

This will cause a dislocation of the equilibrium in such a way,
that this external change is opposed, nihilated
as much as possible.

In other words:

If you disturb a system in equilibrium,

that system will have the tendency to minimalise the effects of that disturbance.

This action continues until a new equilibrium has been reached.

Such an external change, such an influence could be, for example: Below you find more details about this:
  1. changing the amount of a substance that participates in an equilibrium
    for example: adding Hydrogen ions causes a dislocation of the equilibrium, that will take care that (part of) the extra Hydrogen ions will go away again, by stimulating the forward reaction.
    This continues until a new equilibrium is reached.

    1. The beaker contains NH3(aq) (basic environment) with some drops of the indicator methyl red,
      of which the abbreviated formula is: HIn

      Than in the solution the following equilibrium will occur:

          HIn H++ In-
      Red                       Yellow

      The presence of a base (that absorbs H+) means a dislocation of the equilibrium to the right. the yellow color dominates.

    2. Now you add a bit of HCl(aq), the basic environment changes to a less basic or more aced env.

      The acid wil donate H+, so:

      the equilibrium:   HIn H++ In-
                                Red             Yellow

      dislocates to left and the red color will dominate.

      Now again we add some base: (NH3(aq)), the color changes again to yellow

      so the same equilibrium dislocated again to the right:

      HIn H++In-
      Red               Yellow

    3. Doing this can be repeated endless.
      Every time the equilibrium will dislocate to the right of to the left.

      The overall situation in the beaker does only change a very littlebit.
      There is only a small dislocation of the equilibrium to one of the other side, dependent on the environment.

  2. The Volume is changed
    A bigger volume means that more space is created for the participating particles in the equilibrium.
    The principle is that such a change must be opposed by the equilibrium, and that is possible by creating more particles.
    You can change here the equilibrium in different ways:
    1. directly in the case of gases, for example by pulling out the piston giving the cylinder more volume.
    2. indirectly by diluting in the case of liquids.
    In both cases the effect is the same.

    example: I2(g) + H2(g) 2HI(g) (at elevated temp)
    In this example volume change has no influence, because at both sides of the arrows the number of particles is the same. Dislocation is useless.

  3. Adding or withdrawing heat energy Adding extra energy to a chemical equilibrium will also result in an attempt to counteract this change. In this case the equilibrium could try to consume that extra added energy, or: stimulate the endothermic reaction.
    In the case of HI the equilibrium will dislocate to the left. The forward reaction is exothermic.

    [glucose + fructose saccharose + water ΔH < 0]
    The saccharose-equilibrium is exothermic to the right.

    At lower temperatures, less energy will be available and the equilibrium will try to 'produce' extra energy.
    This can be realised by stimulating the exothermic reaction, the one to the right. Or: at lower temperatures, more saccharose will be formed.
All what was said above is a qualitative view.

Youi can also investigate the same effects in a quantitative way, and prove it with the equilibrium constant K.
That K has a fixed value and does not change, whatever you change at the equilibrium (exept temperature).
Those proves can be delevered with mathematical calculations.

Imagine that the amount of I2 is increased.
Look at the mathematical formula for K. If the [I2] increases and K may not change, then, in a mathematical view, the amount of HI must also increase, or: the equilibrium must dislocate.