How does Le Chatelier's Principle work?

[alingy1]

It is said that if there is a change in pressure in an equilibrium state, the chemical reactions that diminish the number of molecules will be favorised. Therefore, there will be less molecules, so the pressure will decrease to reach equilibrium again.

However, since the kinetic energy of the molecules doesn't change before and after the reactions, if there are less molecules, those molecules will be slower but heavier. So, according to me, the pressure wouldn't change because the collisions will be less but each collision will apply more force.

What do you think about that?

 

[Simon Bridge]

I suspect you have slightly misstated the principle:

If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.
http://en.wikipedia.org/wiki/Le_Chatelier's_principle​

... that makes sense since it is an equilibrium reaction that is being considered.
It is possible to have reactions that are not equilibrium reactions.

Per your example - the details depend on how the pressure is changed.

 

[Borek]

the kinetic energy of the molecules doesn't change before and after the reactions

It doesn't?