Le Chatelier's Principle Explained again.

One of my smarter students asked me a question about Le Chatelier's principle--and if he's confused, so is everyone else.

Changes in pressure are attributable to changes in volume. The equilibrium concentrations of the products and reactants do not directly depend on the pressure subjected to the system. However, a change in pressure due to a change in volume of the system will shift the equilibrium.

Once again, let us refer to the reaction of nitrogen gas with hydrogen gas to form ammonia:

N2 + 3 H2 ⇌ 2 NH3 ΔH = −92kJ mol-1
4 volumes ⇌ 2 volumes

Note the number of moles of gas on the left hand side, and the number of moles of gas on the right hand side. When the volume of the system is changed, the partial pressures of the gases change. Because there are more moles of gas on the reactant side, this change is more significant in the denominator of the equilibrium constant expression, causing a shift in equilibrium.


Thus, an increase in pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas.[3] A decrease in pressure due to increasing volume causes the reaction to shift to the side with more moles of gas. There is no effect on a reaction where the number of moles of gas is the same on each side of the chemical equation.

when you increase pressure, it favors the side with more moles--because it spreads the pressure out, and thus, causes the reaction to shift to the side with fewer moles....

is that better?! AHAHAH sorry... i got the excerpt above from wikipedia