The concentration or pressure ratio


would decrease by a factor of (2) / (2) (2) = 1/4, and would become smaller than the equilibrium constant, . More and would react until the imbalance was corrected and the concentration ratio again was equal to . Pressure changes have no effect if the total number of moles of gas is unchanged during the reaction. The HCl synthesis would be unaffected by an increase or decrease in external pressure.

These pressure effects are one example of an important general principle, which, if understood, can save a lot of mathematics. This is Le Chatelier's principle:

If external stress is applied to a system at equilibrium, the equilibrium conditions will shift so as to compensate for, or counteract, the stress.





Right: Henri Louis Le Chatelier (1850 - 1936)