Peter Atkins says

In introductory chemistry, we meet the empirical rule of thumb known as Le Chatelier’s principle:

When a system at equilibrium is subjected to a disturbance, the composition of the system adjusts so as to tend to minimize the effect of the disturbance.

For instance, if a system is compressed, then the equilibrium position can be expected to shift in the direction that leads to a reduction in the number of molecules in the gas phase, for that tends to minimize the effect of compression.Le Chatelier’s principle, though, is only a rule of thumb, and to understand why reactions respond as they do, and to calculate the new equilibrium composition, we need to use thermodynamics.

That Le Chatelier’s principle, is only a rule of thumb, and thermodynamic calculations need to be done to determine new equilibria.

So are there any exceptions to this rule of thumb ??

  • 2
    $\begingroup$ Metastable systems, where the 'disturbance' pushes it over the edge towards a global minimum energy, won't obey it particularly well. $\endgroup$ – Jon Custer Jun 25 '19 at 13:39
  • $\begingroup$ I'll say if you find an exception to Le Chatelier, a closer look will show that you've had a wrong idea (i.e. not the whole picture) about what's actually happening. Very strong hint that you should have a closer look. $\endgroup$ – Karl Jun 25 '19 at 22:02