Pressure and Temperature Effects

Pressure and volume stresses work through Q. Shrinking the container raises every concentration at once, so Q jumps above K. The system responds by shifting toward whichever side has fewer moles of gas, since that side cools the total concentration fastest and brings Q back down to K. K itself never changes; only the position of equilibrium moves. This is why an inert gas added at constant volume does nothing: it raises total pressure, but the partial pressures of the actual reacting species stay exactly where they were, so Q is untouched and nothing shifts.

Temperature is different because it changes K, not just Q. Treat heat like a chemical species: in an exothermic reaction, heat appears on the product side, so adding heat (raising temperature) pushes the system back toward reactants, and removing heat pushes it toward products. Because the equilibrium constant describes a balance that depends on temperature, shifting the balance this way means K itself has a new value at the new temperature — it is not just Q changing to catch up to the old K.

This sits next to Le Chatelier's principle as its quantitative partner: concentration, pressure, and volume stresses are all just ways of knocking Q away from a fixed K, while temperature is the one stress that moves the target K is chasing. Seeing that distinction clearly is what separates a shift in position from a genuine change in the equilibrium constant.