# Why are pressure or volume not able to change the equilibrium constant? [duplicate]

It is stated in my textbook that temperature is the only factor that can change the equilibrium constant. According to the Ideal Gas Law, temperature is dependent on the pressure and volume. Thus, shouldn't pressure and volume change the equilibrium constant too?

• Inasmuch as they change temperature, yes. Short of that, no. – Ivan Neretin May 10 at 18:20
• Yes, it is dependant on pressure and volume, but a change in pressure or a change in volume doesn't necessarily mean a change in temperature. Thus, it doesn't affect the equilibrium constant. – TheGodlyBeast May 10 at 18:29
• I would add to the answer in the linked question, that you should be careful to distinguish between the standard equilibrium constant and equilibrium constants in general. The first do not depend on pressure because (as the linked answer explains, perhaps not entirely clearly) they are defined at a fixed standard pressure (each reagent and product is in pure form at the given T and at the standard pressure, typically 1 bar). – Buck Thorn May 11 at 7:16

Just like any sweeping statements made by the General Chemistry textbooks, this statement is also not completely true. When you work at pressure extremes, as in modern day chromatography, such as 1000 or higher bar, large molecules can change their shape, in that case, one can see a change in retention factors which sort of indicates a change in equilibrium constant with pressure under constant volume.

Just for fun, some solvents can become solids at high pressures-but this happens at another high-pressure level.

Coming back to routine reactions, pressure will not affect equilibrium constants because whenever you try to adjust the pressure in a gaseous reaction, the equilibrium concentrations will change in such a way that their ratio remains constant-hence the equilibrium constant does not change.

One way to think about it as that the equilibrium constant is a ratio of forward and backward rate constant. Changes in pressure or volume will not change the kinetic energy of the molecules but only temperature can change the kinetic energy and hence affect the rate constants.

According to the Ideal Gas Law, temperature is dependent on the pressure and volume.

Mathematically, this is incorrect. You would rather say that pressure and volume of a gas are functions of temperature i.e., the converse of what you wrote.

• Splitting hairs here but if you perform an adiabatic expansion on a gas its temperature drops. – Buck Thorn May 10 at 18:35
• Right, I was thinking about it but didn't want to confuse the student. – M. Farooq May 10 at 18:38
• "the equilibrium concentrations will change in such a way that their ratio remains constant"How is it any different from temperature changing the equilibrium concentrations? Why do pressure change in the way described? – Zratos May 10 at 18:43
• @Zratos, I edited the answer. – M. Farooq May 10 at 20:19
• You should be careful not to confuse the student further, because it is likely that the question arises from a misunderstanding of the difference between equilibrium constants in general and standard equilibrium constants. Also, you invoke Le Chateliers principle as the reason that the equilibrium constant is invariant, but this is not the true reason why the constant does not change. – Buck Thorn May 11 at 7:39