# Why is there no standard temperature?

The standard state for a gas is defined at 1 bar of ideal gas. Why isn't there a corresponding standard state temperature?

• Note that most data published before 1982 used a standard pressure of one ‘standard atmosphere’, i.e. $p=1\ \mathrm{atm}=101325\ \mathrm{Pa}$.
– user7951
Sep 18, 2016 at 13:00

The standard pressure is not an experimental condition, while the temperature is.

The main reason the standard state is defined is because it leads to this very useful equation $$K_p = e^{-\Delta G^\circ/RT}$$ Say you have this reaction: $A \rightleftharpoons B + C$ One way to use this equation is to compute the free energy of 1 mol of $A$, $B$, and $C$ at 1 bar using equations derived for an ideal gas, compute $\Delta G^\circ = G^\circ (B) + G^\circ (C) - G^\circ (A)$, and use that value to predict $K_p$.

If the gasses behave like ideal gasses "in real life" then the measured $K_p$ will match the $K_p$ computed from $\Delta G^\circ$. You can do the measurement at any pressure you want, not just at 1 bar.* The standard state refers to the pressure you use when computing $\Delta G^\circ$. The only thing it has to do with the experimental measurement is that it defines the units you should use for your partial pressures when computing $K_p$

$\Delta G^\circ$ does also depend on temperature, but the temperature you chose should be the same as the experimental conditions. So the temperature is not part of the standard state definition.

But what about "Standard temperature and pressure (STP)?"

Standard temperature and pressure (STP) refers to STP conditions under which $K_p$ is measured, not the pressure used to compute $\Delta G^\circ$. I know, they couldn't have made it more confusing if they tried when they named these things.

*Of course if you do the measurements at very high pressures or low temperatures, then the assumption that the gasses behave ideally will be less valid and the measured $K_p$ will differ more from the $K_p$ computed from Equation 1. However, that is a separate issue unrelated to the standard state because the $K_p$ in Equation 1 refers to the $K_p$ you would measure if the gasses behaved ideally at the pressure and temperature used in the experiment.

• if anyone has a different take on this I would very much like to hear it Sep 18, 2016 at 12:50
• The standard in STP refers to a legal norm so that sets of data can be compared. So, both T and P have nothing to do with the thermodynamic standards that you describe and discuss (and most probably understand a lot better than I). Sep 18, 2016 at 13:05
• The various standard conditions for temperature and pressure (STP etc.) are explained by this xkcd.
– user7951
Sep 18, 2016 at 13:10
• @JanJensen If we had select a different standard pressure e.g. $10$ bar would the equilibrium constant change? Jun 7, 2021 at 18:32