*The standard state pressure is 1 bar, why is there no standard temperature?*

**The short answer**

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

**The long answer**

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.

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