Standard conditions: the fair-comparison rule

Hold your hand near a window on a cold day and you can feel the heat leaking out — yet the same window on a hot day seems to leak heat in. The direction of transfer depends on the temperature difference, not on the window.

Enthalpy changes behave the same way. They depend on:

• the temperature the reaction starts at;
• the pressure (especially if gases are involved);
• the state of each substance — ice, water and steam all store different amounts of energy.

If everyone measured at whatever temperature their lab happened to be, data tables would be nonsense. So one set of conditions is agreed.

The little ⦵ is a promise. It says: this number was measured (or calculated) at 298 K and 101 kPa, with every species in its standard state.

It does not mean "standard" in the everyday sense of "normal" or "typical". It is a precise label.

You will see it attached to the name of the enthalpy change:

• $\Delta H_\text{r}^{\ominus}$ — standard enthalpy change of reaction;
• $\Delta H_\text{f}^{\ominus}$ — standard enthalpy change of formation;
• $\Delta H_\text{c}^{\ominus}$ — standard enthalpy change of combustion;
• $\Delta H_\text{neut}^{\ominus}$ — standard enthalpy change of neutralisation.