Concept:

To compare enthalpy changes across different reactions universally, scientists define "standard conditions". When properties are measured under these conditions, they are referred to as standard enthalpy changes (\( \Delta H^\circ \)).

Formula:

$$ \text{Standard Temperature} = 25^\circ \text{C} = 298 \text{ K} $$
$$ \text{Standard Pressure} = 1 \text{ atm} $$

Solution:

• In chemical thermodynamics, standard conditions specify that the pressure must be 1 atmosphere (or 100 kPa in modern IUPAC definitions, but historically 1 atm).



• The designated standard temperature is strictly \( 25^\circ \text{C} \), which converts to 298 K.



• Thus, standard enthalpy is measured at 298 K and 1 atm.

Why other options are incorrect:

300 K (\( 27^\circ \text{C} \)) and 295 K (\( 22^\circ \text{C} \)) are not accepted IUPAC standard thermodynamic temperatures. 2 atm is non-standard pressure.