Question

Define the standard enthalpy of reaction.

Answer 1

We have some defined standard state of properties like when we say standard pressure means the value of pressure is 1 bar if we say standard temperature then the value of temperature is 298 K.

Complete step by step solution:
We know that the enthalpy change of a system can be defined as the heat absorbed or evolved by the system at constant pressure.
Or we can say that the enthalpy change of a system can also be defined as or calculated as the sum of the change in internal energy of the system and the pressure-volume work done by the system i.e., the work of expansion.
The expression is:
$\Delta H=\Delta U+P\Delta V$
Where $\Delta H$is the change in enthalpy, $\Delta U$ is the change in internal energy, $\Delta V$ is the change in volume and P is the pressure.
So, the enthalpy change of a reaction depends on some conditions under which the reaction is carried out. Hence, it is important to specify the standard condition of the system. We have some defined standard state of properties like when we say standard pressure means the value of pressure is 1 bar if we say standard temperature then the value of temperature is 298 K.
So, we can say that the standard enthalpy of a reaction is defined as the enthalpy change accompanying the reaction when all the reactants and products are taken in their standard state.

Note: Earlier, the standard pressure was taken as 1 atmosphere. But in SI units, $1\text{ atm = 101}\text{.325 kPa}$ whereas $1\text{ bar = 1}{{\text{0}}^{5}}\text{ Pa}$. Thus the conversion of the bar into the SI unit is more convenient. Hence, IUPAC recommends 1 bar as the standard pressure.