Explain the standard enthalpy of reaction.

(N/A) The enthalpy of a reaction depends on the conditions under which the reaction is carried out. Therefore,it is necessary to specify standard conditions.
The standard enthalpy of reaction is defined as the enthalpy change for a reaction when all the participating substances are in their standard states.
The standard state of a substance at a specified temperature is its pure form at $1 \ bar$ pressure.
For example,the standard state of liquid ethanol at $298 \ K$ is pure liquid ethanol at $1 \ bar$; the standard state of solid iron at $500 \ K$ is pure iron at $1 \ bar$. Usually,thermodynamic data are reported at $298 \ K$.
Standard conditions are denoted by adding the superscript $\theta$ to the symbol $\Delta H$,represented as $\Delta H^{\theta}$.