Question
What is standard enthalpy change (∆rH0) of a reaction?
Solution
It should be noted that the magnitude of ∆H for a reaction varies with the temperature and therefore for comparison, the values of ∆H for various reactions are expressed at the standard state. A substance is said to be in the standard state when it is present in its most stable state generally at 298K and under I bar pressure.
The standard enthalpy of a reaction is the enthalpy change for a reaction when all the participating substances (elements and compounds) are in their standard states (i.e. at 298K and 1 bar pressure). It is denoted by ∆r H0. For example
