A) The standard enthalpy of formation (_f^) of a substance is the enthalpy change when one mole of the compound is formed from its constituent elements in their stand

a) The standard enthalpy of formation ($\Delta\text{H}_{\text{f}}^\circ$) of a substance is the enthalpy change when one mole of the compound is formed from its constituent elements in their standard states under standard conditions ($298 \text{ K}$ and $1 \text{ atm}$ or $1 \text{ bar}$).

b)
I) Two types of heat changes represented by the reactions are:

• Standard enthalpy of combustion
• Standard enthalpy of formation

II) An energy level diagram for the reaction represented by equation (i) ($\text{C}_2H_6(g) + \frac{7}{2}O_2(g) \longrightarrow