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}_2\text{H}_6\text{(g)} + \frac{7}{2}\text{O}_2\text{(g)} \longrightarrow