Which of the following equations is used to calculate the standard enthalpy change of a reaction $(\Delta_r H^\circ)$?

• A
  $\Delta_r H^\circ = \sum \Delta_f H^\circ(\text{products}) - \sum \Delta_f H^\circ(\text{reactants})$
• B
  $\Delta_r H^\circ = \sum \Delta_f H^\circ(\text{reactants}) - \sum \Delta_f H^\circ(\text{products})$
• C
  $\Delta_r H^\circ = \sum \Delta_f H^\circ(\text{products}) + \sum \Delta_f H^\circ(\text{reactants})$
• D
  $\Delta_r H^\circ = \frac{\sum \Delta_f H^\circ(\text{products})}{\sum \Delta_f H^\circ(\text{reactants})}$