One method of communicating an enthalpy change involves writing the enthalpy of reaction beside the chemical reaction. The enthalpy change of the reaction is symbolized as Δ_rH and is expressed in kJ. If the reaction occurs at SATP, the standard state symbol is also included in the enthalpy symbol (Δ_rH°). Sometimes, instead of the subscript r, another letter is used to indicate that type of reaction occurring. (Refer to the Learning Tip at the top of page 489 in the textbook.)

It is not necessary to stipulate which substance in the equation is being considered because the enthalpy change (Δ_rH) indicated refers to the reaction equation as a whole.

First, consider the process of photosynthesis:

\( \mathrm { 6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(s) + 6O_2(g) } \)

\( \mathrm { \Delta_rH = +2802.5 kJ } \)

This enthalpy value indicates that 2802.5 kJ of energy are absorbed when 6 moles of CO₂(g) reacts with 6 moles of H₂O(l) to produce 1 mole of C₆H₁₂O₆(s) and 6 moles of O₂(g).

Next, consider the process of cellular respiration:

This enthalpy value indicates that 2802.5 kJ of energy are released when 1 mole of C₆H₁₂O₆(s) reacts with 6 moles of O₂(g) to produce 6 moles of CO₂(g) and 6 moles of H₂O(l).