Which of the following best describes the difference between the heat (q) of a reaction and the reaction enthalpy (ΔH_{rxn})?

• A. q and ΔH_{rxn} both refer to the change in temperature of the system, not the heat exchanged.
• B. ΔH_{rxn} refers to the heat exchanged at constant volume, while q is the heat exchanged at constant pressure.
• C. q is always equal to ΔH_{rxn} for all reactions.
• D. q refers to the heat exchanged under any conditions, while ΔH_{rxn} is the heat exchanged at constant pressure.

Answer: (D)

Verified step by step guidance

1. Understand that heat (q) is a general term for the energy transferred due to temperature difference, and it can occur under various conditions such as constant volume or constant pressure.
2. Recognize that reaction enthalpy (\$\Delta H_{rxn}\$) specifically refers to the heat exchanged during a reaction carried out at constant pressure.
3. Recall the thermodynamic relationship that at constant pressure, the heat exchanged (q_p) is equal to the change in enthalpy, so \$q_p = \Delta H_{rxn}\$.
4. Note that at constant volume, the heat exchanged (q_v) is related to the change in internal energy (\$\Delta U\$), not enthalpy, so \$q_v \neq \Delta H_{rxn}\$ in general.
5. Conclude that \$q\$ can refer to heat exchanged under any conditions, but \$\Delta H_{rxn}\$ is specifically the heat exchanged at constant pressure, making the last statement the best description.