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} are always equal for all reactions.
• B. q refers to the heat at constant volume, while ΔH_{rxn} refers to the heat at constant temperature.
• C. q is the heat exchanged under any conditions, while ΔH_{rxn} is the heat exchanged at constant pressure.
• D. q is the heat exchanged at constant pressure, while ΔH_{rxn} is the heat exchanged at constant volume.

Answer: (C)

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 be measured 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 between heat and enthalpy: at constant pressure, the heat exchanged (\$q_p\$) equals the change in enthalpy (\$\Delta H\$), so \$q_p = \Delta H\$.
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\$ in general.
5. Therefore, the key difference is that \$q\$ can represent heat under any condition, while \$\Delta H_{rxn}\$ specifically represents heat exchanged at constant pressure.