The expression U = q_v implies a condition under which

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9. The expression $\Delta U = q_v$ implies a condition under which According to the First Law of Thermodynamics, $\Delta U = q + w$. When a process occurs at constant volume ($\Delta V = 0$), the pressure-volume work ($w = -P\Delta V$) is zero. Therefore, the change in internal energy ($\Delta U$) is equal to the heat exchanged at constant volume ($q_v$). The term "heat absorbed" refers to a positive $q$.
   • A. The heat absorbed by the system in the course of a reaction occurs at constant volume
   • B. The heat absorbed by the surrounding occurs at constant volume in the course of a reaction