Here are the solutions to the questions: 9. The expression $\Delta U = q_v$ implies a condition under which According to the First Law of Thermodynamics, $\Delta U = q + w$. At constant volume, the work done ($w = -P\Delta V$) is zero because $\Delta V = 0$. Therefore, the change in internal energy ($\Delta U$) is equal to the heat exchanged at constant volume ($q_v$). 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 C. The heat absorbed by the surrounding occurs at varying volume in the course