Should you carry out a chemical reaction under conditions of cons...

Question

Should you carry out a chemical reaction under conditions of constant volume or constant pressure to obtain the largest possible amount of heat, if there is a large increase in the number of moles of gas? Explain.

Accepted Answer

Understand the difference between constant volume and constant pressure conditions. Under constant volume, the heat exchanged is equal to the change in internal energy ($ \Delta U $), while under constant pressure, the heat exchanged is equal to the change in enthalpy ($ \Delta H $).. Recall that for reactions involving gases, the change in enthalpy ($ \Delta H $) is related to the change in internal energy ($ \Delta U $) by the equation: $ \Delta H = \Delta U + \Delta n_{gas}RT $, where $ \Delta n_{gas} $ is the change in moles of gas, $ R $ is the ideal gas constant, and $ T $ is the temperature.. Consider the scenario where there is a large increase in the number of moles of gas ($ \Delta n_{gas} > 0 $). This means that $ \Delta n_{gas}RT $ is positive, making $ \Delta H > \Delta U $.. Since $ \Delta H > \Delta U $ when there is an increase in moles of gas, more heat is absorbed or released under constant pressure conditions compared to constant volume conditions.. Therefore, to obtain the largest possible amount of heat, the reaction should be carried out under constant pressure conditions.