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.

Verified step by step guidance

1. 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 (\( \Delta H \)).
2. 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.
3. 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 \( \Delta H > \Delta U \).
4. 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.
5. Therefore, to obtain the largest possible amount of heat, the reaction should be carried out under constant pressure conditions.

Key Concepts

Enthalpy and Heat Transfer

Enthalpy is a thermodynamic property that reflects the total heat content of a system. In chemical reactions, the change in enthalpy (ΔH) indicates whether heat is absorbed or released. Under constant pressure, the heat exchanged is equal to the change in enthalpy, making it crucial for understanding how heat is produced or consumed during reactions.

Ideal Gas Law and Molar Volume

The Ideal Gas Law (PV=nRT) describes the relationship between pressure (P), volume (V), temperature (T), and the number of moles (n) of a gas. A large increase in the number of moles of gas during a reaction can significantly affect pressure and volume, influencing the conditions under which the reaction occurs and the heat produced.

Constant Volume vs. Constant Pressure Conditions

In a constant volume process, no work is done by the system, and the heat released corresponds to the internal energy change. Conversely, in a constant pressure process, the system can do work on the surroundings, which can lead to greater heat release when gas moles increase. Thus, the choice between these conditions affects the total heat output of the reaction.