Exactly 1.5 g of a fuel burns under conditions of constant pressure and then again under conditions of constant volume. In measurement A the reaction produces 25.9 kJ of heat, and in measurement B the reaction produces 23.3 kJ of heat. Which measurement (A or B) corresponds to conditions of constant pressure? Explain.

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Identify the two types of thermodynamic conditions: constant pressure and constant volume.

Recall that under constant pressure, the heat exchanged is equal to the change in enthalpy (ΔH), while under constant volume, the heat exchanged is equal to the change in internal energy (ΔU).

Understand that for exothermic reactions, the enthalpy change (ΔH) is typically greater than the change in internal energy (ΔU) because it includes work done by the system (expansion work).

Compare the heat values given: 25.9 kJ and 23.3 kJ. The larger value (25.9 kJ) is likely to correspond to the enthalpy change (ΔH) under constant pressure.

Conclude that measurement A, which produces 25.9 kJ of heat, corresponds to conditions of constant pressure.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy and Heat Transfer

Enthalpy is a thermodynamic property that reflects the total heat content of a system at constant pressure. When a reaction occurs at constant pressure, the heat released or absorbed is equal to the change in enthalpy (ΔH). This means that the heat measured in such conditions directly corresponds to the enthalpy change of the reaction.

Internal Energy and Work

Internal energy is the total energy contained within a system, including kinetic and potential energies of the particles. When a reaction occurs at constant volume, any heat exchanged is related to the change in internal energy (ΔU), and work done by the system is zero. Thus, the heat measured in this scenario reflects changes in internal energy rather than enthalpy.

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed. In the context of chemical reactions, this means that the heat produced or absorbed during a reaction can be accounted for by changes in internal energy and work done. Understanding this law helps differentiate between heat measurements at constant pressure and constant volume.

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