What is the difference between the internal energy change ∆E and the enthalpy change ∆H? Which of the two is mea- sured at constant pressure and which at constant volume?

Verified step by step guidance

1. Step 1: Understand the definitions. The internal energy change (∆E) of a system is the difference in energy between the final and initial states of the system. It includes all forms of energy, such as kinetic and potential energy. The enthalpy change (∆H) is the heat absorbed or released by a system at constant pressure.
2. Step 2: Understand the difference. The main difference between ∆E and ∆H is that ∆E includes all forms of energy changes in a system, while ∆H only includes the heat absorbed or released at constant pressure. In other words, ∆H is a measure of the heat flow in or out of a system during a reaction at constant pressure.
3. Step 3: Understand the conditions. The internal energy change (∆E) is measured at constant volume. This is because at constant volume, there is no work done by or on the system, so the change in internal energy is equal to the heat added or removed from the system.
4. Step 4: The enthalpy change (∆H) is measured at constant pressure. This is because most chemical reactions occur at constant pressure, and it is often more convenient to measure the heat flow at constant pressure.
5. Step 5: Remember that while both ∆E and ∆H are state functions (meaning their values depend only on the current state of the system, not on the path taken to reach that state), they measure different aspects of energy change in a system.

Key Concepts

Internal Energy (∆E)

Internal energy (∆E) is the total energy contained within a system, encompassing kinetic and potential energies of the particles. It reflects the energy changes due to heat transfer and work done on or by the system. Internal energy is typically measured at constant volume, where no work is done by expansion or compression.

Enthalpy (∆H)

Enthalpy (∆H) is a thermodynamic quantity that represents the total heat content of a system at constant pressure. It accounts for internal energy and the work done by the system when it expands against external pressure. Enthalpy changes are particularly useful in chemical reactions and phase changes occurring at constant pressure.

Constant Pressure vs. Constant Volume

The distinction between constant pressure and constant volume is crucial in thermodynamics. At constant pressure, the enthalpy change (∆H) is measured, allowing for the inclusion of work done by the system. Conversely, at constant volume, the internal energy change (∆E) is measured, as no work is performed due to volume changes, making it a more straightforward representation of energy changes.