Enthalpy

Definition

Enthalpy is a measure of the total energy of a thermodynamic system, including its internal energy and the work done by or on the system as a result of changes in pressure and volume. It represents the sum of a system's internal energy and the work done on the system by its surroundings or the work done by the system on its surroundings.

5 Must Know Facts For Your Next Test

1. Enthalpy is a key concept in understanding phase changes, as the energy required or released during a phase transition is directly related to the change in enthalpy.
2. The First Law of Thermodynamics states that the change in the internal energy of a system is equal to the sum of the work done on the system and the heat added to the system, which is directly related to the change in enthalpy.
3. In simple processes like isobaric (constant pressure) and isochoric (constant volume) processes, the change in enthalpy is equal to the heat added or removed from the system.
4. Enthalpy plays a crucial role in the operation of heat pumps and refrigerators, as these devices rely on the transfer of heat and the associated changes in enthalpy to achieve their desired cooling or heating effects.
5. The enthalpy of a system can be influenced by factors such as temperature, pressure, and the composition of the system, which must be considered when analyzing thermodynamic processes.

Review Questions

• Explain how the concept of enthalpy is related to phase changes.
  • The change in enthalpy, denoted as $\Delta H$, is directly related to the energy required or released during a phase transition, such as melting, boiling, or vaporization. For example, the enthalpy of vaporization, which is the energy required to convert a liquid to a gas at a constant temperature and pressure, is a key property that determines the energy needed to change the phase of a substance. Similarly, the enthalpy of fusion, which is the energy required to convert a solid to a liquid, is an important factor in understanding phase changes.
• Describe the relationship between enthalpy and the First Law of Thermodynamics.
  • The First Law of Thermodynamics states that the change in the internal energy of a system is equal to the sum of the work done on the system and the heat added to the system. This relationship can be expressed as $\Delta U = Q - W$, where $\Delta U$ is the change in internal energy, $Q$ is the heat added to the system, and $W$ is the work done on the system. Enthalpy, which is defined as $H = U + PV$, where $P$ is the pressure and $V$ is the volume, can be used to simplify this relationship. In the case of constant-pressure processes, the change in enthalpy is equal to the heat added or removed from the system, $\Delta H = Q$.
• Analyze the role of enthalpy in the operation of heat pumps and refrigerators.
  • Heat pumps and refrigerators are devices that rely on the transfer of heat and the associated changes in enthalpy to achieve their desired cooling or heating effects. These devices operate by exploiting the fact that when a substance undergoes a phase change, such as evaporation or condensation, there is a significant change in enthalpy. By manipulating the pressure and temperature of a working fluid, heat pumps and refrigerators can absorb heat from a low-temperature source and release it to a higher-temperature sink, effectively transferring heat against the natural direction of heat flow. This process is driven by the changes in enthalpy associated with the phase changes of the working fluid, which is a key principle underlying the efficient operation of these thermodynamic devices.

Related terms

Internal Energy: The total energy contained within a system, including the kinetic energy of the particles and the potential energy associated with their positions and interactions.

Work: The transfer of energy to or from a system, measured as the product of the force applied and the distance moved in the direction of the force.

Thermodynamic System: A defined region in space that is being studied, with its surroundings being the rest of the universe outside the system.