Enthalpy part 2: Enthalpy transfer

Why internal energy and work are separately accounted for in enthalpy?

Enthalpy is a thermodynamic state function that combines internal energy, U, and the effects of pressure volume, PV work. Work is a path function.

Enthalpy H is expressed as H = U + PV.

Although both internal energy and work involve energy, they play distinct roles in understanding and analysing thermodynamic systems.

Internal Energy:

Internal energy (U) is the sum of the kinetic and potential energies of the particles within a system. It represents the total energy stored within the system, including the energy associated with the molecular motion and interactions. Changes in internal energy relate to the heat transferred to or from the system and the work done on or by the system.

Work:

In thermodynamics, work is the transfer of energy between a system and its surroundings. Work can take various forms, including mechanical work and thermodynamic work. In the context of enthalpy, the work accounted for is the thermodynamic work resulting from changes in pressure and volume. This work contributes to the total energy transfer and affects the enthalpy of the system.

When analysing enthalpy changes in a system, it is essential to consider both internal energy and work separately because they represent different aspects of energy transfer. Internal energy reflects the inherent energy content of the system, while work represents the energy transferred due to changes in the system's external conditions, such as pressure and volume.

Enthalpy has different meanings in different states of matter:

Solids and Liquids:

In solids and liquids, where the effects of changes in pressure and volume are minimal, enthalpy is essentially equivalent to internal energy. Enthalpy in these states mainly reflects the internal energy of the system, considering the kinetic and potential energies of the particles.

Here, H = U.

Gases:

In gases, where changes in pressure and volume significantly impact the system, enthalpy accounts for not only the internal energy but also the work done by or on the system. In this case, enthalpy incorporates the energy associated with changes in volume under constant pressure conditions, making it a combination of internal energy and pressure-volume work.,

Here, H = U+ PV

This distinction highlights the versatile nature of enthalpy in different states of matter and why internal energy and work are separately accounted for in enthalpy. By understanding the specific contributions of internal energy and work in the calculation of enthalpy, one can make more accurate predictions and evaluations of thermodynamic processes, tailored to the specific characteristics of the system under study.

Enthalpy transfer

Enthalpy transfer can take place through two main mechanisms: via changes in internal energy and via work done on or by the system. These mechanisms can transfer energy under different thermodynamic conditions:

Constant pressure:

In a system at constant pressure, enthalpy transfer occurs primarily through changes in internal energy and work transfer together, dH = dU + PdV.

Constant temperature and pressure:

In a system at a constant temperature and constant pressure, enthalpy transfer occurs primarily through changes in work, U = Q - W, at constant temperature U remains constant. Q = W. At constant pressure the enthalpy H = Q. Therefore, at constant temperature and pressure H = W.

Adiabatic process:

In an adiabatic process, there is no heat transfer between the system and its surroundings. Enthalpy transfer in an adiabatic process occurs mainly through internal energy change within the system caused by the work done during expansion or compression.

U = Q - W

or U = W, when there is no change in heat Q.

Constant volume process:

In a system at constant volume, enthalpy transfer occurs primarily through changes in internal energy by heat since there is no change in volume and no work

U = Q - W, when W = 0,

Q = U.

In summary, enthalpy transfer takes place through changes in internal energy and work done on or by the system under different thermodynamic conditions. Understanding how enthalpy transfer occurs helps analysing and to predict the behavior of systems in various thermodynamic processes.

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