Question

What is enthalpy? How enthalpy is related to internal energy?

Answer 1

Hint :In order to answer this question, first we will write the explanation of enthalpy and then we will discuss the formula of enthalpy. And then we will also discuss how enthalpy is related to internal energy.

Complete Step By Step Answer:
Enthalpy is the measurement of energy in a thermodynamic system. The overall content of heat in a system is enthalpy, which is equal to the system's internal energy plus the product of volume and pressure.
Enthalpy is a technical term that indicates the amount of internal energy required to create a system as well as the amount of energy required to make room for it by establishing its pressure and volume and displacing its surroundings.
The developed heat (either absorbed or released) equals the change in enthalpy when a process starts at constant pressure. The total of internal energy, written by $U$ , and the product of volume and pressure, denoted by $PV$ , is enthalpy change, expressed in the following manner:
 $H=U+PV$
Enthalpy can also be thought of as a state function composed entirely of the state functions $P,T,andU$ . It is usually demonstrated by the difference in enthalpy $(H)$ of a process between its initial and final states.
 $\mathrm{\Delta }H=\mathrm{\Delta }U+\mathrm{\Delta }PV$
According to the following equation, the flow of heat $(q)$ at constant pressure in a process equals the change in enthalpy.
 $\mathrm{\Delta }H=q$
 $\ast$ As we discussed above,
 $H=U+PV$
where, $H$ is the enthalpy,
 $U$ is the internal energy,
 $P$ is the pressure and $V$ is the volume.
Consider a process that takes place at constant pressure and only allows pressure-volume work $(w=-P\mathrm{\Delta }V)$ :
The change in enthalpy is then given by:
$$\begin{gathered} \mathrm{\Delta }H=\mathrm{\Delta }U+\mathrm{\Delta }(PV) \\ \Rightarrow \mathrm{\Delta }H=\mathrm{\Delta }U+P\mathrm{\Delta }V \end{gathered}$$
and,
 $$\begin{gathered} \mathrm{\Delta }U=qP+w \\ \Rightarrow \mathrm{\Delta }U=qP-P\mathrm{\Delta }V \end{gathered}$$
Replacing $\mathrm{\Delta }U$ by its expression in the expression of $\mathrm{\Delta }H$
we get:
 $\mathrm{\Delta }H=qP$ at constant pressure.

Note :
Internal energy and enthalpy for a given system are equal at constant pressure. There are two ways that internal energy can change. The first is due to heat transfer – a system can absorb or release heat – and the second is due to work. As a result, the following equation can be used to express internal energy change: $w$ denotes work performed by or on a system.