# What is the enthalpy in thermodynamics? [duplicate]

What does enthalpy tell us about? According to definition; it is the total heat content of a system, if it is the total heat content of a system then what is Internal Energy? Since we know,
$$H = U + PV$$ If we have one work done $d(PV)$ in Internal Energy itself,then what is the need of the second one?

• You should be very careful thinking of things as "heat content". There is no such quantity as the "amount of heat" in an object. Heat is a quantity associated with processes. Mar 22, 2014 at 18:08
• @ZachMcDargh The older name of Enthalpy is "total heat content". Mar 24, 2014 at 8:33

I think you have misunderstood the enthalpy definition, it comes from the second law (conserved energy, if there is constant pressure ($\delta W =pdV$)

$dU=\delta Q-\delta W \Rightarrow dU=\delta Q-pdV$

from the chain rule $d(pV)=Vdp+pdV \Rightarrow pdV=d(pV)-Vdp$, so

$dU=\delta Q -d(pV)+Vdp \Rightarrow d(U+pV)=\delta Q+Vdp \Rightarrow H\equiv U+pV \Rightarrow dH=\delta Q+Vdp$

Also if you have an infinitesimal cuasiestatical process ($\delta Q=TdS$)

$dH=TdS+Vdp$

Also entalphy it si known as flux energy, it is often used in fluid dynamics and movement, because here is conserved. Be careful with the definition of work (could be internal), and internal energy. Internal energy comes from the speed of the particles, but work is in this particular case, it is mechanical, just a $\Delta V$.