is Q =dH in all thermodynamics process
Answers
Answer:
For a simple substance, enthalpy --and any other thermodynamic property--depends on two variables, e.g., temperature T and pressure p.
An adiabatic process is a process in which no heat is exchanged. An adiabatic and reversible process has constant entropy s--it is isentropic.
An isenthalpic process has constant enthalpy, and probably there is a myriad ways to realize such a process. However, the most commonly discussed isenthalpic process is the irreversible adiabatic expansion in a throttling device (e.g., flow is pushed through a small orifice or a porous plug).
The process is adiabatic, since the throttle is so small that there is no time for the throughput to exchange heat. The pressure drops due to friction.
First and second law of thermodynamics for such a process reduce to
h(T1,p1) = h(T2,p2)
s(T1,p1) < s(T2,p2)
where indices 1,2 indicate inflow and outflow values.
Only for an ideal gas, where enthalpy depends on temperature only [that is, h(T)], does temperature stays the same. In general, for an arbitrary substance, pressure drops, and temperature can increase, stay the same ,or drop. This depends on the Joule-Thomson coefficient which is defined as J=(dT/dp) for constant h .
Best know use for a throttle is in refrigerator, where they are used to expand liquid cooling fluid at (roughly) room temperature to create the low temperature in the evaporator.