Is delta H=0 for isothermal process?

[sachin123]

1.>Consider a gas in a vessel with a piston on top.Let it expand to a greater volume.
So,
delta H=delta U+delta(PV)
but delta U is 0 as it is isothermal.
now,as the number of moles of gas remains constant,delta(PV) is nR(delta T) which is again 0.
SO delta H is 0.

Thats what my book says.

BUt,my questions is,when work is done by the gas on the piston,without change in internal energy,then it should have absorbed some energy from the surroundings.(conservation of energy)
Correct?
then shouldn't delta H be a non zero value for an isothermal process?

please help.Thank You

 

[ojs]

Your assumptions are correct but your conclusion is not. Why do you think the system expands? It does work to expand and the energy the system got to do that is the change in Q, but the system lost all the energy it got by doing work and the internal energy stayed constant through the process, hence H is constant also.

 

[sachin123]

Then what's the difference between Q and H?aren't they the same?though their definitions differ,I have not understood any difference between them.Can you clarify it?

 

[ojs]

H (enthalpy) is a state quantity, that is when you go in a circle (from one state, to state two, to state three to state four and end again in state one) then H is the same before and after the circle but Q (heat) is a process function and is dependent on how you do things.

 

[sachin123]

Thank you ojs,
so finally ,delta H is 0 for an isothermal process right?and my assumption that q and H are the same was wrong.