Question about Enthalpy

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Caleb Lim 2M
Posts: 13
Joined: Wed Sep 21, 2016 2:55 pm

Question about Enthalpy

Postby Caleb Lim 2M » Thu Jan 12, 2017 6:18 pm

When looking at the phase change diagram of H2O, is the enthalpy (value of H) value different in the beginning and the end of the second horizontal line? If so, why?

Rachel_Harland_3I
Posts: 19
Joined: Fri Jul 22, 2016 3:00 am

Re: Question about Enthalpy

Postby Rachel_Harland_3I » Thu Jan 12, 2017 6:25 pm

Yes. Although the temperature has not increased, heat is still being supplied. And by definition, enthalpy is the amount of heat being released or absorbed at a constant pressure.

Elan Karlin Dis3H
Posts: 20
Joined: Wed Sep 21, 2016 2:57 pm

Re: Question about Enthalpy

Postby Elan Karlin Dis3H » Thu Jan 12, 2017 6:26 pm

In the phase change diagram for H20, the second horizontal line is meant to symbolize liquid water changing into water vapor. It is horizontal because the temperature is not increasing as heat is going into breaking the hydrogen bonds. The enthalpy for this change is symbolized as kJ/mol and does not change throughout the process until the liquid is completely converted to gas. Then the temperature will rise and the enthalpy changes based on the temperature increase.

Sakina_Qadir_3K
Posts: 10
Joined: Fri Jul 22, 2016 3:00 am

Re: Question about Enthalpy

Postby Sakina_Qadir_3K » Thu Jan 12, 2017 11:36 pm

I also had a question about enthalpy, will there ever be a scenario where we are solving for it but the pressure is not constant?

Myra Kurani 1L
Posts: 10
Joined: Fri Jul 22, 2016 3:00 am

Re: Question about Enthalpy

Postby Myra Kurani 1L » Thu Jan 12, 2017 11:41 pm

According to the textbook, enthalpy allows us to keep track of energy changes only at a constant pressure, so the answer is no I believe.


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