Heat and Enthalpy

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Heat and Enthalpy

Postby AtreyiMitra2L » Mon Jan 15, 2018 10:04 pm

Why is it that heat at constant pressure (enthalpy) is considered a state function but not heat? Thanks!

Kate Zeile 2D
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Re: Heat and Enthalpy

Postby Kate Zeile 2D » Tue Jan 16, 2018 8:41 am

The equation for enthalpy, H, is H=U+PV, where U is internal energy, P is pressure, and V is volume. U, P, and V are all state functions, so enthalpy (with equation H=U+PV) must be a state function as well.

Heat is not a state function because it DOES matter what path is taken to achieve the change in the state of the system. As explained in the textbook, if you wanted to raise the temperature of 100 g of water from 25 degrees Celsius to 30 degrees Celsius, you could do it two ways: you could use an electric heater to heat the water or you could stir the water vigorously to heat it up. In the first case, you added heat to the system in order to raise the temperature, so q would be a positive value. In the second case, when you stirred the water, you supplied work to the system, so q would be 0 since no heat was applied. For both scenarios, you succeeded in increasing the temperature of the water from 25 to 30 degrees Celsius, but q was different in both scenarios, so the path taken DOES matter and therefore heat cannot be a state function.

Ryan Sydney Beyer 2B
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Re: Heat and Enthalpy

Postby Ryan Sydney Beyer 2B » Tue Jan 16, 2018 6:25 pm

Both heat and work are not state functions. Work can't be a state function because it depends on the distance an object is moved, which depends on the pathway. If work is not a state function then heat can't be either based on the first law of thermodynamics. Heat is energy transferred between the system and the surroundings during a process. The amount that transfers depends on the pathway and how the process is happening which makes it not a state function.

Beza Ayalew 1I
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Re: Heat and Enthalpy

Postby Beza Ayalew 1I » Wed Jan 17, 2018 1:10 am

So enthalpy is a state function just because the components to make it up are state functions? Is there a more qualitative explanation why enthalpy is a state function?

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