State function in relation to heat

isochoric/isometric:
isothermal:
isobaric:

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allyz1F
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Joined: Sat Jul 22, 2017 3:00 am

State function in relation to heat

Postby allyz1F » Mon Jan 15, 2018 6:36 pm

In the textbook, it says "Because heat is not a state function, we should not speak of a system as possessing a certain amount of heat." (the same claim is said for work). But yet when we describe reactions, we assign it a quantitative value such as +750 kJ or 5 J, etc. Can someone explain this reasoning, to me it seems counterintuitive.

Jakob von Morgenland 2C
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Re: State function in relation to heat

Postby Jakob von Morgenland 2C » Mon Jan 15, 2018 7:13 pm

I believe the value assigned is the enthalpy of the reaction, which is a state function, and is different from heat. Also, joules and kilojoules are units of energy, which gives way to understanding that we are tracking the change in energy of the function, which is the enthalpy.

MSkye Goldwater 2K
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Re: State function in relation to heat

Postby MSkye Goldwater 2K » Thu Jan 18, 2018 8:11 pm

q is the general symbol we use for heat and it is a path dependent function. H is the general symbol for enthalpy and it has the definition of H = U + PV which is derived from the 1st law of thermodynamics. H is a state function while q is a path dependent function. This is why in many of the problems we are asked to calculate delta H but in almost none of the problems are we asked to calculate delta q.

Max Mazo 2C
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Re: State function in relation to heat

Postby Max Mazo 2C » Thu Jan 18, 2018 11:47 pm

Adding to the other posts, when we calculate the change in internal energy of a system, ∆U = Q + W, Q represents the energy that the system either loses or gains as heat, not the heat the system possesses.


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