q isn't a state function

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Liam Maxwell 2E
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Joined: Fri Sep 29, 2017 7:07 am

q isn't a state function

Postby Liam Maxwell 2E » Tue Jan 23, 2018 12:01 am

In a review session we were going over things that were and were not state functions. What came up was q and w were not state functions. I understand work but could someone give an example as to why heat isn't a state function.

Yashaswi Dis 1K
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Joined: Fri Sep 29, 2017 7:04 am

Re: q isn't a state function

Postby Yashaswi Dis 1K » Tue Jan 23, 2018 2:06 am

Heat results because of a temperature difference. We know temperature is a state function because you can do T (final) - T (initial), but it doesn't really make sense to find delta q = q(final) - q(initial) because q is dependent on the pathway. Think of it like this: Every molecule motion due to thermal energy matters so you can't really exclude that pathway to get to the final thermal energy motion of the molecules which is essentially heat, q.

I hope that helps!

Daniisaacson2F
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Joined: Sat Jul 22, 2017 3:00 am

Re: q isn't a state function

Postby Daniisaacson2F » Wed Jan 24, 2018 8:49 pm

Q is not a state function because throughout a reaction, the heat changes; therefore, you cannot solve for q with qfinal-qinitial. With work, it is the same thing. The amount of work done on a system is not solved for just by the final and initial work.

Jenny Cheng 2K
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Re: q isn't a state function

Postby Jenny Cheng 2K » Sat Jan 27, 2018 7:14 pm

q isn't a state function because it isn't solely dependent on the initial and final states; the value of q depends on the pathway taken to reach the final q.
Here is an example of why heat is not a state property:
Consider raising the temperature of 50.0g of water from 25.0°C to 50.0°C. One method to raise the temperature is to supply energy as heat through an electric water heater. The heat can be calculated from the specific heat capacity of water: q = (4.184 J * °C^-1 * g^-1)(50.0g)(50.0°C - 25.0°C) = +5.23 kJ. Another way to raise the temperature is to stir the water vigorously until +5.23 kJ of work has been supplied.
In the first case, q=+5.23 kJ. In the second case q=0. The final state of both scenarios is the same, but q is different. Thus, heat is not a state function since it is not dependent on only the final and initial conditions; the path taken to reach the final condition will affect q.


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