The sign of q

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Abigail Urbina 1K
Posts: 102
Joined: Thu Jul 27, 2017 3:01 am

The sign of q

Postby Abigail Urbina 1K » Thu Jan 25, 2018 2:53 pm

For questions involving heat transfers between a system and its surroundings, I'm a little bit confused on how you determine where the negative sign goes (which q is going to take the negative sign).

If q is negative, doesn't it mean that the temperature is going down, meaning it is losing heat?
And if q is positive, doesn't it mean that the temperature is increasing, meaning that it is gaining heat?

In the example Dr. Lavelle provided us during lecture 4 where 1.0 M HCl and 1.0 M NaOH are mixed in a calorimeter and we are asked to find how much heat is released, I'm a little bit confused as to why he set it up as qsys=-qsurroundings. Shouldn't qsys be negative since the system is losing heat? He was saying that the surroundings gain the heat that the system gives off, so I'm a bit confused...

Kelly Kiremidjian 1C
Posts: 62
Joined: Fri Sep 29, 2017 7:04 am

Re: The sign of q

Postby Kelly Kiremidjian 1C » Thu Jan 25, 2018 3:05 pm

q is postitive is heat is added to the system and q is negative is heat is removed from the system.

Kayla Danesh 1F
Posts: 38
Joined: Thu Jul 13, 2017 3:00 am

Re: The sign of q

Postby Kayla Danesh 1F » Thu Jan 25, 2018 4:53 pm

Yes, you are correct that this is an exothermic reaction and the system is releasing energy. The statement "qsys=-qsurroundings" is just to help us solve for the missing variable. This statement though is the equivalent of saying that "-qsys=qsurroundings." What one gains is what the other loses.

Michael Lee 2I
Posts: 51
Joined: Fri Sep 29, 2017 7:05 am

Re: The sign of q

Postby Michael Lee 2I » Thu Jan 25, 2018 5:17 pm

qsys + qsurr = 0 is a "perfect system" with no extra released/absorbed.

Jimmy Zhang Dis 1K
Posts: 30
Joined: Fri Sep 29, 2017 7:05 am

Re: The sign of q

Postby Jimmy Zhang Dis 1K » Fri Jan 26, 2018 10:57 am

I think the system should be thought of as the calorimeter which is gaining heat and thus the q is considered positive

Justin Bui 2L
Posts: 51
Joined: Fri Sep 29, 2017 7:06 am

Re: The sign of q

Postby Justin Bui 2L » Mon Jan 29, 2018 1:15 am

Yeah, since the equation of q in the system is closed, the sign's position really just depends on whether the reaction is exo or endothermic.


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