### sign of q

Posted:

**Thu Feb 01, 2018 4:19 pm**When calculating entropy how do you know when qrev is negative in the equation?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=131&t=26694

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Posted: **Thu Feb 01, 2018 4:19 pm**

When calculating entropy how do you know when qrev is negative in the equation?

Posted: **Thu Feb 01, 2018 5:43 pm**

Qrev is the heat being transferred into or out of the system. If heat is leaving the system, the q would be negative. If heat enters the system, then q is positive. This makes sense since if there is more energy in the system, you can expect there to be an increase in entropy.

Posted: **Thu Feb 01, 2018 8:02 pm**

It might help to think of it using the familiar concept q(system)=-q(surroundings). A system's surroundings gain the heat that a system loses. Because a gain in heat is an increase or positive value, the system must have the opposite sign when it loses heat. So if heat is leaving a system, q will be negative.

Posted: **Fri Feb 02, 2018 1:50 pm**

I also have trouble with the sign of q. I found that it helps me to try to think whether the q applies to the system or surrounding and determine what is losing and what is gaining.

Posted: **Sat Feb 03, 2018 3:32 pm**

The system's q sign and the surrounding's q sign will always be opposites since whatever the system is losing is what the surroundings gain.

Posted: **Sun Feb 04, 2018 8:09 pm**

When qrev is negative, it means that the system is losing heat, so the surroundings will gain heat and vice versa if qrev is positive. You can think of it in the context of exothermic and endothermic: if a system is endothermic (taking in heat), then qrev will be positive and if a system is exothermic (releasing heat), then qrev will be negative.

Posted: **Sun Feb 04, 2018 8:16 pm**

The sign of delta S depends on the direction of heat flow. Delta S is simply q_{rev} divided by T. q_{rev} is negative whenever the system is losing heat. From that information, you then know that the surroundings gain heat. If the system is gaining heat, then q_{rev} is positive.

Posted: **Sun Feb 04, 2018 9:30 pm**

q of system= -q of surroundings