## the sign of q

$\Delta U=q+w$

Shannon Wasley 2J
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### the sign of q

How do we determine whether the sign on q is positive or negative, if we are only given the number and not the sign? For example, if it said that it required 5.5 kJ of heat. How would we know if that value was positive or negative?

DianaTrujillo2K
Posts: 40
Joined: Fri Sep 29, 2017 7:04 am

### Re: the sign of q

If it is said that system requires heat then the sign of q will be positive because the system is absorbing heat. You can tell the sign of q based on the wording. If a system cools down or gives off heat then the sign of q will be negative.

Carlos Gonzales 1H
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### Re: the sign of q

When q is positive, heat has been absorbed by the system. When q is negative, the system has lost heat. If the system requires 5.5 kJ of heat that means that for the system to work it will have to absorb 5.5 kJ of heat. This means that q in this case will be positive.

Shanmitha Arun 1L
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### Re: the sign of q

Simply put, Q is positive if heat is added to the system, and negative if heat is removed.

Ashley Garcia 2L
Posts: 33
Joined: Fri Sep 29, 2017 7:05 am

### Re: the sign of q

If the system loses/releases heat and warms the surroundings then q is negative (exothermic), if the system gains/absorbs heat and cools the surroundings then q is positive (endothermic). Therefore, in your example if an reaction " required 5.5 kJ of heat. How would we know if that value was positive or negative?" the sign would be positive because the system is requiring 5.5 kJ of heat to be put into the system.

Nicole Anisgard Parra 2H
Posts: 39
Joined: Sat Jul 22, 2017 3:01 am

### Re: the sign of q

The wording in the problem will give you a hint as to whether or not the value of q is positive or negative. When a system requires energy, the value of q is positive (endothermic, system absorbs energy). When a system releases energy, the value of q is negative (exothermic, system releases energy).
The way I visualize it is by drawing a diagram; if the heat/energy is going away from the system, then the system is losing that energy and it is thus negative.
Hope that hopes :)