the sign of q


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Shannon Wasley 2J
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the sign of q

Postby Shannon Wasley 2J » Sun Jan 21, 2018 11:05 pm

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
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Re: the sign of q

Postby DianaTrujillo2K » Sun Jan 21, 2018 11:14 pm

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

Postby Carlos Gonzales 1H » Sun Jan 21, 2018 11:15 pm

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

Postby Shanmitha Arun 1L » Sun Jan 21, 2018 11:37 pm

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

Ashley Garcia 2L
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Re: the sign of q

Postby Ashley Garcia 2L » Mon Jan 22, 2018 3:51 pm

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
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Re: the sign of q

Postby Nicole Anisgard Parra 2H » Mon Jan 22, 2018 8:49 pm

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 :)


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