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Heat Released/Absorbed

Posted: Sun Jan 22, 2017 7:56 pm
by Carla Cruz Medina 1L
Is heat absorbed always negative and heat released always positive? Why is that so?

Re: Heat Released/Absorbed  [ENDORSED]

Posted: Sun Jan 22, 2017 8:03 pm
by Maggie Bui 1H
Hm, I think you have them mixed up? Heat released is negative and heat absorbed is positive.

This all goes back to the concept of a system and the surroundings. The system is what we're focusing on while the surroundings are everything else. We think of change in energy from the perspective of the system. Therefore, when we say the system releases heat, we think of that as energy leaving the system, hence the negative sign. Conversely, when the system absorbs heat, energy is entering the system, hence the positive sign.

Re: Heat Released/Absorbed

Posted: Sun Jan 22, 2017 8:08 pm
by Rika Jitosho 2E
Hi Carla,

It was helpful for me to remember that qp = H = enthalpy. qp gives a measure of heat released or absorbed at a constant pressure.

Enthaply, or H will be negative if the system is losing heat; therefore the system is cold, and the surroundings, hot. This type of reaction would be an exothermic reaction.
Enthalpy, or H will be positive if the system is gaining heat. This makes the system hot, and the surroundings cold. This type of reaction would be an endothermic reaction.