# 1st Law of Thermodynamics?

• B

## Main Question or Discussion Point

So it is stated that exothermic reactions are the ones where the heat of reaction is positive and endothermic reactions are the ones where the heat of reaction is negative.

My question is: why is this so, when in the 1st law of thermodynamics, where
ΔU=Q+W it is stated that Q is positive if the system absorbs heat from the environment and Q is negative if the system releases heat to the environment ?

Related Other Physics Topics News on Phys.org
dRic2
Gold Member
So it is stated that exothermic reactions are the ones where the heat of reaction is positive and endothermic reactions are the ones where the heat of reaction is negative.
It depends on the reference you choose. If you consider the heat absorbed by the system to be positive then the exothermic reactions are the one where the heat of reaction is negative . Otherwise if you consider the heat produced by the system to be positive the exothermic reaction have positive heat of reaction.

It depends on the reference you choose. If you consider the heat absorbed by the system to be positive then the exothermic reactions are the one where the heat of reaction is negative . Otherwise if you consider the heat produced by the system to be positive the exothermic reaction have positive heat of reaction.
I thought it's not a matter of considering, I thought that it's a convention that if the system produces heat, then heat is negative, because it's lost.

So if we get an exam question, such as "Is heat negative or positive in an exothermic reaction" , then what would the answer be ?

dRic2
Gold Member
I thought that it's a convention that if the system produces heat, then heat is negative, because it's lost.
It is a convention, you said yourself. The importance thing is to say that the heat is lost. The sing should be indifferent (for you) as long as it is coherent with your calculation.

By the way, worldwide, exothermic reaction have negative $\Delta H$ of reaction (in a test I would say so) for the reason you said, because that's the convention most used.

Chestermiller
Mentor
The reaction being exothermic means that heat has to be removed from the system in order to hold the temperature of the system constant at its initial value. The reaction being endothermic means that heat has to be added to the system to hold the temperature of the system constant at its initial value.