# Violation of 2nd Law of Thermodynamics

## Main Question or Discussion Point

Chemical reaction have 2 classifications: Exothermic and Endothermic.
Exothermic is when the chemical give off heat in the reaction.
Endothermic is when the chemical obsorb heat in the reaction.

So... If one exploit Endothermic reaction by keep taking heat from the low temperature heat sink and later use it to produce a higher temperature environment. Have we not indirectly violate the 2nd law of thermodynamics?

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Mapes
Homework Helper
Gold Member
The Second Law states that the total entropy in the universe tends to increase. Can you explain further how you think your thought experiment violates this statement? It isn't clear how an endothermic reaction could be used to "produce a higher temperature environment."

I think you're misunderstanding the law--endothermic rxns only 'go' when the entropy increase makes up for the drop in temp.

russ_watters
Mentor
In an exothermic reaction, the amount of energy released is equal to the amount of energy stored in the chemical bonds. That's a demonstration of the laws of thermodynamics, not a violation of them.

I think you're misunderstanding the law--endothermic rxns only 'go' when the entropy increase makes up for the drop in temp.
Well, there are many interpretations of the 2nd law, but I will put up what I most see it as.

from winki
"In simple terms, the second law is an expression of the fact that over time, ignoring the effects of self-gravity, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how far along this evening-out process has progressed.

There are many versions of the second law, but they all have the same effect, which is to explain the phenomenon of irreversibility in nature."

Alright, endo reaction take heat from the surrounding. Let say we do the reaction at 60F. Later we break the bond at 70F. We then release some heat to 70F environment. That means we have taken heat from lower source (60F) and tranfer it to higher source (70F). Heat cannot flow from cold to hot. Yet, it happens.

Mapes
Homework Helper
Gold Member
The Second Law says that heat does not spontaneously flow from a colder reservoir to a hotter reservoir. And that didn't happen in your example. You heated the substance from 60°F to 70°F. Then you applied energy to break the chemical bonds, because the endothermic reaction is less likely to run in reverse at a higher temperature. These additions of external energy were needed because the process is not spontaneous, and the result is a net increase in entropy in the universe. The Second Law isn't violated.

It's important to realize that an endothermic (or exothermic) reaction isn't necessarily spontaneous. A reaction is endothermic (exothermic) if the change in internal energy $\Delta U$ is positive (negative). It's spontaneous (non-spontaneous) if the change in free energy $\Delta G$ is negative (positive), and

$$\Delta G=\Delta U+p\Delta V-T\Delta S.$$

If you think about the signs of $\Delta U$ and $\Delta S$, you'll see that your endothermic reaction at 60°F could not possibly run in reverse spontaneously at 70°F.

Hm... this just sum up to "the chemical absorb heat to becomes more stable". Interesting.

HallsofIvy