chemistry.stackexchange.com/questions/7696/how-do-i-distinguish-between-internal-energy-and-enthalpy/7700#7700 has a good insight:
To summarize, internal energy and enthalpy are used to estimate the thermodynamic potential of the system. There are other such estimates, like the Gibbs free energy G. Which one you choose is determined by the conditions and how easy it is to determine pressure and volume changes.
Adds up chemical energy and kinetic energy.
Wikipedia mentions however that the kinetic energy is often negligible, even for gases.
The sum is of interest when thinking about reactions because chemical reactions can change the number of molecules involved, and therefore the pressure.
To predict if a reaction is spontaneous or not, negative enthalpy is not enough, we must also consider entropy via Gibbs free energy.
TODO understand more intuitively how that determines if a reaction happens or not.
At least from the formula we see that:
  • the more exothermic, the more likely it is to occur
  • if the entropy increases, the higher the temperature, the more likely it is to occur
    • otherwise, the lower the temperature the more likely it is to occur
A prototypical example of reaction that is exothermic but does not happen at any temperature is combustion.
Video 1.
Lab 7 - Gibbs Free Energy by MJ Billman (2020)
. Source. Shows the shift of equilibrium due to temperature change with a color change in a HCl CoCl reaction. Unfortunately there are no conclusions because its student's homework.
I think these are the ones where , i.e. enthalpy and entropy push the reaction in different directions. And so we can use temperature to move the Chemical equilibrium back and forward.
Video 1.
Demonstration of a Reversible Reaction by Rugby School Chemistry (2020)
. Source. Hydrated copper(ii) sulfate.

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