how is the enthalpy of formation of a substance connected to the thermodynamic stability of that substance?

Answer 1

enthalpy of formation affects the thermodynamic stability of any resulting compound by altering electron configuration, Gibbs free energy, temperature and entropy. Although the formation of a compound decreases entropy (a thermodynamically non-favorable affect) by combining otherwise seperate atoms or molecules, the stability of the product depends mostly on whether it was an endothermic or exothermic reaction. An endothermic rxn will use G (Gibbs free energy) or heat to overcome the Ea (energy of activation) barrier. This not only reduces entropy, but also temperature or heat in the system thus resulting in a less-stable product than the reagents. If the reaction were exothermic (emitting heat), entropy would decrease, but the energy released into the surounding environment would be available to do work as heat or freed energy (think kinetic). This result would be favorable as it will lead to an overall increase in entropy throughout the system, even though the reaction lead to a more stable product (based upon electron configurations not explained here).

The equation:
G = H - TS
is used to explain this phenomena.
G = Gibbs free energy (energy available to do work)
H = Enthalpy of formation
T = Temp. in K
S = Entropy.

Answer 2

Some substances give off heat and others consume heat based on the sign of the enthalpy of formation.

http://dbhs.wvusd.k12.ca.us/webdocs/Thermochem/StandardEnthalpyFormation.html

In simpler terms - reactions that give off heat tend to increase the entropy of a system and decrease the stability while reactions that consume heat tend to decrease entropy and increase stablity.

Gibbs Free Energy is not a topic covered in a high school chemistry class and is barely covered in college chemistry courses.

Answer 3

Hi. H = E + PV. Assuming Energy added to Pressure / Volume results in a lower energy state then as long as no additional energy is added, the new state is stable. Just a guess.