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Thermodynamics and Stability


Heat of Reaction ΔHro

An exothermic reaction An endothermic reaction

 

However, there are certain types of reactions that are particularly common for making thermodynamic comparisons:

Heat of Combustion, ΔHco

Heat of combustion diagram for ethane
Heat of Formation, ΔHf o
Heat of formation diagram for ethane
Heat of hydrogenation, ΔHho
Heat of hydrogenation diagram for ethene

Hess's Law

Although not specifically covered in most organic text books, Hess's Law is very useful when investigating the thermodynamics of reactions.  Hess's Law can be expressed by the following expression and diagram:

"The enthalpy change for a reaction (ΔHro) that converts starting materials to products is independent of the reaction pathway"

Representation of Hess's Law For the reaction SM ? P which can either proceed directly or via an intermediate, I, the overall enthalpy change for the converssion of SM to P must be the same.

i.e.   ΔHSM-Po = ΔHSM-Io + ΔHI-Po

Essentially, it can be treated as the addition of 2-dimensional vectors, and CARE is required with the signs of the ?Ho terms.


© Dr. Ian Hunt, Department of Chemistry, University of Calgary