Return to Contents Chapter 2 : Alkanes Ch 2 contents

Thermodynamics and Stability


There are two very important fundamental thermodynamic equations that you should know and understand that relate various thermodynamic terms:


DG = -RT ln K

Heat of Reaction,  DHro

An exothermic reaction An endothermic reaction

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

Heat of combustion, DHco

Heat of combustion diagram for ethane
Heat of formation, DHf o
Heat of formation diagram for ethane
Heat of hydrogenation, DHho
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 (DHro) that converts starting materials to products is independent of the reaction pathway"
Representation of Hess's Law For the reaction SM to 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.   DHSM-Po = DHSM-Io + DHI-Po

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

previous page
next page
organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary