Alkanes are described as kinetically stable because they usually react very slowly, but as thermodynamically unstable because reactions such as combustion produce more stable products and release energy. This distinction is relevant to both SL and HL Chemistry.
| Type of stability | What it means for alkanes |
|---|---|
| Kinetic stability | A reaction has a high activation energy, so it proceeds extremely slowly unless sufficient energy is supplied. |
| Thermodynamic stability | The relative energies of reactants and products determine whether the overall reaction is energetically favorable. Alkanes are less stable than their combustion products in the presence of oxygen. |
Alkanes contain strong, relatively non-polar CtoC and CtoH bonds. To begin a reaction, bonds must first be broken, and bond breaking requires energy. At room temperature, few collisions have enough energy to overcome the activation energy, so a mixture of an alkane and oxygen may remain unchanged for a long time.
However, once combustion is initiated by a spark, the formation of strong C=O bonds in carbon dioxide and OtoH bonds in water releases more energy than is absorbed in breaking the reactant bonds. For methane:
The reaction is strongly exothermic, so the products have lower enthalpy than the reactants. This explains the alkane's thermodynamic instability relative to its oxidation products.
This also links to Reactivity 3.3: electron sharing reactions. In radical substitution, ultraviolet light causes homolytic fission of a halogen molecule, producing radicals that initiate reaction with the otherwise unreactive alkane.
A common misconception is that thermodynamically favorable means fast. It does not: thermodynamics concerns the energetic favorability and equilibrium position, while kinetics concerns reaction rate and activation energy.
Exam technique: In an “explain” question, state both ideas: products are energetically more stable, but a high activation-energy barrier prevents rapid reaction. Do not claim that breaking bonds releases energy.