Enthalpy change for different alcohols.

Investigation –Enthalpy change for different alcohols:

An alcohol is a series of organic, homologous compounds, Alcohols may be classified according to which carbon of the alkyl group is bonded to the hydroxyl group with the general formula CnH2n+1OH. The alcohol reacts with the oxygen in the air to form water and carbon dioxide:

The structure of the molecules in this reaction is:

This is an exothermic reaction, as heat is given out from the formation of water and carbon dioxide. This is because there is more energy given out when bonds are broken, furthermore less energy is absorbed when bonds are formed therefore the energy left over after the formation (water and carbon dioxide) is given out to the surrounding as heat. This can be shown in an energy level diagram:

The values of enthalpy change for any exothermic reaction is always negative as the diagram above shows that energy is 'lost' as heat. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. The bonds that are formed in an exothermic reaction can be of two types. The first could be ionic, where a metal is produced. Ionic bonding involves electrons transferring from one atom to the other consequently leaving an electrostatic force between them. The other form of bonding is covalent where atoms share electrons to complete their outer orbit. An example is Methanol where four hydrogen atoms each share an electron with a carbon atom.

For any reaction that is carried out directly at a constant pressure, the heat flow is exactly equal to the difference between enthalpy of products and that of the reactant. The symbol delta H ( H ) is given to symbolise enthalpy change.

H Reaction = H Products - H Reactant

Combustion of methanol.