Energy From Fossil Fuels

Bond Making and Bond Breaking

During chemical reactions, energy is either released to the environment (exothermic reaction) or absorbed from the environment (endothermic reaction). During chemical reactions, bonds are broken in the reactants and new ones are made in the products. Bond-breaking is an endothermic process and bond-making is an exothermic process. The average bond dissociation energies of some chemical bonds are shown in the following table.

For any chemical reaction, the overall energy change, the enthalpy of reaction(DH), is the difference of all the energy absorbed in bond-breaking and all the energy released in bond-making.

DH = S BE(bonds broken)- SBE(bonds formed)

The energy produced in exothermic reactions can be used directly or be converted to other energy forms.

Redox Reactions

An oxidation-reduction or redox reaction is one in which electrons are transferred between two species. The rusting of iron is an example of a redox reaction.

In this reaction iron metal losses electrons (is oxidized) and the oxygen gains electrons (is reduced).

In a redox reduction something is always oxidized while another species is reduced.

Oxidation is easy to recognize when an element changes changes oxidation state to become an ion. However when dealing with organic molecules, atoms are joined by covalent bonds rather than ionic ones so oxidation is not as easily recognized.

In organic molecules, an atom is oxidized if during a reaction it becomes bonded to a more electronegative element which pulls electron density away from it.

In general, oxidation occurs if the oxygen content of a covalently bonded molecule increases or if the hydrogen content decreases.The carbon atom in methane (CH₄) is more reduced than the carbon atom in carbon dioxide (CO₂₎

Combustion of Fossil Fuels

Combustion is a reaction with oxygen. In the case of the combustion of fossil fuels, the combustion reaction is what we think of as a burning process. In the combustion reaction, the species reacting with the oxgyen is oxidized (because oxygen is very electronegative). Fossil fuels are composed primarily of hydrocarbons (molecules containing primarily carbon hydrogen bonds). In these molecules carbon is in a very reduced state. During the combustion reaction, the hydrocarbon molecules are converted to carbon dioxide and water.

Every mole of methane (16 g) releases 810 KJ of energy on burning.

Combustion energetics can be estimated from the bond energies for all the classifications of fossil fuels. The amount of energy released is dependent on the oxidation state of the carbons in the hydrocarbon which is related to the hydrogen/carbon ratio. The more hydrogen per carbon, the lower the oxidation state and the more energy that will be released during the oxidation reaction. Thus the greater the H/C ratio, the more energy release on combustion.

Petroleum has a large saturated hydrocarbons content. For estimating energy content, we can consider the molecule to consist of multiple -CH₂-units. For calculating the expected energy release per CH₂ unit, the energy of two C-H bonds and one C-C bond (since the carbon joins two neighboring groups and we are averaging) would be used which gives an energy content similar to that of methane ("natural gas"). However, petroleum also can contain a significant fraction of aromatic molecules depending on the nature of the petroleum distillate product. For example crude oil has an energy content of about 45.2 kJ/g while gasoline has a higher value of about 48.1 kJ/mole because it contains a smaller fraction of aromatics.

Coal is composed of primarily aromatic hydrocarbons, so we can consider the molecule to consist of multiple -CH- units. The actual average energy release per gram of coal from combustion is less than the predicted value since coal contains significant amounts of water and minerals. Hard coals such as bituminous or anthracite have larger energy content (29-33 kJ/g) than the soft sub-bituminous or lignite coals (17-21 kJ/g).

 The average hydrogen/carbon ratios show that the degree of unsaturation increases from natural gas through petroleum to coal. The amount of carbon dioxide released per mole also increases as the amount of unsaturation increases. Since carbon dioxide is a greenhouse gas, from this data, it would appear that burning coal would have a larger greenhouse effect than burning natural gas.

Ethanol (CH₃CH₂OH, a biomass derived fuel) is more highly oxidized than a hydrocarbon since it contains an oxygen and gives off significantly less energy on combustion than petroleum. Gasohol is a gasoline/ethanol mixture.