Question

Combustion reactions of fossil fuels provide most of the energy needs of the world. Why are combustion reactions of fossil fuels so exothermic?

Short answer

Combustion reactions of fossil fuels are highly exothermic due to the energy difference between the bonds formed and the bonds broken during the reaction. Fossil fuels contain strong C-H and C-C bonds in their hydrocarbon makeup, which break during combustion, releasing energy. The reaction forms new C=O bonds in CO₂ and H-O bonds in H₂O, releasing even more energy. The net energy release in the form of heat is significant, making these reactions particularly useful for providing the world's energy needs.

Step-by-step solution

Understand Exothermic Reactions

An exothermic reaction is a chemical reaction that releases energy in the form of heat. It occurs when the energy required to break the bonds in the reactants is less than the energy released upon the formation of new bonds in the products. The difference between these energies results in the release of heat.

Fossil Fuels Composition

Fossil fuels, such as coal, oil, and natural gas, are composed mainly of hydrocarbons (molecules containing hydrogen and carbon atoms) and other carbon-based compounds. These hydrocarbons, like methane (CH4) and octane (C8H18), are rich in energy due to the strong carbon-hydrogen (C-H) and carbon-carbon (C-C) bonds present in the molecules.

Combustion Reactions

In a combustion reaction, a hydrocarbon reacts with oxygen to produce carbon dioxide (CO2), water (H2O) and heat. The general equation for a combustion reaction can be represented as: \[ C_xH_y + O_2 \rightarrow CO_2 + H_2O + \text{Heat} \]

Breaking and Forming Bonds

During the combustion of fossil fuels, C-H and C-C bonds in hydrocarbons are broken, and new bonds—carbon-oxygen (C=O) bonds in CO2 and hydrogen-oxygen (H-O) bonds in H2O—are formed. The energy required to break the C-H and C-C bonds is less than the energy released during the formation of C=O and H-O bonds.

Highly Exothermic Reaction

The difference between the energy required to break the original bonds in the hydrocarbons and the energy released upon the formation of new bonds in CO2 and H2O accounts for the highly exothermic nature of combustion reactions. The large amount of heat released during these reactions makes them particularly useful for providing the significant amount of energy required to meet the world's energy needs.