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Fossil fuels are hydrocarbon compounds that have formed over millions of years from the remains of plants and animals. They include coal, oil, and natural gas and contain a high amount of chemical energy stored in their bonds.

An exothermic reaction is a chemical reaction that releases energy (mostly in the form of heat) into the surrounding environment. These reactions occur when the energy stored in the bonds of the reactants is greater than the energy stored in the bonds of the products.

Combustion is a type of chemical reaction that occurs between a fuel (usually a hydrocarbon) and an oxidizing agent (usually oxygen from the air), releasing heat and producing products such as carbon dioxide and water.

In the combustion of fossil fuels, the carbon and hydrogen atoms present in the fuel combine with oxygen from the air to form carbon dioxide (CO2) and water (H2O). This process involves breaking the chemical bonds between carbon, hydrogen, and oxygen and forming new bonds in the CO2 and H2O molecules.

The combustion reactions of fossil fuels are exothermic because the energy released during the formation of new CO2 and H2O bonds is greater than the energy required to break the original carbon, hydrogen, and oxygen bonds. This excess energy is released as heat, making the reaction exothermic. The energy released in the form of heat during combustion reactions drives various processes such as generating electricity in power plants and providing heat for our homes and vehicles. In conclusion, combustion reactions of fossil fuels are exothermic because the formation of new chemical bonds in the products (CO2 and H2O) releases more energy than the energy required to break the initial bonds in the reactants (carbon and hydrogen in the fuel and oxygen in the air).