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FIg 1 Chem bonds

Chemical Bonds

"Hydrocarbon molecules have energy-rich bonds"


In thermochemical conversion processes, bonds btw adjacent carbon, hydrogen, and oxygen molecules are broken down to release their chemical energy (the energy stored in the bonds of atoms & molecules) [EIA, 2023].


For any chemical reaction, the overall energy change, i.e., the enthalpy change (DH), is the difference btw i) the energy absorbed in bond-breaking (an endothermic process: absorbs energy from its surroundings) and ii) the energy released in bond-making (an exothermic process: releases energy to its surroundings).  Table 1 shows selected bond energies, i.e., the amount of energy required to break one mole (about 6×10^23) of such bond.  As shown in Figure 1, the enthalpy change for the reaction 2H2 + O2 => 2H2O is 470 kJ/mol.


Crude oil (petroleum) is made up of carbon (82–87%, by weight) & hydrogen (10–14%), and, to a lesser extent, oxygen & nitrogen (about 1.5% each), indicating that it contains mainly carbon-hydrogen & carbon-carbon bonds.  Table 2 shows the energy content of hydrogen & fossil fuels.

Table 1: Chemical bond energies

bonds average v3a 3 v5 v7.png

Figure 1: Enthalpy change (DH) in 2H2 + O2 => 2H2O reaction

released energy v3.png

Table 2: Energy content of hydrogen & fossil fuels

density H2 v345_edited.jpg
Tab 2 energy densities
Fig 1 bons H2O
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