C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + energy (6.2.5)is the process that we and most other organisms use to obtain energy. So photosynthesis
enables the capture of energy from photons of visible light, a form of electromagnetic
energy, and its conversion to chemical energy.
Even the tractor shown in Figure 6.1 runs indirectly on solar energy. That is because in
eons past, plants and microscopic photosynthetic organisms (phytoplankton) performed
photosynthesis to produce large quantities of biomass. Subjected to high temperatures
and pressures in the absence of air underground, this biomass got converted to oils
(petroleum), coal, and organic matter closely associated with rocks called kerogen, the
source of shale oil. It is these fossil fuels upon which the world depends for most of its
energy today.
6.3. Storage and Release of Energy By Chemicals
Consider the burner on a kitchen range fueled by natural gas. The flame is obviously
hot; something is going on that is releasing heat energy. The flame is also giving off light
energy, probably as a light blue glow. A chemical reaction is taking place as the methane
in the natural gas combines with oxygen in the air,
CH 4 + 2O 2 → CO 2 + 2H 2 O + energy (6.3.1)to produce carbon dioxide and water. Most of the energy released during this chemical
reaction is released as heat, and a little bit as light. It is reasonable to assume that the
methane and oxygen contain stored energy as chemical potential energy and that it is
released in producing carbon dioxide and water. Common sense tells us that it would be
hard to get heat energy out of either of the products. They certainly won’t burn! Water is
used to put out fires, and carbon dioxide is even used in fire extinguishers.
The potential energy contained in chemical species is contained in the chemical
bonds of the molecules that are involved in the chemical reaction. Figure 6.4 shows the
kinds of bonds involved in methane, elemental oxygen, carbon dioxide, and water and
the energy contained in each. The bond energies are in units of the number of kilojoules
(kJ) required to break a mole (6.02 × 023 ) of the bonds (kJ/mol). The same amount of
energy is released when a mole of a bond is formed. By convention, energy put into a
system is given a positive sign and energy released is denoted by a negative sign.
To calculate the energy change when a mole of methane reacts with oxygen as shown
in Reaction 6.3.1, the difference is taken between the sum of the energies of the bonds
in the products and the sum of the energies of the bonds in the reactants. Examination of
Reaction 6.3.1 and Figure 6.4 shows the following total bond energies in the products:
1 mol CO 2 × 2 mol C=O × 799 kJ = 1598 kJ
mol CO 2 mol C=O140 Green Chemistry, 2nd ed