Green Chemistry and the Ten Commandments

2 mol H 2 O × 2 mol O-H × 459 kJ = 1836 kJ

mol H 2 O mol O-H

Total bond energy in products = 1598 kJ + 1836 kJ = 3434 kJ

O C O O H

H

O H

H

C=O 799 kJ/mol

O-H 459 kJ/mol

H C H

H

H

• O O O O

C-H 411 kJ/mol O=O 494 kJ/mol

Figure 6.4. Bonds and their energies in the chemical species involved when methane burn in oxygen to
produce carbon dioxide and water.

A similar calculation gives the total bond energies in the reactants:

1 mol CH 4 × 4 mol C-H × 411 kJ = 1644 kJ

mol CH 4 mol C-H

2 mol O 2 × 1 mol O=O × 494 kJ = 988 kJ

mol O 2 mol O=O

Total bond energy in reactants = 1644 kJ + 988 kJ = 2632

The difference in bond energies between products and reactants is

3434 kJ - 2632 kJ = 802 kJ

This calculation states that, based upon considerations of bond energy, alone, the
energy released when 1 mole of CH 4 reacts with 2 moles of O 2 to produce 1 mole of
CO 2 and 2 moles of H 2 O, is 802 kJ. This is an exothermic reaction in which heat energy
is released, so it is denoted as -802 kJ. This value is close to the value that would be
obtained by experimentally measuring the heat energy released by the reaction, assuming
all the reactants and products were in the gas phase (a significant amount of heat energy
is released when vapor-phase water condenses to liquid). For the most part, therefore,
the amount of heat energy released in a chemical reaction, and the amount of potential
chemical energy contained in the reactants is equal to the difference between the total
bond energies of the products and those of the reactants.

Chap. 6. Energy Relationships 141