2 O=O Bonds Are Being Broken These Require

Bond Energies

When chemical changes occur, bonds are rearranged. The bonds in the reactants are broken and new bonds are formed to produce the products.

ex CH4(g) + 2 O2(g) à CO2(g) + 2 H2O(l)

bonds being broken: 4 C-H bonds are being broken both of

2 O=O bonds are being broken these require

energy

bonds being formed: 2 C=O bonds being formed both of

4 H-O bonds being formed these release

energy

The heat of the reaction is the difference between the energy needed to break the bonds in the reactants and the energy released in the formation of the bonds in the products.

Energy to Break Bonds / Energy Released When Bonds Formed
Bond / Bond Energy / Energy / Bond / Bond Energy / Energy
C-H / 413 kJ/mole / 1652 kJ/mole / C=O / 805 kJ/mole / 1610 kJ/mole
O=O / 498 kJ/mole / 996 kJ/mole / H-O / 464 kJ/mole / 1856 kJ/mole
Total Energy / 2648 kJ/mole / Total Energy / 3466 kJ/mole

DH = Total Energy to Break Bonds – Total Energy Given Off

DH = (2648 kJ/mole) – (3466 kJ/mole)

DH = - 818 kJ/mole

* Note – the bond energy values are averages and may vary as molecules become larger or smaller.

C3H8(g) + 5 O2(g) à 3 CO2(g) + 4 H2O(l)

Energy to Break Bonds / Energy Released When Bonds Formed
Bond / Bond Energy / Energy / Bond / Bond Energy / Energy
C-C / 347 kJ/mole / 694 kJ/mole / C=O / 805 kJ/mole / 4830 kJ/mole
C-H / 413 kJ/mole / 3304 kJ/mole / H-O / 464 kJ/mole / 3712 kJ/mole
O=O / 498 kJ/mole / 2490 kJ/mole
Total Energy / 6488 kJ/mole / Total Energy / 8542 kJ/mole

DH = Total Energy to Break Bonds – Total Energy Given Off

DH = (6488 kJ/mole) – (8542 kJ/mole)

DH = - 2054 kJ/mole