Bond Energies
Non-metal elements combine to form molecules
by sharing pairs of electrons in covalent bonds.
Every covalent bond requires a certain amount
of energy to break it.
This is known as the bond energy.
Since the breaking of a covalent bond requires
energy, the process is said to be endothermic.
i.e. heat (energy) is absorbed.
………….………..continued
For example, to break up the formula mass of
hydrogen molecules, H2, (Mr=2 x 1=2),or 2 grams
of hydrogen molecules requires 436 kJ (kilojoules)
of energy.
+
H = + 436 kJ
Conversely, the forming of a covalent bond
releases energy, and this process is said to be
exothermic. i.e. heat (energy) is evolved.
……………………..……continued
For example, when the formula mass of chlorine
molecules,(Mr = 2x35.5 = 71), or 71 grams
of chlorine molecules are formed, 242 kJ of
energy are released.
+
H = - 242 kJ
Hence, bond energies can be used to calculate
the overall heat change for a chemical reaction.
That is, we can calculate to what extent a chemical
reaction is exothermic or endothermic.
separate atoms
Energy
x
reactants
y
z
products
Progress of reaction
x = Total energy needed to break the bonds in the reactants.
y = Total energy released when the bonds are formed in the products.
z = Overall energy change for the reaction; i.e. exothermic
…………………….summary
You must start any calculation by writing a
balanced symbols equation;
– preferably showing all the covalent bonds
H
H + Cl Cl
2 H Cl
Carry out your calculations in the from of a
‘balance sheet’
1 x H-H bond = 436 kJ
1 x Cl-Cl bond = 242 kJ
Total
= 678 kJ
endothermic
2 x H-Cl bond = 2 x 431
= 862 kJ
exothermic
…continued
………………..continued
 Overall, more heat is evolved in forming the new
covalent bonds than is required to break the original
covalent bonds:
 Heat change for the reaction is:
862 – 678 = 184 kJ
Since this is heat evolved, we say:
H = -184 kJ (exothermic)
Why should we study bond energies?
• Bond energies can be used in industry to predict
the heat change for untried chemical reactions.
• This allows the chemical engineer to modify the
process to allow for excessive amounts of heat
being evolved; e.g. introduce cooling at
appropriate stages.
• Also, if it is known that large amounts of heat
(energy!) are to be released, it may be
appropriate to ‘trap’ this energy for use
elsewhere; i.e. to improve the economics of the
process.
………..continued
• Bond energies are also useful in determining the
efficiency of fuels; i.e. by calculating the heat
(energy) evolved per gram of fuel.
See the homework question in the workbook!
Tips for HWK
H
H
The structure of ethanol is: H C
C
H H
and it burns in oxygen
to form carbon dioxide and water only
O H