When chemical change occurs bonds are rearranged. The bonds in the reactants are broken and new bonds are formed to produce the products. Look at the reaction in the example provided in the previous section on heats of formation, the bonds in the propane and the oxygen need to break and new bonds in the carbon dioxide and oxygen have to form. 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.
These bond energies termed average bond energies (of dissociation) are determined by examining the heats of various reactions in which each type of bond is broken.
For Example:
a) The dissociation of hydrogen (hydrogen gas becoming 2 separate hydrogen atoms).
H2 (g)
2 H (g)
H =
+ 436.4 kJ
The bond dissociation for a H __ H bond is 436.2 kJ.
b) The bond energy of a C__ H bond is determined by examining many reactions in which hydrocarbons like methane gas are broken into carbon and hydrogen atoms.
CH4 (g)
C (s)
+
4 H (g)
H =
+ 1650 kJ
Since four C __ H bonds have to be broken the average energy for each bond is + 412.5 kJ. These are average values except for diatomic gases like Hydrogen.
Using this method of examining reactions chemists have been able to estimate many bond energies for the breaking of bonds. When the same bond is formed the bond energy is equal and opposite. For example when a C __ H bond forms the energy released is - 412.5 kJ.
(Lists of these energies can be found in many books or on the net)
The formula used in the previous section with heats of formation can now be rewritten to calculate H using bond energies.
Examine the two formula's side by side. How are they different? How are they the same?
Answer


Hess's Law formula using bond energies
H
=
Energy of bonds broken
__
Energy of bonds made
Calculating H using bond energies.
To calculate H using bond energies we can use the following procedure.
Step 1) Determine the number (in moles) and type of bonds broken and formed from the balanced equation. This may require you to draw the molecules to identify how many of each bond type are present.
Step 2) Multiply the number of bonds by the average bond energy given in the table to determine the energy change.
Step 3) Plug values into the above formula to calculate the H of the overall reaction.
Example : Calculate the energy of the reaction for the burning of methane in oxygen to form carbon dioxide gas and water gas, using heats of formation. The balanced equation is given below. Use the following list of bond energies
C3H8(g) + 5 O2(g) ----> 3 CO2 (g) + 4 H2O(g)
Steps 1 and 2
1) Determine the number and types of bonds broken and formed.
2) Determine the energy change.
Bonds Broken (reactants)
Type
#
Bond Energy
Energy
C - C
2
347 kJ/mol
+ 694 kJ
C - H
8
413 kJ/mol
+3,320 kJ
O = O
5
498 kJ/mol
+2,490 kJ
Total
+6,488 kJ
Bonds formed (products)
O - H
8
464
- 3,712 kJ
C = O
6
805
- 4,830 kJ
Total
- 8,542 kJ
Step 3)
H = Energy of Bonds Broken - Energy of Bonds formed
H = 6,488 kJ - 8,542 kJ
H = - 2,054 kJ
 
 


Calculations of Heats of Reactions using bond energy
a) Comparison of Methods; Examine the following calculation of a heat of reaction that compares the value calculated with heats of formation with that calculated using Bond energies. Calculate the % difference between the values
Method
H
Examine the heat of reaction ( H ) calculated using the two methods above for the complete combustion of propane.
Calculate the % difference between the values using the following formula.
% Difference = |M1 - M2| ÷ Average of M1 and M2
M1 is the H using heat of formation
M2 is the H using bond energies
How far apart are the values calculated by the two methods ? Answer
Heat of formation :
- 2,044.5 kJ
Bond energies:
- 2,054.0 kJ
b) Complete the following activity comparing calculations using Heats of Formation with that of Bond Energies
notepad
Heats of Combustion
a) Using the two methods and tables below, determine the heats of complete combustion for the following substances. Heat of combustion means that each substance combines with oxygen ( O2 ) and the products are carbon dioxide gas and water vapour.
Show all your work.
Methane ( CH4(g) )
Methanol (CH3OH (l) )
Gasoline ( C8H18 (l) )
Hydrogen ( H2 (g) )
b) Compare the values for the H using % difference.
c) Provide some reasons why the values for bond energy and heat of formation are not the same.
Heat of formations
Bond Energies
Answer