10.02.3 Using Bond Energies (HT)
Syllabus
During a chemical reaction:
energy must be supplied to break bonds in the reactants
energy is released when bonds in the products are formed.
The energies needed to break bonds & released when bonds form can be calculated from bond energies.
The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
Students should be able to calculate the energy transferred in chemical reactions using bond energies supplied.
What does this mean?
How do we calculate energy changes for reactions?
To estimate the energy change in a reaction we can simply:
add up all the energy we would have to put in to break the bonds in the reactants.
add up all the energy it would released when the bonds in the products form.
Find the difference between the two totals
If more energy goes in than comes out = Endothermic = Positive value
If less energy goes in than comes out = Exothermic = Negative value
Energy change of Reaction = Total Energy In - Total Energy Out
You will always be given a chart containing all the bond energies you need, like the one below.
Example 1
Energy added to break bonds = one C=C + four C-H + 1 H-H
= 614 + 1652 + 436 = + 2702 kJ/mol
Energy released when bonds form = six C-H + one C-C
= 2478 + 348 = - 2826 kJ/mol
So, more released than added.
Energy change = - 124 kJ/mol (exothermic)
Example 2
Energy in = (5xC-H) +(C-C) + (C-O) + (O-H) + (3xO=O)
= + 4732
Energy out = (4x C=O) + (6xO-H) = - 8762
Energy Change = + 4732 - 8762
- 4030 kJ/mol
This reaction is very exothermic.
Which is why ethanol makes a good fuel
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