1.7 Chemical Bond Energy
As we have seen, whenever a chemical reaction occurs, there is always a change in energy. Where does this energy come from?
In any chemical substance, energy is stored in the chemical bonds that hold atoms together to form molecules (intramolecular bonds), and in the bonds that hold individual molecules together in the solid and liquid state (intermolecular bonds). During a chemical change, these bonds are rearranged - some bonds are broken, other bonds are formed.
Energy is required
to break chemical bondsEnergy is released
when new bonds formAlmost all chemical reactions involve changes in which some bonds are both broken and formed. Thus, some energy will be needed to break bonds, but energy will also be released as new bonds form. When we write a chemical equation that includes the energy change, the equation shows the net difference in energy change.
During exothermic reactions there is a net release of energy. More energy is given off than is put into the reaction.
For example, consider what happens at a molecular level when hydrogen and fluorine gas combine to produce hydrogen fluoride:
As atoms rearrange, both H—H and F—F bonds must be broken while H—F bonds must form. In this particular reaction, more energy is released when HF bonds form than is needed to break the other bonds. For this particular reaction, the net difference is a release of 546 kJ of energy so the energy term (546 kJ) appears on the product side of the equation:
H2(g) + F2(g) → 2 HF(g) + 546 kJ
Endothermic reactions require a net input of energy. More energy is needed to break bonds than is given off when new bonds form.For example consider the when sulfur trioxide decomposes into sulfur trioxide and oxygen. Energy is needed to break apart the SO3 molecule, but is released when the bonds within SO2 and O2 are formed. In this case, more energy is needed to break bonds than is released. The energy term (198 kJ) is written on the reactant side of the equation.
2 SO3(g) + 198 kJ → 2 SO2(g) + O2(g)
To help you understand the concept of net energy change consider this analogy:
Say you have a lemonade stand.
On Day 1, you had to spend $8.00 to buy your supplies - lemonade, sugar, cups. At the end of the day you had $10.00. What is your profit, or net difference for the day? You had a profit of $2.00. This would be like an exothermic reaction - there is more money at the end of the day than was initially put in.
On Day 2, the supplies cost $15.00, but you only sold $10.00 worth of lemonade. Not such a good day - a net loss of $5.00. This is like an endothermic reaction - more was put in then was gained at the end.
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