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The bond energy is a measure of the amount of energy needed to break apart one mole of covalently bonded gases. The SI units used to describe bond energy is kiloJoules per mole of bonds (kJ/mol).
When a chemical reaction occurs, molecular bonds are broken and other bonds are formed to make different molecules. For example, the bonds of two water molecules are broken to form hydrogen and oxygen.
\[ 2H_2O \rightarrow 2H_2 + O_2\]
Bonds do not break and form spontaneously-an energy change is required. The energy input required to break a bond is known as bond energy. While the concept may seem simple, bond energy serves a very important purpose in describing the structure and characteristics of a molecule. It can be used to determine which Lewis Dot Structure is most suitable when there are multiple Lewis Dot Structures.
When a bond is strong, there is a higher bond energy because it takes more energy to break a strong bond. This correlates with Bond Order and Bond Length. When the Bond Order is higher, bondlength is shorter, and the shorter the bondlength means a greater the Bond Energy because of increased electric attraction. In general, the shorter the bondlength, the greaterthe bond energy.
Think about it this way: it is easy to snap a pencil, but if you keep snapping the pencil it gets harder each time since the length of the pencil decreases. A higher bond energy (or a higher bond order or shorter bond length) means that a bond is less likely to break apart. In other words, it is more stable than a molecule with a lower bond energy. With Lewis Structures then, the structure with the higher bond energy is more likely to occur.
The diagram depicts how the atoms of Nitrogen break and bond with one another. The breakage and formation of bonds is similar to a relationship: you can either get married or divorced and it is more favorable to be married.
Enthalpy is the total change in energy in a thermodynamic system. Energy is either released or absorbed depending on the reaction that is taking place. Enthalpy is related to Bond Energy because an energy change is required to break bonds. More specifically, bond energy measures the energy that is added to the system to break bonds. Bond Energies can be used to determine if a reaction is endothermic or exothermic.
The same bond can appear in different molecules, but it will have a different bond energy in each molecule because the other bonds in the molecule will affect the bond energy of the specific bond. So the bond energy of C-H in methane is slightly different than the bond energy of C-H in ethane. We can calculate a more general bond energy by finding the average of the bond energies of a specific bond in different molecules to get the average bond energy.
Average Bond Energies (kJ/mol)
|Single Bonds||Multiple Bonds|
|C = C|| |
|C ≡ C|| |
|O = O|| |
| ||C = O*|| |
|C ≡ O|| |
| ||N—O|| |
|N = O|| |
|N = N|| |
|N ≡ N|| |
|C ≡ N|| |
| ||C = N|| |
| ||Si—H|| |
| || |
| ||Cl—Cl|| |
| || |
| ||Cl—Br|| |
| || |
| ||Br—Br|| |
| || |
*C == O(CO2) = 799
When more bond energies of the bond in different molecules that are taken into consideration, the average will be more accurate. Keep in mind that:
What is the enthalpy change for this reaction and is it endothermic or exothermic?
\[H_2(g)+I_2(g) \rightarrow 2HI(g)\]
First look at the equation and determine what bonds exist.
Because we're dealing with net change, we only need to look at 1 mol of H-H, I-I, and H-I bond. Then examine the bond breakage which is located in the reactant side:
Then we look at the bond formation which is on the product side:
Hess's Law relates to this equation as it depicts how the energy of the overall reaction is equal to the sum of the individual steps involving energy change.
which is an output (released) energy = 872.8 kJ/mol + 498.7 kJ/mol = 1371.5 kJ/mol.
Total energy difference is 1840 kJ/mol – 1371.5 kJ/mol = 469 kJ/mol, which indicates that the reaction is endothermic and that 469 kJ of heat is needed to be supplied to carry out this reaction.
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