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Bond Energies

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).

Introduction

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, Bond Length is shorter, and the shorter the Bond Length means a greater the Bond Energy because of increased electric attraction. 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. 

Bond Breakage/Formation

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. 

From the diagram above, it is clearly seen that energy is released when atoms form bonds while energy is absorbed to break apart bonds, which is why breaking bonds is positive and forming bonds is negative. It takes energy and stress to get "divorced." Atoms are much happier when they are "married" and release energy because it is easier and more stable to be in a relationship. The energy change is negative because the system is gives off energy when a bond is formed. 

Enthalpy

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. We can use Bond Energy to determine if a reaction is endothermic or exothermic.

  • If the reactants have weak bonds, while the products have strong bonds then the reaction is exothermic (enthalpy change < 0). There is a small amount of energy needed to break the bond (smaller bond energy) and a bigger energy released when strong bonds form. A negative enthalpy  change means that the system released energy. 
  • If the reactants have strong bonds, but the products have weak bonds its an endothermic reaction. (enthalpy change > 0). The energy required to break the reactant bonds is greater than the energy released when the product bonds form. 

Average Bond Energy

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. 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: 

  • It also isn't as accurate as a certain bond-dissociation energy.
  • Double bonds have higher energy bonds in comparison to a single bond. However that doesn't make the energy twice the amount of a single bond, it is simply just higher.

Average Bond Energies (kj/mol):

 

Bond Energy (kj/mol)
Bond
Bond Energy
(kj/mol)
Bond
Bond Energy (kj/mol)
Bond
 297
H-I
347
C-C
163
N-N
364
H-Br
611
C=C
418
N=N
368
H-S
837
C:::C
946
N:::N
389
H-N
305
C-N
222
N-O
414
H-C
615
C=N
590
N=O
431
H-Cl
891
C:::N
 
436
H-H
360
C-O
 
464
H-O
736
C=O
 
565
H-F
339
C-Cl
 
151
I-I
142
O-O
 
159
F-F
498
O=O
 
193
Br-Br
 
243
Cl-Cl
 
 
-Average bond energies are the averages of bond dissociation energies. For example the average bond energy of O-H in H2O is 464 kj/mol. This is due to the fact that the H-OH bond requires 498.7 kj/mol in order to dissociate, while the O-H bond needs 428 kj/mol. 
 
(498.7 kj/mol +428 kj/mol)/2=464 kj/mol.
 
Example
Consider this reaction
H2(g)+I2(g) → 2HI(g)

First look at the equation and determine what bonds exist.

There's an H-H bond, I-I bond, and 2 H-I bonds (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:

1 mol H-H bonds → 436 kj/mol

1 mol I-I bonds → 151 kj/mol

The sum is 587 kj/mol.

Then we look at the bond formation which is on the product side :

1 mol H-I bond → 297 kj/mol

The sum is 297 kj/mol.

The net change of the reaction is therefore 587-297= 290 kj/mol. Since it's a positive number you know that the reaction is endothermic.  

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.

Problems

  1. What is the definition of bond energy? When is energy released and absorbed?
  2. If the bond energy for H-Cl is 431kj/mol. What is the overall bond energy of 2HCl?
  3. Using the bond energies given in the chart above, find the enthalpy change for: O2 (g)+ N2 (g) → 2NO(g)
  4. Is the reaction written above exothermic or endothermic? Explain.
  5. Which bond in this list has the highest bond energy? The lowest? H-H, H-O, H-I, H-F.

Solutions

1. Bond energy is the energy required to break a bond that exists between two atoms. Energy is given off when the bond is broken, but is absorbed when a new bond is created.

2. Simply multiply the average bond energy of H-Cl by 2. This leaves you with 862 kj/mol.

3. The enthalpy change deals with one mole of O-O, N-N, and N-O bond.

The sum of the bonds being broken is 142 kj/mol + 163 kj/mol= 305 kj/mol

O-O bond:142 kj/mol

N-N bond: 163 kj/mol

The sum of the bonds being created is -222 kj/mol because there is one mole of N-O.

The enthalpy of the reaction is 305 kj/mol-222 kj/mol= +83 kj/mol.

4. For this question simply look at the number you calculated as your enthalpy of reaction. Is it positive or negative? It is positive so this means that it is in fact endothermic. It requires energy in order to create bonds.

5. H-F would have the highest bond energy since the difference in electronegativity is the largest there. Likewise, H-H would have the lowest bond energy since the electronegativity is the same.

References

  1. Petrucci, Ralph H., Harwood, William S., Herring, F. G., and Madura Jeffrey D. General Chemistry: Principles and Modern Applications. 9th ed. Upper Saddle River: Pearson Education, Inc., 2007.
  2. Carruth, Gorton, Ehrlich, Eugene. "Bond Energies." Volume Library. Ed. Carruth, Gorton. Vol 1. Tennessee: Southwestern, 2002.
  3. Petrucci, Ralph H., Bissonnette, Carey, Herring, F.G., and Madura Jeffrey D. General Chemistry: Principles and Modern Applications. 10th ed. Upper Saddle River: Pearson Education, Inc., 2010
  4. "Bond Lengths and Energies." UWaterloo, n.d. Web. 21 Nov 2010. <http://www.science.uwaterloo.ca/~cch...20/bondel.html>
  5. "Bond Energy." N.p., n.d. Web. 21 Nov 2010. <http://users.rcn.com/jkimball.ma.ult...ndEnergy.htmll>.

Contributors

  • Kim Song (UCD), Donald Le (UCD)

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Last Modified
00:58, 18 Feb 2014

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