Bond Energies

Atoms bond together to form compounds because in doing so they attain lower energies than they possess as individual atoms. A quantity of energy, equal to the dispute between the energies of the bonded atoms and the energies of the divide atoms, is released, normally as heat. That is, the bonded atoms have a lower department of energy than the individual atoms do. When atoms combine to make a compound, energy is constantly given off, and the compound has a lower overall energy .
When a chemical reaction occurs, molecular bonds are broken and other bonds are formed to make different molecules. For model, the bonds of two water molecules are broken to form hydrogen and oxygen .
\ [ 2H_2O \rightarrow 2H_2 + O_2\ ]
Energy is constantly required to break a shackle, which is known as chemical bond energy. While the concept may seem elementary, adhere energy serves a identical authoritative aim in describing the structure and characteristics of a molecule. It can be used to determine which Lewis Dot Structure is most desirable when there are multiple Lewis Dot Structures.

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Energy is constantly required to break a bond. Energy is released when a bond is made .

Although each atom has its own feature adhesiveness energy, some generalizations are possible. For example, although the exact respect of a C–H attachment energy depends on the particular atom, all C–H bonds have a bond energy of approximately the same value because they are all C–H bonds. It takes roughly 100 kcal of energy to break 1 gram molecule of C–H bonds, so we speak of the chemical bond energy of a C–H bond as being about 100 kcal/mol. A C–C bond has an approximate adhere energy of 80 kcal/mol, while a C=C has a alliance department of energy of about 145 kcal/mol. 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 adhere energy .

Table 1: Average Bond Energies (kJ/mol)

Single Bonds
Multiple Bonds
H—H
432
N—H
391
I—I
149
C = C
614
H—F
565
N—N
160
I—Cl
208
C ≡ C
839
H—Cl
427
N—F
272
I—Br
175
O = O
495
H—Br
363
N—Cl
200

C = O*
745
H—I
295
N—Br
243
S—H
347
C ≡ O
1072

N—O
201
S—F
327
N = O
607
C—H
413
O—H
467
S—Cl
253
N = N
418
C—C
347
O—O
146
S—Br
218
N ≡ N
941
C—N
305
O—F
190
S—S
266
C ≡ N
891
C—O
358
O—Cl
203

C = N
615
C—F
485
O—I
234
Si—Si
340

C—Cl
339

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Si—H
393

C—Br
276
F—F
154
Si—C
360

C—I
240
F—Cl
253
Si—O
452

C—S
259

F—Br
237

Cl—Cl
239

Cl—Br
218

Br—Br
193

*C == O ( CO2 ) = 799
When a bond is strong, there is a higher bond energy because it takes more department of energy to break a impregnable bail. This correlates with bond order and bond duration. When the Bond ordain is higher, adhere distance is shorter, and the shorter the bond length means a greater the Bond Energy because of increase electric attraction. In general, the shorter the bond length, the greater the bond energy.
The average adhesiveness energies in Table T3 are the averages of chemical bond dissociation energies. For exemplar the average bind 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 to dissociate, while the O-H bond needs 428 kJ/mol .
\ [ \dfrac { 498.7\ ; kJ/mol + 428\ ; kJ/mol } { 2 } =464\ ; kJ/mol\ ]
When more shackle energies of the bond in different molecules that are taken into consideration, the average will be more accurate. however ,

  • Average bonds values are not as accurate as a molecule specific bond-dissociation energies.
  • Double bonds are higher energy bonds in comparison to a single bond (but not necessarily 2-fold higher).
  • Triple bonds are even higher energy bonds than double and single bonds (but not necessarily 3-fold higher).

Bond Breakage and Formation

When a chemical reaction occurs, the atoms in the reactants rearrange their chemical bonds to make products. The newly arrangement of bonds does not have the same full energy as the bonds in the reactants. therefore, when chemical reactions occur, there will always be an play along energy deepen .
Figure 1: (left) Exothermic Reactions. For an exothermic chemical reaction, energy is given off as reactants are converted to products. (right) Endothermic Reactions. For an endothermic chemical reaction, energy is absorbed as reactants are converted to products.
In some reactions, the energy of the products is lower than the energy of the reactants. frankincense, in the course of the reaction, the substances lose energy to the surrounding environment. such reactions are exothermic and can be represented by an energy-level diagram in Figure 1 ( left ). In most cases, the energy is given off as heat ( although a few reactions give off energy as sparkle ). In chemical reactions where the products have a higher energy than the reactants, the reactants must absorb energy from their environment to react. These reactions are endothermic and can be represented by an energy-level diagrams like Figure 1 ( right ) .
technically Temperature is Neither a reactant nor product
It is not uncommon that textbooks and instructors to consider estrus as a independent “ species ” in a reaction. While this is rigorously faulty because one can not “ add or remove inflame ” to a reaction as with species, it serves as a convenient mechanism to predict the chemise of reactions with changing temperature. For exercise, if inflame is a “ reactant ” ( \ ( \Delta { H } > 0 \ ) ), then the reaction favors the formation of products at raise temperature. similarly, if hotness is a “ product ” ( \ ( \Delta { H } < 0 \ ) ), then the reaction favors the geological formation of reactants. A more accurate, and hence preferred, description is discussed below . Exothermic and endothermic reactions can be thought of as having energy as either a `` product '' of the reaction or a `` reactant. '' exothermic reactions releases energy, so energy is a product. endothermic reactions require energy, so energy is a reactant . exemplar \ ( \PageIndex { 1 } \ ) : exothermic vs. Endothermic Is each chemical chemical reaction exothermic or endothermic ?

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  1. \(2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(ℓ)} + \text{135 kcal}\)
  2. \(N_{2(g)} + O_{2(g)} + \text{45 kcal} \rightarrow 2NO_{(g)}\)

Solution
No calculates are required to address this question. precisely search at where the “ estrus ” is in the chemical reaction .

  1. Because energy is released; this reaction is exothermic.
  2. Because energy is absorbed; this reaction is endothermic.

exercise \ ( \PageIndex { 1 } \ )
If the bond energy for H-Cl is 431 kJ/mol. What is the overall attachment energy of 2 moles of HCl ?
Answer
Simply multiply the median adhere energy of H-Cl by 2. This leaves you with 862 kJ/mol ( Table T3 ) .
exercise \ ( \PageIndex { 2 } \ ) : generation of Hydrogen Iodide
What is the enthalpy change for this reaction and is it endothermic or exothermic?

What is the heat content variety for this reaction and is it endothermic or exothermic ? \ [ H_2 ( guanine ) +I_2 ( deoxyguanosine monophosphate ) \rightarrow 2HI ( gigabyte ) \ ]
Solution
inaugural front at the equation and identify which bonds exist on in the reactants .

  • one H-H bond and
  • one I-I bond

nowadays do the lapp for the products

  • two H-I bonds

then identify the bond energies of these bonds from the table above :

  • H-H bonds: 436 kJ/mol
  • I-I bonds: 151 kJ/mol

The union of enthalpies on the reaction side is :
436 kJ/mole + 151 kJ/mole = 587 kJ/mol .
This is how much energy is needed to break the bonds on the reactant side. then we look at the adhere constitution which is on the product side :

  • 2 mol H-I bonds: 297 kJ/mol
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The total of enthalpies on the merchandise side is :
2 adam 297 kJ/mol= 594 kJ/mol
This is how much energy is released when the bonds on the product side are formed. The internet change of the reaction is consequently
587-594= -7 kJ/mol .
Since this is negative, the chemical reaction is exothermic .
exemplar \ ( \PageIndex { 2 } \ ) : decay of Water
Using the bond energies given in the chart above, find the heat content transfer for the thermal decomposition of water :
\ [ 2H_2O ( gram ) \rightarrow 2H_2 + O_2 ( g ) \ ]
Is the chemical reaction written above exothermic or endothermic ? Explain .
Solution
The heat content transfer deals with breaking two gram molecule of O-H bonds and the formation of 1 mole of O-O bonds and two moles of H-H bonds ( Table T3 ) .

  • The sum of the energies required to break the bonds on the reactants side is 4 x 460 kJ/mol = 1840 kJ/mol.
  • The sum of the energies released to form the bonds on the products side is
    • 2 moles of H-H bonds = 2 x 436.4 kJ/mol = 872.8 kJ/mol
    • 1 moles of O=O bond = 1 x 498.7 kJ/mil = 498.7 kJ/mol

which is an end product ( released ) energy = 872.8 kJ/mol + 498.7 kJ/mol = 1371.5 kJ/mol .
sum energy deviation is 1840 kJ/mol – 1371.5 kJ/mol = 469 kJ/mol, which indicates that the chemical reaction is endothermic and that 469 kJ of inflame is needed to be supplied to carry out this reaction .
This reaction is endothermic since it requires energy in decree to create bonds .

Summary

Energy is released to generate bonds, which is why the heat content transfer for breaking bonds is positive. Energy is required to break bonds. Atoms are much happier when they are “ marital ” and release energy because it is easier and more stable to be in a relationship ( for example, to generate octet electronic configurations ). The heat content change is damaging because the system is releasing energy when forming bond .

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. For more practice problems: http://www.chalkbored.com/lessons/chemistry-11/bond-energies-worksheet.pdf

Contributors and Attributions

  • Kim Song (UCD), Donald Le (UCD)
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