Applications of Bond Energies


Applications of Bond Energies are listed below:-

1. Determination of enthalpy of reaction

The bond energies can be used for determining enthalpies of reactions Delta H_{Reaction} = E(B.E)_R- E(B.E.)_P for example, we want to determine the enthalpy of the following reaction:

HC \equiv CH(g) +2H_2(g) \to H_3C- CH_3(g) \hspace{5mm}\Delta H = ?

 

In this reaction a triple bond (C \equiv C) breaks in acetylene and two H — H bonds break in 4H. In turn, one C C- bond and four C — H bonds are formed inC_2H_6. Therefore

\Delta H = [\Delta H_{C- C} +4 \Delta H_{C-H}] + [\Delta H_{C \equiv C} + 2\Delta H_{H- H}]

 

= -[3.47.3 + 4 \times 416.2] + [811.7 + 2 \times 435.1] \\[3mm] =-2.12.1 + 1681.9 \\[3mm] = -330.2 kJ mol^{-1}

 

2. Determination of enthalpies of formation of compounds

The bond energies can be used for determining enthalpies of formation of compounds. For example, we want to determine the enthalpy of formation of ethyl alcohol:

Determination of enthalpies of formation of compounds

Determination of enthalpies of formation of compounds


The formation of ethyl alcohol involves:

(i) Breaking of 3H-H bonds to give 6H, breaking bf 1/2O-O bond to give O and sublimation of 2 atoms of C(s) to give 2C(g).

(ii) Formation of one C-C bond, five bonds, one C-0 bond and one bond.

Therefore,

\Delta H_f = [3 \Delta H_{H- H} + \dfrac{1}{2} \Delta H_{O- O} + 2 \Delta H_{C(s) \to C(g)}]- [\Delta H_{C- C} + 5\Delta H_{C- H} + \Delta H_{C-O} + \Delta H_{O- H}]

 

= 3 \times 435.1 + \dfrac{1}{2} \times 489.5 + 2 \times 719.6]- [347.3 + 5 \times 416.2 + 351.4 + 464.4]

 

2989.2- 3244.1 = -254.9 k J mol^{-1}

 

3. Determination of resonance energy

Resonance energy is taken to be equal to the difference between the actual bond energy of the molecule and that of the most stable of the resonating structures. Let us determine the resonance energy of benzene.

The dissociation of benzene may be given as:

C_6H_6(g) \to 6C(g) + 6H(g)

 

According to Kekule's structure there are three C-C bonds, three C = C bonds and six C-H bonds in benzene. Therefore dissociation energy of benzene may be given as:

\Delta H_d = 3 \Delta H_{C-C} + 3 \Delta H_{C = C} + 6\Delta H_{C-H}

 

= 3 \times 347.3 + 3 \times 615.0 + 6 \times 416.2 \\[3mm] = 5384.1 k J mol^{-1}

 

Thus, dissociation energy of benzene is 5384.1 kJ mol^{-1}. But experimental value is known to be 5535.1 kJ mol^{-1}. Evidently, the resonance energy of benzene is 5384.1 = 151 kJ \hspace{2mm} mol^{-1}.



Related posts:

  1. Bond Energy Worksheet Bond energy is used to describe the strenght of a...
  2. Hess's law of constant heat summation It states that, "The amount of heat evolved or absorbed...
  3. Hydrogen-bond Hydrogen-Bond [Introduced By Latimer and Rodebush] It may be defined...
  4. Covalent Bond Covalent bond: Such bonds are formed by mutual sharing of...
  5. Odd Electron Bond Odd Electron Bond It may be defined as, "The bonds...