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Chemistry · Ch 5 — Thermodynamics

Bond Enthalpy

5.5(c)

Bond Enthalpy

Bond Enthalpy

Chemical reactions involve the breaking and making of chemical bonds. Energy is required to break a bond, and energy is released when a bond is formed. It is possible to relate the heat of reaction to changes in energy associated with breaking and making of chemical bonds.

Two different terms are used in thermodynamics with reference to enthalpy changes associated with chemical bonds:

  1. Bond dissociation enthalpy
  2. Mean bond enthalpy

Diatomic Molecules

Consider the process in which the bonds in one mole of dihydrogen gas (H₂) are broken:

H2(g)→2H(g);ΔH–HH⊖=435.0 kJ mol−1\text{H}_2(g) \rightarrow 2\text{H}(g); \quad \Delta_{\text{H–H}} H^\ominus = 435.0\ \text{kJ mol}^{-1}

The enthalpy change involved in this process is the bond dissociation enthalpy of the H–H bond.

Definition: The bond dissociation enthalpy is the change in enthalpy when one mole of covalent bonds of a gaseous covalent compound is broken to form products in the gas phase.

For diatomic molecules, the bond dissociation enthalpy is the same as the enthalpy of atomization. This is true for all diatomic molecules.

Examples:

Cl2(g)→2Cl(g);ΔCl–ClH⊖=242 kJ mol−1\text{Cl}_2(g) \rightarrow 2\text{Cl}(g); \quad \Delta_{\text{Cl–Cl}} H^\ominus = 242\ \text{kJ mol}^{-1}

O2(g)→2O(g);ΔO=OH⊖=428 kJ mol−1\text{O}_2(g) \rightarrow 2\text{O}(g); \quad \Delta_{\text{O=O}} H^\ominus = 428\ \text{kJ mol}^{-1}

Polyatomic Molecules

In the case of polyatomic molecules, bond dissociation enthalpy is different for different bonds within the same molecule, even if the bonds are of the same type.

Consider methane, CH₄. The overall thermochemical equation for its atomization reaction is:

CH4(g)→C(g)+4H(g);ΔaH⊖=1665 kJ mol−1\text{CH}_4(g) \rightarrow \text{C}(g) + 4\text{H}(g); \quad \Delta_a H^\ominus = 1665\ \text{kJ mol}^{-1}

In methane, all four C–H bonds are identical in bond length and energy. However, the energies required to break the individual C–H bonds in each successive step differ:

CH4(g)→CH3(g)+H(g);ΔbondH⊖=+427 kJ mol−1\text{CH}_4(g) \rightarrow \text{CH}_3(g) + \text{H}(g); \quad \Delta_{\text{bond}} H^\ominus = +427\ \text{kJ mol}^{-1}

CH3(g)→CH2(g)+H(g);ΔbondH⊖=+439 kJ mol−1\text{CH}_3(g) \rightarrow \text{CH}_2(g) + \text{H}(g); \quad \Delta_{\text{bond}} H^\ominus = +439\ \text{kJ mol}^{-1}

CH2(g)→CH(g)+H(g);ΔbondH⊖=+452 kJ mol−1\text{CH}_2(g) \rightarrow \text{CH}(g) + \text{H}(g); \quad \Delta_{\text{bond}} H^\ominus = +452\ \text{kJ mol}^{-1}

CH(g)→C(g)+H(g);ΔbondH⊖=+347 kJ mol−1\text{CH}(g) \rightarrow \text{C}(g) + \text{H}(g); \quad \Delta_{\text{bond}} H^\ominus = +347\ \text{kJ mol}^{-1}

Summing these four steps gives the total atomization enthalpy:

CH4(g)→C(g)+4H(g);ΔaH⊖=1665 kJ mol−1\text{CH}_4(g) \rightarrow \text{C}(g) + 4\text{H}(g); \quad \Delta_a H^\ominus = 1665\ \text{kJ mol}^{-1}

In such cases, we use the mean bond enthalpy of the C–H bond. For CH₄:

ΔC–HH⊖=14(ΔaH⊖)=14(1665 kJ mol−1)=416 kJ mol−1\Delta_{\text{C–H}} H^\ominus = \frac{1}{4}(\Delta_a H^\ominus) = \frac{1}{4}(1665\ \text{kJ mol}^{-1}) = 416\ \text{kJ mol}^{-1}

Tip

Mean bond enthalpy is an average value. It is calculated by dividing the total atomization enthalpy by the number of bonds of that type being broken. It differs slightly from compound to compound but not by a great deal.

Using Hess's law, bond enthalpies can be calculated. Bond enthalpy values of some single and multiple bonds are given in the tables below.

Relating Reaction Enthalpy to Bond Enthalpies

The reaction enthalpies are very important quantities as these arise from the changes that accompany the breaking of old bonds and formation of new bonds. We can predict the enthalpy of a reaction in the gas phase if we know different bond enthalpies.

ΔrH⊖=∑bond enthalpiesreactants−∑bond enthalpiesproducts\Delta_r H^\ominus = \sum \text{bond enthalpies}_{\text{reactants}} - \sum \text{bond enthalpies}_{\text{products}}

This relationship is particularly more useful when the required values of ΔfH⊖\Delta_f H^\ominus are not available. …

Table 5.3Some Mean Bond Enthalpies at 298 K (kJ mol$^{-1}$)

(a) Single bonds (row–column bond enthalpy)

HCNOFSiPSClBrI
H435.8414389464569293318339431368297
C347293351439289264259330276238
N159201272209201243
O138184368351205201
F155540490327255197
Si176213226360289213
P213230331272213
S213251213
Cl243218209
Br192180
I151

(b) Multiple bonds

| Bond | Enthalpy | Bond | Enthalpy | …

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