Chapter U5.104, Problem 6E

Solution

(a)

Interpretation: The total energy required to break the bonds in the reactant molecules needs to be calculated.

Concept Introduction : The total bond energy can be defined as the summation of individual bond energies of the bonds in the molecule.

The total energy required to break the bonds in the reactant molecules is 1358 kJ.

The bond energy of is 432 kJ/mol, is 142 kJ/mol

In the reaction,

  ${\text{2H}}_{\text{2}}\text{+}{\text{O}}_{\text{2}}\to {\text{2H}}_{\text{2}}\text{O}$

The total bond energy can be calculated as the sum of bond energies of and bonds.

Total energy (E)= 2 bond) + bond)

  $\begin{array}{c}\text{E=}\text{2×}\text{432}\text{+}\text{494}\\ \text{=}\text{1358}\text{kJ}\end{array}$

(b)

Interpretation: The total energy released by the formation of bonds in the product molecule needs to be calculated.

Concept Introduction : The total bond energy can be defined as the summation of individual bond energies of the bonds in the molecule.

The total energy released by the formation of bonds in the product molecule is 1836 kJ.

In the reaction,

  ${\text{2H}}_{\text{2}}\text{+}{\text{O}}_{\text{2}}\to {\text{2H}}_{\text{2}}\text{O}$

The total bond energy of the product molecule can be calculated as the bond energy of molecule.

The bond energy of is 459 kJ/mol.

So,

  $\begin{array}{c}\text{Total}\text{energy}\left(\text{E}\right)\text{=}\text{4}\left(\text{H-O}\text{bond}\text{energy}\right)\\ \text{=}\text{4×}\text{459}\\ \text{=}\text{1836}\text{kJ}\end{array}$

(c)

Interpretation: The reaction is endothermic or exothermic needs to be determined using the value of (a) and (b).

Concept Introduction : The total bond energy can be defined as the summation of individual bond energies of the bonds in the molecule.

If the energy value is positive, then the reaction is an endothermic reaction and if the energy value is negative, the reaction is an exothermic reaction.

The reaction,

  ${\text{2H}}_{\text{2}}\text{+}{\text{O}}_{\text{2}}\to {\text{2H}}_{\text{2}}\text{O}$ is an endothermic reaction.

From (a) and (b), we can determine that the reaction is endothermic or exothermic by calculating the heat of the reaction.

So,

  $\begin{array}{c}\text{Heat}\text{of}\text{the}\text{reaction}=(-1358+1836)\text{kJ}\\ =478\text{}\text{kJ}\end{array}$

Therefore, the reaction is an endothermic reaction.

The reaction is an endothermic reaction because, the heat of reaction is $478\text{}\text{kJ}$ which is a positive value.