Question

From the given data what is the bond enthalpy of \[Cl - Cl\] bond
A.\[70\;{\text{kcal}}\]
B.\[80\;{\text{kcal}}\]
C.\[67.75\;{\text{kcal}}\]
D.\[56.75\;{\text{kcal}}\]

Answer 1

Hint:To answer this question, you should recall the concept of bond dissociation enthalpy and heat of reaction. Bond dissociation enthalpy is a measure of the strength of the chemical bond between any two species. The heat of the reaction can be written as the difference between reactants and product’s bond enthalpy.

Complete step by step answer:
The reaction given in the question is:
\[C{H_4}\left( g \right) + C{l_2} \to C{H_3}Cl\left( g \right) + HCl\left( g \right)\;\]and has \[\Delta H = - 25\;{\text{kCal}}\].
The heat of the reaction can be written as the difference between reactants and product’s bond enthalpy.
For this reaction:
\[\Delta H = 4B.E\left( {C - H} \right) + B.E.\left( {Cl - Cl} \right) - 3B.E.\left( {C - H} \right)\; - B.E.\left( {C - Cl} \right) - B.E.\left( {H - Cl} \right) = - 25\;{\text{kCal}}\]
Substituting the respective values of bond energy, we get:
\[x + y - 103 - 81 = - 25x = \left( {\dfrac{9}{5}} \right)y\]
After solving this we will have the value of required bond dissociation energy as:
\[y = 159 \times \dfrac{5}{{14}} = 56.75\;{\text{kcal}}\].

Therefore, we can conclude that the correct answer to this question is option D.

Note:
The value of bond enthalpy assesses the energy required to break or form the bond. Combined bond enthalpy for all broken and newly formed bonds during the reaction describes a total change in the energy of the system which is called change in enthalpy. This value of bond enthalpy i.e. positive or negative, can determine whether the reaction is endothermic or exothermic. : In ideal cases, it can be said that any bond is hundred percent ionic or covalent. But in reality, any bond shows partial ionic and covalent character. The components of any molecular: the cations polarize the anions and try to pull the electronic charge towards themselves and this increases the electric charge between the two ions. This is the case with a covalent bond, that is, an electron charge density is build up around the nuclei.