Question

Calculate the bond energy of $Cl - Cl$ bond from the following data:
$C{H_4} + C{l_2} \to C{H_3}Cl + HCl$ ; $\Delta H = - 100.3kJ$
Also, the bond enthalpies of $C-H$ , $C-Cl$ , $H-Cl$ bonds are 413, 326 and 431 kJ/mol respectively.

Answer 1

Bond enthalpy can be understood as the amount of energy that is required to either make or break the bond between two atoms. When bonds are formed, energy is released from the system, whereas when bonda rebroken, energy is supplied to the system.

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
According to the laws of thermodynamics, the bond enthalpy when forming a bond is positive whereas the bond enthalpy when breaking the bond is negative.
The chemical equation that has been given to us is:
$C{H_4} + Cl \to C{H_3}Cl + HCl$
From this equation, we can observe that on the reactant side, one $C-H$ bond is broken from the methane molecule, whereas one $Cl - Cl$ bond is broken from the chlorine molecule.
One the other side of the reaction, we can see the formation of one $C-Cl$ bond to form chloroform, and one H – Cl bond to form Hydrochloric acid. Also, the enthalpy of the reaction is given to be equal to -100.3kJ. Hence, the equation for the enthalpy of the reaction can be given as:
$\Delta {H_{reaction}} = \Delta {H_{products}} - \Delta {H_{reac\tan ts}}$
$\Rightarrow$ $- 100.3 = [\Delta H(C{H_3}CI) + \Delta H(HCI)] - [\Delta H(C{H_4}) + \Delta H(C{I_2})]$
$\Rightarrow$ $- 100.3 = \left[ {\left( {326} \right) + \left( {431} \right)} \right] - \left[ {\left( {413} \right) + \left( {Cl - Cl} \right)} \right]$
$\Rightarrow$$$Cl - Cl$$ = 444.3 kJ/mol

Note: We can easily measure the heat absorbed or evolved by a chemical reaction. Errors occur because some of the heat may escape before it can be measured – we need ideally to measure the total heat energy evolved or absorbed, which can be done if all the heat is transferred to a liquid whose temperature we measure, however some heat is inevitably lost to the environment.