Question

1. a. Use the bond energy data provided to calculate AH.. for the following reaction. H-H + Cl-Cl → 2 H-CI Bond Bond Energy H-H 436 kJ/mol Cl-CI 243 kJ/mol H-CI 431 kJ/mol b. Based on your answer to part a, is the reaction an exothermic reaction or an endothermic reaction? Provide a brief explanation to support you choice of answer. An endothermic reaction occurs in a cylinder with initial volume of 1 L. 5 kJ of heat is transferred and the internal energy change (DE) is -5 kJ. The only work occurring is the expansion or contraction of the cylinder against an external pressure of 1 atm. What is the final volume of the cylinder?

Answer 1

1

H-H +Cl-Cl $\rightarrow$2HCl

from the reaction we see the stoichiometry of hydrogen is 1 and chlorine is 1 and HCl is 2

in terms of moles

1 mole of H₂ react with 1 mole of chlorine to give 2 moles of HCl

Bond dissociation energy BDE for 1 mole of HCl is 431 kJ

so for 2 moles it would be 2 *431kJ= 862 kJ

Bond formation energy BFE for 1 mole of HCl is -431 kJ

so for 2 moles it would be 2 *-431kJ= -862 kJ

Bond dissociation energy of H_H for 1 mole BDE= 436 kJ

bond dissociation energy of Cl-Cl for 1 mole BDE= 243kJ

$\Delta H_{reactant}=BDE_{Hydrogen}+BDE_{chlorine}$

$\Delta H_{reactant}=$ (436+243) kJ =679 kJ

by hess law

$\Delta H_{rxn}=$(-862+679)kJ =-183 kJ

The enthalpy change for the reaction will be ΔH = -183 kJ.

since the value of $\Delta H_{rxn}=$ -183 kJ is negative so the reaction is exothermic in nature

1 mole of H-H and Cl-Cl bonds are broken. ΔH_reactant = +(436 kJ + 243 kJ) = +679 kJ

Bond breaking requires energy, so we expect the value for ΔH to be positive for this step.
Two moles of H-Cl bonds are formed and Bond breaking liberates energy, so we expect the ΔH for this portion of the reaction to have a negative value. ΔH_product = -2(431 kJ) = -862 kJ

thus we see that for breaking hydrogen and chlorine bond the energy required is less than the energy of formation of H-Cl bond involves liberation of energy so it is exothermic i.e. formation of HCl is favourable.