Question
SOLVED PROBLEM 2. Calculate the bond energy of HCl given that H-H bond energy is 433 kJ mol^-1 C1-Clbond 242kJmol^-1 and Delta H_(f) for HCl is -91kJmol^-1
Answer
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Vance
Elite · Tutor for 8 years
Answer
The energy change during the reaction, which is the sum of the bond energies of the products minus the bond energies of the reactants, is the heat of formation ΔHf for HCl.So, Bond energy of H-Cl = (Energy to break H-H + energy to break Cl-Cl + energy for formation of 2HCl)/2.Which means, = (433 kJ/mol + 242 kJ/mol - 2 * (-91 kJ/mol))/2 = 428.5 kJ mol^-1
Explanation
To calculate bond energy, we can use Hess's law, which states that the total enthalpy change during the reaction of a system is the same whether the process occurs in one step or several. In this case, the reaction we're looking at is the formation of one mole of HCl. Given that the bond energies for H-H and Cl-Cl are 433 kJ/mol and 242 kJ/mol respectively, and the heat of formation (ΔHf) for HCl is -91 kJ/mol, we can calculate the bond energy of HCl.In the case of this problem, we recognize that the formation of H-Cl from H2 and Cl2 involves the breaking of one mole each of H-H and Cl-Cl bonds and then formation of two moles of H-Cl bonds. The energy required to break one mole of H-H bonds is +433 kJ (endothermic, so we call it positive), and for a mole of Cl-Cl bonds it's +242 kJ (also endothermic, so positive), and energy released when two mole of H-Cl bonds form is 2* ΔHf = -2*91 kJ (the heat of formation of one mole of a compound from its elements is exothermic, hence negative).Given all this, we can use Hess's law to create an equation.