Use the table of bond dissociation energies to calculate and compare the enthalp

Question

Use the table of bond dissociation energies to calculate and compare the enthalpy of monobromination and monochlorination for the given alkyl position. CH3 CH3 Number CH3 Br HBr k.J/ mol CH3 CH3 CH3 CH3 Number CH3 kJ/mol CH3 CH3 Bond Dissociation Energies (for A-B A. + Bond broken | , kJ/mol | Bond broken H-H CH3CH2CH2-H 423 (CH3)2CH-H 413 (CH3)3C-hH AH 432 CH3CH2CH2-Cl 354 (CHs)2CH-CI 355 349 243 AH, kJ/mol Bond broken H-Br CH3CH2CH2-Br 294 (CHs)2CH-Br 298 (CH3)3C-Bir Br-Br 436 366 H-Cl 400 292 (CH3)3C-CI CI-CI 193

Explanation / Answer

Answer –

We are given monobromination and monochlorination reaction and we need to calculate the enthalpy of reaction using the bond dissociation energy.

We know the formula for calculating the enthalpy from bond dissociation energy

Hrxn = sum of bond breaking energy – sum of bond forming energy

For first reaction – monobromination

Hrxn = [H (CH3)3C-H + H Br-Br ] – [H (CH3)3C-Br + H H-Br]

           = (400 + 193 ) – ( 292 + 366)

           = -6.5*101 kJ/mole

For the second reaction – monochlorination

Hrxn = [H (CH3)3C-H + H Cl-Cl ] – [H (CH3)3C-Cl + H H-Cl]

           = (400 + 243 ) – ( 349 + 432)

           = -1.38*102 kJ/mole

So the enthalpy of monochlorination is more than monobromination. When monochlorination is reaction then there is evolved 138 kJ/mol energy and when monobromination reaction then there are evolved only 65 kJ/mol energy.

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