Calculate the H value for the addition of Br2 to ethene. (H for bond of ethene =

Question

Calculate the H value for the addition of Br2 to ethene. (H for bond of ethene = 62 kcal/mol)

Table 5.1 Experimental Bond Dissociation Enthalpies Y-Z Y Z DH Bond Bond DH (kcaVmol) (kJ/mol (kcaVmol) (kJ/mol) 104.2 CH 05.0 439 436 CH,CH -H 58.0 243 (CH3 2CH-H 404 (CH3 3C 96.5 51.0 214 136.3 570 103.2 CH CH 90.2 377 432 87.5 Br (CH 2CH-CH3 88.6 37 71.3 298 (CH)3 C- CH 87.5 366 114.8 83.6 350 H2C CH2 230.4 84.7 HC ECH 964 CH3CH Cl 355 (CH 3 Br 70.7 296 H2C CH-H 463 110.7 57.1 239 C-H 133.3 558 CH,CH2-1 563 235 HC (CH) 2CH- CI 85.1 356 Ho-H 118.8 497 (CH3 3C-Cl 1848 355 436 CHO H 104.2 70.3 CH3- Br 294 CH3-OH 92.1 386 CH3CH2-Br 70.5 295 (CH)2CH 72.0 301

Explanation / Answer

Energy is required to break the bonds and released when new bonds are formed. Delta H is the net sum of these energies.

Ethene reacts with Br2 to form 1,2-dibromo ethane.

So bonds broken are

Br - Br

And pi bond of ethene

Bonds formed are

Two C-Br

Delta H = 53.5 + 62 + ( 2 × -70.3)

Delta H = -25.1 Kcal

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