Question

Given the bond energies below, estimate the enthalpy change for the addition of Br2 across the double bond in ethene. H2C=CH2 + Br2 à H2BrCCBrH2

Answer 1

Bonds broken:

In C2H4: you break the C=C (which requires +612 kJ) and four C-H bonds.(4 x 414 kJ = 1656 kJ); total = 2268 kJ

In Br2: you break the Br-Br bond (193 kJ).

Total energy required to break bonds = 2268 kJ + 193 kJ = +2459 kJ

Now will be released when the bonds of the products are formed.

There is a formation of one C-C bond (-347 kJ), two C-Br bonds (2 x -276 kJ = -552 kJ), and four C-H bonds (4 x -414 kJ = -1656 kJ); total = -2551 kJ.

Net energy change = energy required to break bonds + energy released when bonds are formed
= +2459 kJ + (-2551 kJ) = -92 kJ/mol