Using average bond energy values estimate ΔH for the following gas phase reaction CH2=CH2 +Br2→BrCH2CH2Br?

Answer 1

In your textbook, you will find a table of bond energies for different bonds.

Looking at the reaction, you need to determine which bonds in the reactants have to be broken and which bonds in the products have to be formed. Breaking a bond is always endothermic, and forming a bond is exothermic.

So, comparing the products and the reactants, you'll need to BREAK: a C=C bond, and a Br-Br bond. To form the products, you will need to FORM: C-C bond and 2 C-Br bonds.

Paying attention to the signs, add up all the enthalpies, and you've got your answer.