Example 8.13

[Angela 1K]

The questions asks the estimate the enthalpy of the reaction between bromine and propene to form 1,2-dibromopropane. The enthalpy of vaporization of Br₂ is 29.9 kJ/mol, and that of CH₃CHBrCH₂Br is 35.61 kJ/mol. The reaction is
Br₂(l) + CH₃CH=CH₂ --> CH₃CHBrCH₂Br(l)

In the first step on the solution, they explained that you first need to break 1 mol C=C bonds in CH₃CH=CH₂ (average value 612 kJ/mol) and 1 mole Br-Br bonds in Br₂ (193 kJ/mol). They also explained that you need to form 1 mol C-C bonds (average value 348 kJ/mol) and 2 mol C-Br bonds (average value 276 kJ/mol).

I have two questions regarding this question.

1. How do you know which atoms you need to break/form during this reaction?
2. Where did they get the average energy required to break/form the bonds?

Thanks!

[AtreyiMitra2L]

I have two questions regarding this question.

1. How do you know which atoms you need to break/form during this reaction?
2. Where did they get the average energy required to break/form the bonds?

Alright, so for the first one. Draw out the lewis structures of each of the reactants and products. On the reactants side, determine the bonds broken. You can do this by closely looking at the reactants and comparing it to the products. Because the bonds were broken on the products side, we know bonds must be formed on the products side. Look for these. For the second question, i think they got the average energy I believe through empirical experiments. Please correct me if I'm wrong.

[SitharaMenon2B]

You can figure out which bonds need to be broken and which ones need to be formed by drawing out the lewis structures of the reactants and products.