Br2(l) --> 2Br(g)

[Amy Kumar 1I]

For the reaction Br2(l) --> 2Br(g), how do you know which phase change enthalpies to use (why is it delta h vap + delta h bond)?

[JonathanS 1H]

We know to use the phase change from a liquid to gas, H_vap because that is what is demonstrated in the equation. We then use the H for breaking bonds because the Br₂ becomes 2Br.

[Tyler Angtuaco 1G]

You use the heat/enthalpy of vaporization since the reactant is being vaporized into gas. You use bond enthalpy since the bond in the diatomic molecule Br2 breaks, forming two bromine atoms in the reaction.