# Can you please explain how to use bond energies to determine the change in heat for reactions, or maybe post a link to a video on thermodynamics/ thermochemistry?

Apr 9, 2015

We use Hess's Law when we use bond energies to calculate heats of reaction.

We break all the bonds to form atoms, and then we reassemble the atoms to form new bonds.

For example, in the reaction

$\text{H"_2 + "Cl"_2 → 2"HCl}$,

we break an H-H bond and a C-Cl bond and form two H-Cl bonds.

ΔH_"rxn" = BE_"H-H" + BE_"Cl-Cl" -2BE_"H-Cl"

We use a table of bond energies like the one below.

and get

ΔH_"rxn" = "436 kJ + 242 kJ – 2 × 431 kJ" = "-184 kJ"

The genera formula is

ΔH_"rxn" = ΣΔH_"bonds broken" - ΣΔH_"bonds formed" = ΣBE_"reactants" – ΣBE_"products"

$\textcolor{red}{\text{EXAMPLE:}}$

Use bond energies to calculate ΔH_"rxn" for the bromination of ethylene.

$\text{H"_2"C=CH"_2 + "Br-Br" → "Br-CH"_2"-CH"_2"-Br}$

$\textcolor{red}{\text{Solution:}}$

ΔH = (cancel(4BE_"C-H") + 1BE_"C=C" + 1BE_"Br-Br") – (cancel(4BE_"C-H") + 1BE_"C-C" + 2BE_"C-Br")

We can ignore the C-H bonds, because they are just being broken and re-formed.

ΔH = (1BE_"C=C" + 1BE_"Br-Br") – (1BE_"C-C" + 2BE_"C-Br") = "(614 kJ + 193 kJ) – (348 kJ + 2×276 kJ)" = "-93 kJ"

Here's a video on the use of bond energies.