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
"H"_2 + "Cl"_2 → 2"HCl" 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" Δ H rxn = B E H-H + B E Cl-Cl − 2 B E 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" Δ H 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" Δ H rxn = Σ Δ H bonds broken − Σ Δ H bonds formed = Σ B E reactants – Σ B E products
color(red)("EXAMPLE:") EXAMPLE:
Use bond energies to calculate ΔH_"rxn" Δ H rxn
"H"_2"C=CH"_2 + "Br-Br" → "Br-CH"_2"-CH"_2"-Br" H 2 C=CH 2 + Br-Br → Br-CH 2 -CH 2 -Br
color(red)("Solution:") Solution:
ΔH = (cancel(4BE_"C-H") + 1BE_"C=C" + 1BE_"Br-Br") – (cancel(4BE_"C-H") + 1BE_"C-C" + 2BE_"C-Br") Δ H = ( 4 B E C-H + 1 B E C=C + 1 B E Br-Br ) – ( 4 B E C-H + 1 B E C-C + 2 B E 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" Δ H = ( 1 B E C=C + 1 B E Br-Br ) – ( 1 B E C-C + 2 B E C-Br ) = (614 kJ + 193 kJ) – (348 kJ + 2×276 kJ) = -93 kJ
Here's a video on the use of bond energies.
VIDEO
