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?

Answer 1

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"#,

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"#

#color(red)("EXAMPLE:")#

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

#"H"_2"C=CH"_2 + "Br-Br" → "Br-CH"_2"-CH"_2"-Br"#

#color(red)("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.

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Answer 2

To use bond energies to determine the change in heat for reactions, you first need to calculate the total bond energy of the reactants and products. Then, you subtract the total bond energy of the reactants from the total bond energy of the products. The difference represents the change in heat for the reaction.

Here is a link to a video on thermodynamics/thermochemistry: Thermochemistry: Crash Course Chemistry #20

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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