Bond Enthalpy
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Definition:
Bond enthalpy (or average bond enthalpy) refers to the amount of energy stored in the chemical bonds between any two atoms in a molecule. Bond enthalpies have been experimentally determined and can be found in a Table of Bond Enthalpies.
Tables of common average bond enthalpy (PDF file)
The change in energy during a chemical reaction is the difference between how much energy it takes to break chemical bonds and how much energy is released when bonds form. So if we know how much energy is stored in these chemical bonds, we can calculate the change in energy for a reaction. The energy change will equal:
Amount of energy required to break the bonds of the reactant molecules - Amount of energy released when the bonds of the products form
If we know how much energy is stored in the chemical bonds, we can calculate the energy change of a reaction. It is important to realize that bonds are broken on the reactant side of the equation, and bonds are formed on the product side. This will give us an equation to use that is very similar to our Hess's Law formula, but with one important different - when using Hess's Law, we subtract the reactants from the products. But when using bond enthalpies, we will subtract the products from the reactants. Be careful to remember these differences.
To solve bond enthalpy questions, you'll need to be able to draw the structural formulas of models, something you likely learned in Chemistry 20. You'll need to know which atoms are bonded together, and if single, double, or triple bonds are involved. We'll keep our molecules pretty simple here.
Example:
Using bond enthalpies, provided in the table below, calculate the heat of reaction, ΔH, for:
½ H2(g) + ½ Cl2(g) → HCl(g)
Given the following bond enthalpies:H — H -> 436 17:02, 17 August 2009 (UTC)~Cl — Cl -> 243 kJ 17:02, 17 August 2009 (UTC)~H — Cl -> 433 kJ
(you can find a more complete list of bond enthalpies in the [bond enthalpy table)]
Solution:
Draw structural formulas for all molecules. Pretty simple for these molecules: Chemical formula Structural Formula ~~H2: H — H ~~Cl2: Cl — Cl ~~HCl: H — Cl
Using the balancing coefficients in the balanced equation and the structural formulas, determine how much energy is required to break all of the bonds of the reactants, and how much energy is released when all of the product bonds form:
H - H 1× 436 = 436
Cl - Cl 1 × 243 = 243
H-Cl 1 × 433 = 433
We still need to use the balancing coefficients from the balanced equation: For example, 1 mole of H - H bonds requires 436 kJ but in our balanced equation only ½ mole of H - H bonds is involved, which will require only 218 kJ (436 × 0.5): ΔH = Σ (bonds broken) – Σ (bonds formed) ~~ ΔH = [½(436) + ½(243)] – [(433)] ~~ ΔH = 339.5 – 433 = __-93.5 kJ __