1.2.6 Bond Enthalpies

Bond Enthalpy There is another way to calculate the heat of reaction, using bond enthalpies. 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. (see table)

Bond Enthalpy The net 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:

Bond Enthalpy The amount of energy required to break the bonds of the reactant molecules — The amount of energy released when the bonds of the products form

Bond Enthalpy 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.

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 1. 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 kJ Cl — Cl2 43 kJ H — Cl 433 kJ (from bond table)

Example 2: Using the bond enthalpies provided, calculate the heat of reaction, ΔH, for: C2H4(g) + H2(g) → C2H6(g) Given the following table of bond enthalpies: C — H 413 kJ C = C6 14 kJ C —C 348 kJ H — H 436 kJ

assingment 1.2.6- assignment