Bond Enthalpies

What is the simplified process of chemical reactions?

1. energy is absorbed

2. bonds are broken

3. energy absorbed

4. bonds are formed

5. energy released

Define Bond Enthalpy

quantity of energy needed to break one mole of bonds in gaseous molecules under standard conditions, via homolytic fission

Which process is endothermic?

• a. 2Cl (g) —> Cl2 (g)

• b. Na (g) —> Na+ (g) + e-

• c. Na+ (g) + Cl- (g) —> NaCl (s)

• d. Na (g) —> Na (s)

b. Na (g) —> Na+ (g) + e-

the stronger the bond…

the shorter the length of the bond

How do you get ∆H Using Bond Enthalpies?

△H

= sum of energy added per mole (reactants) + sum of energy released per mole (products)

= n(ΣBE bonds broken) - n(Σ BE bonds formed)

= ΣB.E.reactants - Σ B.E. products

What are the steps to calculate Bond Enthalpy?

1. Write your balanced chemical equation

2. Draw the simplified Lewis structures of reactants and products.

3. Use these values to determine the enthalpy change.