McMurry (kap. 6,8) - Describing a reaction: bond dissociation energies

Question 1

The enthalpy change during a bond formation and breakage.

When a bond is formed, heat is released (because the products are more stable and have stronger bonds then reactants - that is why they form in the first place) - (NEGATIVE delta-H)

When a bond is broken, heat is absorbed (because the products are less stable and have weaker bonds then the reactants).

Answer 1

yes.

Question 2

What is the bond dissociation energy (D)?

Answer 2

The amount of energy needed to break a given bond to produce two radical fragments. Called bond strength or bond dissociation energy.

Each specific bond has its own characterized strength - Table 6.3 (page 166) you can see.

Question 3

What is the connection between bond strength and chemical reactivity - in an exothermic reaction?

Answer 3

Weak reactant bonds
Stronger product bonds

That is because in an exothermic reaction - heat is released - and making bonds in products release heat and breaking a bond in the reactants absorbs heat.

Therefore the top statement must be true for the whole reaction to release heat.

Question 4

Can you give biological examples of molecules that have more release of heat when breaking a bond in the molecule?

Answer 4

ATP generate a lot more energy then Glycerol-3-phosphate when broken a bond.

That is because ATP has relatively weak bonds that require a small amount of heat to break, thus leading to a larger release of heat when a strong new bond forms in a reaction.

The bond in glycerol-3-phosphate is stronger in reactant compared to ATP.