4.4 - Bond enthalpies

Question 1

Example of calculating enthalpy change

Answer 1

Question 2

Calculating reaction enthalpy changes

Answer 2

• ΔHr= Σ(bond enthalpies of bonds broken)−Σ(bond enthalpies of bonds formed)
• ΔHr= Σ(bond enthalpies in reactants)−Σ(bond enthalpies in products)

Question 3

Mean bond enthalpies

Answer 3

• the energy required to break one mole of a specified type of covalent bond in a gaseous molecule
• always positive as breaking bonds requires energy (endothermic)
• represents an average across different molecular environments

Question 4

What do reactions involve?

Answer 4

• breaking and forming bonds
• breaking bonds in reactant molecules (endothermic reaction requiring energy)
• forming bonds to create the product molecules - exothermic process which releases energy