Kaupapa / Learning Outcome To be able to use bond energies to calculate ∆H of a reaction.

Bond energy The bond energy is the amount of energy required to break the bonds of one mole of a particular bond when both reactants and products are in their gaseous states.

Why would bond energies be average values?

Bond energies are average values This is because the energy required to break a bond depends on the environment the bond is in.

Bond energies A chemical reaction is a combination of bond breaking and bond forming Bond breaking has a +ve ∆H (endothermic) Bond forming has a –ve ∆H (exothermic)

Calculating ΔrH using bond energies Step 1: draw the balanced equation showing the structure of the molecules Step 2: Show all the bonds you would have to break to get the individual atoms Step 3: Show all the bonds you need to form to make the products Step 4: Use data from a data table to write in the bond energies for each bond Step 5: Calculate the total Energy using the equation: ΔrH = Σ(energy of bonds broken) – Σ(energy of bonds formed)

Example: Calculate the ΔrH for the reaction above given the following bond energies Step 1

Step 2 Bonds Broken Bonds Formed C-H *6 C-C *1 Cl-Cl *1

Step 3 Bonds Broken Bonds Formed C-H *6 C-H *5 C-C *1 C-Cl*1 Cl-Cl *1 H-Cl*1

Step 4 Bond Energy C - H 413 kJ mol-1 C – Cl 339 kJ mol-1 Cl – Cl Bonds Broken Bonds Formed C-H *6 6*413 C-H *5 5*413 C-C *1 1*437 C-Cl*1 1*339 Cl-Cl *1 1*242 H-Cl*1 1*431 Total 3157 3272 Bond Energy C - H 413 kJ mol-1 C – Cl 339 kJ mol-1 Cl – Cl 242 kJ mol-1 H – Cl 431 kJ mol-1 C - C 437 kJ mol-1 These will always be given

ΔrH = 3157 kJ mol-1 - 3272 kJ mol-1 = - 115 kJ mol-1 Step 5 Bonds Broken Bonds Formed C-H *6 C-H *5 C-C *1 C-Cl*1 Cl-Cl *1 H-Cl*1 Bonds Broken: Total 3157 kJ mol-1 Bonds Formed: Total 3272 kJ mol-1 ΔrH = Σ(energy of bonds broken) – Σ(energy of bonds formed) ΔrH = 3157 kJ mol-1 - 3272 kJ mol-1 = - 115 kJ mol-1