Do now! Can you stick the slides in your book please (cut round the edges, but don’t cut out each individual slide – it takes too long!).

Bond energies

Let’s make some molecules! One molecule of methane and two molecules of oxygen

Combustion of methane?

Combustion of methane CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g)

Combustion of methane CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g) All reactions involve bond breaking and bond making as the atoms “swap partners”

Bond breaking - endothermic Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.

Bond making - exothermic Energy is always released when a bond is formed. Bond making is always exothermic.

Bond energies The energy released when a bond is formed or absorbed when it is broken is called the bond energy. e.g. the C-H bond in methane has a bond energy of 413 KJ/mol

Examples of bond energies BondBond energy KJ/mol H-H436 Cl-Cl242 H-Cl431 C-H413 C-C347 C-O335 O=O498

Energy level diagrams

Exothermic reaction The energy need to break the bonds is less than the energy released when new bonds are made “reaction path” energy CH 4(g) + 2O 2(g) C + 4H + 4O CO 2(g) + 2H 2 O (l) Energy needed to break bonds Energy released by forming bonds Energy released

Endothermic reaction The energy need to break the bonds is more than the energy released when new bonds are made “reaction path” energy NH 4 NO 3(s) + H 2 O (l) Energy needed to break bonds Energy released by forming bonds NH 4 NO 3(l) Energy absorbed

ΔH – Energy change in a complete reaction If heat is given out, the reaction has lost energy so ΔH is negative

ΔH – Energy change in a complete reaction If heat is absorbed (reaction gets colder), the reaction has gained energy so ΔH is positive

Calculating ΔH CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g)

Calculating ΔH CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x x 498 = = 2658 KJ/mol

Calculating ΔH CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x x 498 = = 2658 KJ/mol Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x x -805 = = KJ/mol

Calculating ΔH CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x x 498 = = 2658 KJ/mol Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x x -805 = = KJ/mol Overall Energy change = = -808 KJ/mol (Exothermic)

Let’s try some questions! Chemistry for you (OLD) P198 Qs 7,8,9.

How well have you understood?

Bond energies?

Energy level diagrams?

Using bond energies in calculations?

Homework Read pages 160 to 163 Answer ALL questions on those pages Due Monday 11 th Jan