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

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Combustion of methane

CH4(g) + 2O2(g) 2H2O(l) + CO2(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

Bond Bond energy KJ/mol

H-H 436

Cl-Cl 242

H-Cl 431

C-H 413

C-C 347

C-O 335

O=O 498

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”

ener

gy

CH4(g) + 2O2(g)

C + 4H + 4O

CO2(g) + 2H2O(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”

ener

gy

NH4NO3(s) + H2O (l)

Energy needed to break bonds

Energy released by forming bonds

NH4NO3(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

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Bonds broken = 4 x (C-H) + 2 x (O=O)

= 4 x 413 + 2 x 498

= 1662 + 996 = 2658 KJ/mol

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Bonds broken = 4 x (C-H) + 2 x (O=O)

= 4 x 413 + 2 x 498

= 1662 + 996 = 2658 KJ/mol

Bonds made = 4 x (O-H) + 2 x (C=O)

= 4 x -464 + 2 x -805

= -1856 + -1610 = -3466 KJ/mol

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Bonds broken = 4 x (C-H) + 2 x (O=O)

= 4 x 413 + 2 x 498

= 1662 + 996 = 2658 KJ/mol

Bonds made = 4 x (O-H) + 2 x (C=O)

= 4 x -464 + 2 x -805

= -1856 + -1610 = -3466 KJ/mol

Overall Energy change = 2658 + -3466 = -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 11th Jan