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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
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