do now! can you stick the slides in your book please (cut round the edges, but don’t cut out each...
TRANSCRIPT
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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!).
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Bond energies
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Let’s make some molecules!
One molecule of methane and two molecules of oxygen
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Combustion of methane?
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Combustion of methane
CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
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Combustion of methane
CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
All reactions involve bond breaking and bond making as the atoms “swap partners”
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Bond breaking - endothermic
• Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.
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Bond making - exothermic
• Energy is always released when a bond is formed. Bond making is always exothermic.
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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
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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
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Energy level diagrams
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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
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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
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ΔH – Energy change in a complete reaction
If heat is given out, the reaction has lost energy so ΔH is negative
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ΔH – Energy change in a complete reaction
If heat is absorbed (reaction gets colder), the reaction has gained energy so ΔH is positive
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Calculating ΔH
CH4(g) + 2O2(g) 2H2O(l) + CO2(g)
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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
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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
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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)
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Let’s try some questions!Chemistry for you (OLD)
P198 Qs 7,8,9.
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How well have you understood?
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Bond energies?
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Energy level diagrams?
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Using bond energies in calculations?
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Homework
• Read pages 160 to 163
• Answer ALL questions on those pages
• Due Monday 11th Jan