Can you guess the topic for today?
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Transcript of Can you guess the topic for today?
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Can you guess the topic for today?
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Bond energies
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Combustion of methane
One molecule of methane and two molecules of oxygen
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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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Chemical Energy
• When chemical reactions occur, bonds are first broken and then formed.
• The amount of energy released / absorbed during a chemical reaction can be measured and calculated from the bond energies
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Bond Energy
• Energy is absorbed by atoms when their bonds break.
(+∆H) - bond dissociation
• Energy is released when bonds form between atoms.
(-∆H) - bond formation
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Bond EnergyBond
Energy (kJ/mol)
H - H 436
C - H 413
N - H 393
436 kJ is required to disrupt H-H
436 kJ is released when H-H is formed
The higher the bond energy, the more work is required to break the bond, the more stable it is
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Bond EnergyBond
Energy (kJ/mol)
BondEnergy (kJ/mol)
H - H 436 N - N 160
C - H 413 N = O 631
N - H 393 N triple N 941
P - H 297 N - O 201
C - C 347 N - P 297
C - O 358 O - H 464
C - N 305 O - S 265
C - Cl 397 O - Cl 269
C = C 607 O - O 204
C = O 805 C - F 552
O = O 498 C - S 259
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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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Calculating Energy of a Reaction
H2 + Cl2 2HCl
Breaking H bond = 436 kJ/mol
Breaking Cl bond = 242 kJ/mol
H-Cl bond forming = -431 kJ/mol
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Reactions and Energy
If heat is generated during a reaction ≡ exothermic
If heat is absorbed during a reaction ≡ endothermic
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Calculating Energy of a ReactionH2 + Cl2 2HCl
Moles of Bonds Broken
Energy absorbed (kJ)
Moles of Bonds Formed
Energy released (kJ)
1 H-H @ 431.2 kJ
436 2 H-Cl @427 kJ 854
1 Cl-Cl @ 243
679 854
Heat of Reaction is 679 – 854 = -175kJ; energy is released during the reaction
Endothermic or exothermic?
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Chemical Reactions and Energy
H2 + Cl2 Bonds are broken HCl
Energy releasedEnergy released
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Calculating chemical energy
N2 + O2 2NO
Breaking one N bond = 946 kJ/mol
Breaking one O bond = 498 kJ/mol
Forming 2 N-O bonds = 2 x 631 kJ/mol = 1262 kJ/mol
Net energy released = (946 + 498) + 2(-631) = +182 kJ/mol
Endothermic or exothermic?
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Chemical Reactions and Energy
N2 + O2 Bonds are broken NO
Energy absorbedEnergy absorbed
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Calculating chemical energy
2H2 + O2 2H2O
Breaking H bond = 436 kJ/mol
Breaking O bond = 498 kJ/mol
O-H bond forming = -464 kJ/mol
Net energy released = ((2*436) + 498) + 4(-464) = -442 kJ/molWhen the net energy value is positive, the reaction is endothermic.