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Can you guess the topic for today?. Bond energies. Combustion of methane. One molecule of methane and two molecules of oxygen. 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) - PowerPoint PPT Presentation
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Can you guess the topic for today?
Bond energies
Combustion of methane
One molecule of methane and two molecules of oxygen
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.
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
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
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
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
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)
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
Reactions and Energy
If heat is generated during a reaction ≡ exothermic
If heat is absorbed during a reaction ≡ endothermic
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?
Chemical Reactions and Energy
H2 + Cl2 Bonds are broken HCl
Energy releasedEnergy released
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?
Chemical Reactions and Energy
N2 + O2 Bonds are broken NO
Energy absorbedEnergy absorbed
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.