Bond….
Bond Energy
• All physical stuff is made of …– Matter! (Chemicals – atoms and
molecules)
• In Chemical reactions, atoms rearrange to form new substances – (new molecules)
Conservation
• When new substances are made, the atoms themselves do not change.– They break their old bonds and form new ones.– Mass is CONSERVED!
• ENERGY is stored in a chemical bonds
Bond energies
Let’s make some molecules!
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.
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-H 464
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
2H2(g) + O2(g) 2H2O(l)
Calculating ΔH
2H2(g) + O2(g) 2H2O(l)
Bonds broken = 2 x (H-H) + 1 x (O=O)
= 2 x 436 + 1 x 498
= 872 + 498 = 1370 KJ/mol
Calculating ΔH
2H2(g) + O2(g) 2H2O(l)
Bonds broken = 2 x (H-H) + 1 x (O=O)
= 2 x 436 + 1 x 498
= 872 + 498 = 1370 KJ/mol
Bonds made = 4 x (O-H)
= 4 x -464
= -1856 KJ/mol
Calculating ΔH
2H2(g) + O2(g) 2H2O(l)
Bonds broken = 2 x (H-H) + 1 x (O=O)
= 2 x 436 + 1 x 498
= 872 + 498 = 1370 KJ/mol
Bonds made = 4 x (O-H)
= 4 x -464
= -1856 KJ/mol
Overall Energy change = 1370 + (-1856) = -486 KJ/mol
(Exothermic)
Energy in the Chemical Equations
Because Energy is released (exothermic), include it as a product:
2H2(g) + O2(g) 2H2O(l) + Energy
• If Energy is absorbed (endothermic), you would include it as a reactant!
How well have you understood?
Bond energies?
Energy level diagrams?
Using bond energies in calculations?
Let’s try Another!CH4(g) + 4Cl2(g) CCl4(g) + 4HCl(g)
Bond Bond energy KJ/mol
C-H 413
H-Cl 431
Cl-Cl 242
C-Cl 328
C-C 347
C-O 335
O=O 498
Homework
• Homework Book pages 60 – 61
• Due TOMORROW
• Lab Tomorrow as well!
Combustion of methane?
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