2. Energy LevelsEnergy Levels
Exothermic reactions
• Energy is given out
• The products have
less energy than the
reactants
Combustion and neutralisation are exothermic
7. Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
energy given out
∆H is negative
8. Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
• Energy is taken in
• The products have
more energy than
the reactants
The energy is taken in from the surroundings
18. ∆∆HH
How much energy is given out or taken in?
• Energy is needed to break chemical bonds
• Energy is given out when bonds are made
∆H is the difference between the energy
needed to break the bonds in the reactants,
and the energy given out when new bonds are
made in the products
19. Bond energiesBond energies
The energy needed to break a chemical bond
• Different chemical bonds have different
bond energies
Chemical bond Bond energy, kJ/mole
H―H 436
O=O 498
O―H 464
20. Working out ∆HWorking out ∆H
Draw an energy level diagram with all the
reactants and products on it
energy
course of reaction
2H2 + O2
2H2O
21. Working out ∆HWorking out ∆H
Show all the bonds in the reactants
energy
course of reaction
2H2O
H―H
H―H
O=O+
22. Working out ∆HWorking out ∆H
Show all the bonds in the products
energy
course of reaction
H―H
H―H
O=O+
H H
O
H H
O
23. Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436
436
O=O+
H H
O
H H
O
24. Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436
436
498+
H H
O
H H
O
25. Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436
436
498+
H H
O
464 464+
26. Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436
436
498+
464 464+
464 464+
27. Working out ∆HWorking out ∆H
Add the reactants’ bond energies together
energy
course of reaction
464 464+
464 464+
1370
28. Working out ∆HWorking out ∆H
Add the products’ bond energies together
energy
course of reaction
1370
1856
29. Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
1370
1856
―
30. Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
1370
1856
-486
―
31. Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
∆H = -486
32. Working out ∆HWorking out ∆H
Summary
• The energy values have units of kJ/mole
• In the exam, you will be given the energy
values and all the bonds to make or break
∆H is energy in – energy out
• Energy goes in to break bonds
• Energy goes out when bonds are made
