3. 1. An exothermic reaction is one which
releases heat energy to the
surroundings
2. The temperature of the surroundings
increases
Exothermic reactionsExothermic reactionsExothermic reactionsExothermic reactions
12. 1. An endothermic reaction is on which
takes in heat energy from the
surroundings
2. The temperature of the surroundings
decreases
Endothermic reactionsEndothermic reactionsEndothermic reactionsEndothermic reactions
20. Endothermic and exothermic reactionsEndothermic and exothermic reactions
Step 1: Energy must be
SUPPLIED to break bonds:
Step 2: Energy is RELEASED
when new bonds are made:
A reaction is EXOTHERMIC if more energy is RELEASED
then SUPPLIED. If more energy is SUPPLIED then is
RELEASED then the reaction is ENDOTHERMIC
21. Energy level diagramsEnergy level diagrams
Energy
level
Reaction progress
Activation
energy
Energy given
out by
reactionUsing a catalyst
might lower the
activation energy
22. Exothermic vs endothermic:Exothermic vs endothermic:
EXOTHERMIC – more
energy is given out than is
taken in (e.g. burning,
respiration)
ENDOTHERMIC –
energy is taken in but
not necessarily given out
(e.g. photosynthesis)
23. Burning MethaneBurning Methane
CH4 + 2O2 2H2O + CO2
To burn methane
you have to break
all of these bonds:
And then you
have to make
these ones:
25. Bond energiesBond energies
C-H = 435 Kj
O=O = 497 Kj
Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol
H-O = 464 KjC=O = 803 Kj
Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol
Total energy change = 2734-3462 = -728 KJ/mol
26. Drawing this on an energy diagram:Drawing this on an energy diagram:
2734 Kj
3462 Kj
More energy is given out (3462) than is given in (2734) –
the reaction is EXOTHERMIC. The total (“nett”) energy
change is –728 Kj. An endothermic reaction would have a
positive energy change.
-728 Kj
27. But…But…
… you don’t need to know about bond
energies or how to calculate energy
changes for the exam!