1. Endothermic and exothermic reactions

2. Endothermic and exothermic reactions
Objectives:
To explain the difference between an exothermic and an endothermic reaction
Outcomes:
All students will classify reactions observed in previous lesson as either exothermic or endothermic
Most students will explain the terms exothermic and endothermic reaction in terms of bond making and bond breaking
Some students will perform calculations to deduce whether a reaction is endo or exo thermic

3. Endothermic
Exothermic
Temperature rise
Temperature drop

4. Endothermic Exothermic Temperature rise Temperature drop Reactants
Products
Energy
Heat given out
Products
Energy
Heat taken in
Reactants

5. What happens in a chemical reaction?
CH4 + 2O2→ CO2 + 2H2O H
H–C–H O=O O=O → O=C=O H–O–H H–O–H
Bonds between the reactants have to be broken:
4 x C-H bonds
2 x O=O bonds
This requires energy to be put into the system ie this stage is endothermic
Bonds between products have to be made:
2 x C=O
4 x H-O
Energy is given out when bonds are made ie this stage is exothermic

6. Endothermic- breaking bonds
Exothermic- making bonds
Reactants
Products
Energy
Heat taken in
Reactants
Products
Energy
Heat given out

7. So: why is a reaction exothermic or endothermic overall?
CH4 + 2O2→ CO2 + 2H2O H
H–C–H O=O O=O → O=C=O H–O–H H–O–H
Bonds between the reactants have to be broken:
4 x C-H bonds
2 x O=O bonds
This requires energy to be put into the system ie this stage is endothermic
Bonds between products have to be made:
2 x C=O
4 x H-O
Energy is given out when bonds are made ie this stage is exothermic
It depends on the individual bond energies

8. Energy
Energy given out
Energy taken in
Reactants
CH4 + 2O2
Products
CO2 + 2H2O

9. Exothermic or endothermic overall?
CH4 + 2O2→ CO2 + 2H2O H
H–C–H O=O O=O → O=C=O H–O–H H–O–H
Bonds between the reactants have to be broken (energy in):
4 x C-H bonds x O=O bonds
= (4 x 412 kJ/mole) = 1648 kJ/mole = (2 x 496 kJ/mole) = 992 kJ/mole
Total energy put in = = 2640 kJ/mole
Bonds between products have to be made (energy out):
2 x C=O x H-O
= (2 x 743 kJ/mole) = 1486 kJ/mole =(4 x 463 kJ/mole) = 1852kJ/mole
Total given out = = 3338kJ/mole
Overall energy change:
kJ/mole – 3338 kJ/mole = -698 kJ/mole

10. Energy given out = - 3338 kJ/mole
Energy taken in = kJ/mole
Energy
Reactants
CH4 + 2O2
Energy change
= kJ/mole
Products
CO2 + 2H2O

11. Calculating energy changes
Energy in (bonds broken)
Energy out (bonds made)
Energy difference = energy in – energy out