The document discusses exothermic and endothermic chemical reactions. Exothermic reactions release energy to the surroundings and feel hot, while endothermic reactions absorb energy from the surroundings and feel cold. A reaction is exothermic if more energy is released during bond making than used for bond breaking, and endothermic if more energy is used for bond breaking than released during bond making. Energy diagrams show exothermic reactions releasing energy overall and endothermic reactions absorbing energy overall.

1. Key Points
● When a chemical reaction happens, energy is transferred to or from the
surroundings.
● When energy is transferred to the surroundings, this is called an exothermic
reaction and usually feels hot.
● When energy is taken in from the surroundings, this is called an endothermic
reaction and usually feel cold.
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2. Exothermic or endothermic?
● If a reaction is exothermic overall, then more energy has been released
during bond making than has been used for bond breaking.
● If a reaction is endothermic overall, then more energy has been used during
bond breaking than has been released during bond making.
2

3. Energy level diagram – Exothermic reaction
3
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = −ve

4. 4
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = −ve
Energy is
being used to
break the
bonds in the
reactants
Energy level diagram – Exothermic reaction

5. 5
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = −ve
New bonds are
formed here
and energy is
released
Energy level diagram – Exothermic reaction

6. Energy level diagram – Exothermic reaction
6
Energy (kJ)
Reactants
Products
Reaction time
Ea
More energy is
released in bond
making than is
required for bond
breaking, so this
reaction is
exothermic overall

7. What is the minimum amount of
energy needed for a reaction
called?
Activation energy

8. Is bond breaking exothermic or
endothermic?
Endothermic

9. Is bond making exothermic or
endothermic?
Exothermic

10. Energy level diagram – Endothermic reaction
10
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = +ve

11. Energy level diagram – Endothermic reaction
11
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = +ve
Energy is
being used to
break the
bonds in the
reactants

12. Energy level diagram – Endothermic reaction
12
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = +ve
New
bonds are
formed
here and
energy is
released

13. Energy level diagram – Endothermic reaction
13
Energy (kJ)
Reactants
Products
Reaction time
Ea
Overall energy
change = +ve
More energy is taken
in during bond
breaking than is
released from bond
making, so this
reaction is
endothermic overall