2. C2.5 Exothermic and endothermic
reactions
Chemical reactions involve energy transfers.
Many chemical reactions involve the release of
energy. For other chemical reactions to occur,
energy must be supplied.
BTEOTSSSBAT:
Know the difference between endothermic and
exothermic reactions
Know some example of endothermic and
exothermic reactions
4. What happens when a chemical
reaction takes place?
What happens to the bonds?
5. Exothermic Reaction
• Exothermic reactions transfer energy to
the surroundings.
• The energy is usually transferred as heat
energy, causing the reaction mixture and
its surroundings to become hotter.
• Can you think of any examples?
6. • Combustion (burning)
• Many oxidation reactions,
for example rusting
• Neutralisation reactions
between acids and alkalis
• Hand warmers
10. Endothermic Reaction
• These are reactions that take in energy
from the surroundings.
• The energy is usually transferred as heat
energy, causing the reaction mixture and
its surroundings to get colder.
• Can you think of any examples?
20. A reusable hand warmer contains a solution of sodium acetate trihydrate.
When the hand warmer is activated, the sodium acetate trihydrate
crystallises and heat is released. The word equation for this reaction is
shown below:
1. What type of reaction is this?
(1 mark)
i. Using the information above, explain why hand warmers that
contain sodium acetate trihydrate are reusable.
(2 marks)
Exothermic
The reaction is reversible. (1 mark)
So the solid sodium acetate trihydrate that is formed when the
hand warmer is activated can be converted back into sodium acetate trihydrate
solution (1 mark).
21. ii. Suggest how the hand warmer could be reset after use, so that it
is ready to be used again. Explain your answer.
(2 marks)
The conversion of sodium acetate trihydrate solution to solid sodium
acetate trihydrate is exothermic, so the reverse reaction (the conversion
of solid sodium acetate trihydrate back to sodium acetate trihydrate
solution) must be endothermic (1 mark).
This means the hand warmer could be reset by heating it (1 mark).
Editor's Notes
Bonds are broken – requires energy ENDOTHERMIC
Bonds are made – releases energy EXOTHERMIC
Where does the energy come from?
Exothermic:
sodium hydroxide and hydrochloric acid
copper(ii) sulfate and powdered zinc
Endothermic:
citric acid and sodium hydrogencarbonate
ethanoic acid and sodium carbonate
water and ammonium nitrate
1. The polystyrene cup insulates and prevents energy losses. Using a lid also prevents heat loss.
2. Using more concentrated solutions would make the temperature changes greater.
3. You could not use this apparatus because there is no solution present to absorb the energy produced (in the case of the exothermic reaction).
(Also, the polystyrene cup would burn.)
4. If a reaction is exothermic overall then the amount of energy needed to break the bonds in the reactants is less than the amount of energy released when the bonds are made in the products.
