This document discusses endothermic and exothermic reactions. Endothermic reactions absorb energy from their surroundings in the form of heat. Examples include melting ice and dissolving salts in water. Exothermic reactions release energy, usually as heat, warming their surroundings. Examples are combustion reactions and the neutralization of acids and bases. The heat of reaction, ΔH, indicates whether a reaction is endothermic (ΔH positive) or exothermic (ΔH negative). Bond breaking requires energy while bond formation releases energy.

1. ENDOTHERMIC AND EXOTHERMIC REACTIONS
Presentation by Karabo Mdaka

2. Chemical Reactions
All chemical reactions involve bond breaking
and bond forming.
Energy is needed to break bonds and
released when bonds are formed.
Chemical reactions are accompanied by a
change in energy, mainly in the form of heat.

3. A reaction in which heat is taken in is endothermic.
These are reactions the 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.
 . A few reactions that give off gases are highly endothermic
- get very cold.
Dissolving salts in water is another process that is often
endothermic

4. Continuation…..
• The cold reaction products start to gain heat from the surroundings and eventually return
to room temperature.
• The reactants gain energy.
• This comes from the substances used in the reaction and the reaction gets cold.
• Eventually heat is absorbed from the surroundings and the mixture returns to room
temperature.
• Overall the chemicals have gained energy

6.  These are examples of endothermic reactions:
• Melting ice cubes
• melting solid salts
• evaporating liquid water
• converting frost to water vapour (melting, boiling, and evaporation, in general, are endothermic
processes)
• making an anhydrous salt from a hydrate
• forming a cation from an atom in the gas phase
• splitting a gas molecule
• separating ion pairs
• cooking an egg
• baking bread

7.  A reaction in which heat is given out is exothermic.
 Thee reactions transfer energy to the surroundings.
 The energy is usually transferred as heat energy, causing the reaction mixture and its
surroundings to become hotter.
 An exothermic reaction is one which releases heat energy to the surroundings.
 The temperature of the surroundings increases.
 Require less energy is to break bonds to form new ones.

9.  Combustion (burning)
 Many oxidation reactions, for example rusting
 Neutralisation reactions between acids and alkalis
 Hand warmers

10.  The heat of reaction, ΔH, of a chemical
 reaction is the heat in kilojoules released or
 absorbed when the number of moles of
 reactants indicated, in the balanced equation
 describing the reaction, react completely.
For an exothermic reaction ΔH is negative(-)
 For an endothermic reaction ΔH is positive(+)

11.  Bond energy is the amount of energy in kilojoules needed to break one mole of bonds
of the same type, all species being in the gaseous state.
 The average C-H bond energy in methane is 412kj mol-1 i.e. E(C-H) = 412kj mol-1 .
 The energy of a particular bond type can vary.
 It is usual to quote the average bond energies.

12. Type of reaction Exothermic Endothermic
Energy absorbed or released Released Absorbed
Relative energy of reactants and
products
Energy of reactants greater than
energy of product
Energy of reactants less than
energy of product
Sign of ΔH Negative (i.e. <0<0) Positive (i.e. >0>0)

14. Testing your understanding:
 Thermal decomposition of marble?
 Combustion of methane?
 Photosynthesis?
 Neutralisation of an acid and an alkali?
 Rusting of iron?
 Respiration?
Follow the link below to watch a video explaining the two types of reactions:
https://www.youtube.com/watch?v=eJXL0IrbtqE

15. REFERENCES
• Bishop S. (2015). C 2.5 exothermic and endothermic reactions. Available from Slideshare at
https://www.slideshare.net/sbishop2/c2-4-7-exoandendo (accessed 03 September 2018).
• Brown M. (*). Exothermic and endothermic reactions. Available from slideshare at https://www.slideshare.net/brownm7/54-
exothermic-and-endothermic-reactions (accessed 30 August 2018).
• Cartlidge (2013). Energy change during chemical reactions. Available from slideshare at
https://www.slideshare.net/cartlidge/energy-change-during-chemical-rcts (accessed 30 August 2018)
• Mrcool3 (2013). Notes gb lab 08 endothermic and exothermic reactions. Available from slideshare at
https://www.slideshare.net/MrCool3/notes-gb-lab-08-endothermic-and-exothermic-reactions (accessed 03 September 2018)
• Smith M.B. (2010). Lecture 17.1- Endothermic vs. Exothermic. Available from slideshare at
https://www.slideshare.net/elmochem/lecture-171-endothermic-vs-exothermic (accessed 30 August 2018)