MYP 10

Unit 10.4.5 - Why is energy released in an exothermic reaction


  • To understand that bond breaking is an endothermic process
  • To understand that bond formation is an exthermic process
  • To determine the enthalpy change of burning ethanol using bond energies

Class activity

The class builds ethanol and oxygen molecules using the molecular models kit.

They then break the models to individual atoms using their own energy.


The bond energies are revealed and the groups calculate the energy used in breaking C2H5OH + 3O2

Discussion and reminder of the law of conservation of energy. The energy required to break a bond is equal in magnitude but opposite in sign to the energy released by forming a bond.

The students then build the products of the combustion, 2CO2 + 3H2O

They then calculate the energy released by the formation of the products.

The difference between the two values gives the energy of the reaction

Student follow up

Answer the question:

Why is heat energy released in the course of an exothermic reaction?

The answer obtained does not agree with the value of -1340 kJ mol-1 given in the data book. Why not?

The data book value is given for all compounds in the normal state under standard conditions.

The bond energy calculation assumes that everything is a gas.

Look up the enthalpy of vaporisation of ethanol and water to correct your answer.

What is the new corrected value?



Teaching notes

Literature value for the enthalpy of combustion of ethanol = 1340 kJ mol-1


NIST chemistry webbook

Calorimetry animation

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