Unit 10.4.5 - Why is energy released in an exothermic reaction
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?
Literature value for the enthalpy of combustion of ethanol = 1340 kJ mol-1