2019 A Level H2 Chemistry Paper 1 Question 12 - Compare Enthalpy Change of Formation, Atomisation and Combustion
Let's move on to Question 12 for 2019 A Level H2 Chemistry Paper 1.
We need to look at each option and determine which statement is the correct one.
A. Enthalpy change of reaction 1 is enthalpy change of combustion of C6H12(l)
Enthalpy change of combustion of C6H12(l) is to burn 1 mole of C6H12 in excess oxygen to form products in standard states ie CO2(g) and H2O(l).
Since the product formed is H2O in gaseous state instead, this statement is false.
B. Enthalpy change of reaction 2 is enthalpy change of formation of C6H12(l)
Enthalpy change of formation is to form 1 mole of C6H12(l) from its constituent elements in their standard states, ie graphite C(s) and H2(g).
Take note 9O2(g) is unchanged during this reaction so we can ignore this.
Reaction 2 is describing formation of C6H12 correctly hence statement B is true.
C. Enthalpy change of reaction 3 is 6 x enthalpy change of atomisation of H2(g)
Enthalpy change of atomisation is to form 1 mole of H(g) atom from its elements.
Since 12 H(g) atoms are formed in reaction 3, the enthalpy change should be 12 x enthalpy change of atomisation instead.
Hence statement C is false.
D. Enthalpy change of reaction 4 is 6 x [enthalpy change of formation of CO2(g) + enthalpy change of formation of H2O(g)]
The problem with this statement is involving enthalpy change of formation of H2O(g).
The reactants for formation of H2O should be elements H2(g) and O2(g).
Since we have H(g) atoms instead of H2(g) elements in reaction 4, statement D is false.
Finally we can conclude that the correct statement and answer is option B.
Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore
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