In the previous blog post, i have introduced to you the usefulness of Hess’ Law and Energy Cycle Diagram to determine the enthalpy changes of reactions that cannot be determined directly through experiments.
The two most common applications are:
A) Calculations involving Enthalpy Changes of Formation
B) Calculations involving Enthalpy Changes of Combustion
Let us look at two exam-based questions and see how we apply it to solve the questions.
A) Calculations involving Enthalpy Changes of Formation
Question 1:
Calculate the enthalpy change for the reaction CaF2 + H2SO4 → 2HF + CaSO4, given that the enthalpies change of formation of CaF2, H2SO4, HF and CaSO4 are -1220, -814, -271 and -1434 kJmol-1 respectively.
Solutions:
Energy Cycle Diagram:
By Hess’ Law, ΔHrθ = 2(-271) + (1434) – (-1220) – (-814)
= +58 kJmol-1
B) Calculations involving Enthalpy Changes of Combustion
Question 2:
Calculate the standard enthalpy change of formation of CH4 given than the standard enthalpies change of combustion of methane, graphite and hydrogen are -890.2, -393.4 and -285.7 kJmol-1 respectively.
Solutions:
Energy Cycle Diagram:
By Hess’ Law, ΔHfθ = -393.4 + 2(-285.7) – (-890.2)
= -74.6 kJmol-1
Hope the above examples are useful in your learning of this topic on Chemical Energetics.
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I love the examples they were really helpful