Energetics and Thermodynamics Part I: Hess’ Law and Energy Cycles

One of the most daunting task in the chapter of Energetics and Thermodynamics in A level H2 Chemistry is the construction of energy cycles and energy level diagrams.

In this discussion, we will look into 2 different techniques in constructing energy cycles and how to calculate the required enthalpy change.

Let’s take a look at 2 past year prelim questions

1. Use relevant data from the Data Booklet and the following experimental data to construct an appropriate energy cycle and calculate the lattice energy of magnesium carbonate.
   Enthalpy change of atomization of magnesium = +148 kJmol^-1
   Sum of 1st and 2nd electron affinities of oxygen = +657 kJmol^-1
   Enthalpy change of formation of magnesium carbonate = -1096 kJmol^-1
   Enthalpy change of formation of CO2 (g) = -393 kJmol^-1
   O^2- (g) + CO2 (g) → CO3^2- (g); ∆H = – 778 kJmol^-1

    

   [DHS 2012 P3]

2. Chlorophyll is an extremely important bio-molecule which allows plants to absorb energy from light. It consists of a chlorin ring with a magnesium ion, Mg²⁺, at the centre of the ring.With the aid of an energy cycle, calculate the enthalpy change of hydration of Mg²⁺ using the following data and appropriate data from the Data Booklet
   Enthalpy change of solution of MgCl2 = -153 kJmol^-1
   Enthalpy change of hydration of Cl^– = -362 kJmol^-1
   Enthalpy change of formation of MgCl2 = -642 kJmol^-1
   Enthalpy change of atomisation of Mg = +148 kJmol^-1
   First electron affinity of Cl = -364 kJmol^-1

    

   [MJC 2012 P3]

Comments :

1. The difference between these 2 questions is that the equations for the unknown enthalpy for the 1st question contains 2 participating compounds/elements/ions but for the 2^nd question, only 1.
2. Thus, the way we should start constructing the cycle might be different for the 1^st and 2^nd question.

Step by step technique for question 1 (DHS 2012 P3)

For a step by step guide to solving the question, click below:

Step by Step Technique

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