Further thermodynamics

The topics covered in this module are studied in the second year of an A level course and depend upon the thermodynamics learned in the first year (please refer to the list on Energetics).

This topic further develops the idea of Hess cycles studied in year one further by examining the concept of lattice enthalpy and its calculation using Born - Haber cycles. Lattice enthalpy is then used in the determination of enthalpies of solution along with data on enthalpies of hydration, another application of Hess law.

Traditionally, Born-Haber cycles are represented as energy level diagrams, but it is crucial for students to appreciate that they are in fact complex Hess cycles and can be manipulated as such. This makes calculations of the physical parameters from Born-Haber cycles much simpler, and links them to ideas that students are already familiar with.

A number of new definitions of enthalpy changes must be learned, and as with year one thermodynamics, these must be accurate. Equations again must have the correct stoichiometry and state symbols. Students often find this aspect somewhat daunting but the sooner definitions are learned, the easier manipulations become.

The second major thrust of this module is the determination of reaction feasibilty which, in the final analysis, is the whole objective of thermodynamics as applied to chemical reactions.

The second law of thermodynamics (in any spontaneous change in an isolated system the total entropy increases) is invoked to allow an understanding of reaction fesibility to be developed. Entropy is first introduced qualitatively, and then developed quantitatively and combined with the enthalpy change for a reaction to arrive at the Gibbs function . Students must be able to manipulate the Gibbs function (or equivalent) in order to predict reaction feasibility and this requires careful accounting and conversion of quantities which students sometimes find confusing (enthalpy changes are in kJmol-1 whereas entropy changes are in JK-1mol-1).

In some syllabuses the module is further extended by linking quantitatively, the Gibbs function to the standard equilibrium constant for a reaction and to the standard cell potential for an electrochemical cell.

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