# Further thermodynamics

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

The module 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 (some syllabuses don't go so far as this, yet they are effectively using the same idea to deduce feasibilty of chemical reactions). 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^{-1}mol^{-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.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or other recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.’

## Links and Resources

### Unit S3: Chemical Energetics

This comprehensive resource provides much detailed background material on the topics covered in this module. The resource would be useful for teachers when planning lessons, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. An invaluable resource for planning, and for setting example problems to support teaching and learning.

### Born-Haber Cycles

This is a straightforward resource giving students practice on constructing Born-Haber cycles from given data. The resource also revises the calculation of enthalpy of reaction from standard enthalpies of formation.

### Problem 9: Cool Drinking

This is a nice resource giving students an opportunity to apply their understanding of the concepts developed in this module to a real world problem, that of designing a system to cool drinks using an endothermic reaction. It is designed to be carried out in groups, and fit into a time span of two hours. Some preparatory work is needed and students sheets are provided for this. Teacher notes including answers to the preparatory questions and a suggested solution to the problem are included.

### The Enthalpy and Entropy Changes on the Vaporisation of Water

This simple experiment is described as a demonstration. However, given the availability of sufficient electric kettles there is no reason why it can't be carried out as a class practical.

A kettle is boiled for a specified time and the steam allowed to escape. The mass of water lost as steam is measured by weighing and the energy required to exaporate the mass of water calculated from the time elapsed in boiling and the power rating of the kettle (students need to understand or have explained the link here). This data is then used to calculate the entropy change for the process and usefully consolidates the rigorous definition of entropy. This is a suggested activity on the AQA syllabus.

### The Second Law of Thermodynamics

This activity, aimed at gifted and talented students, explores in more detail the relationship between enthalpy, entropy and feasibility of reaction. Students are presented with a number of cards bearing statements which they have to arrange in a sensible sequence in oder to produce a flow chart of ideas in a logical sequence to summarise the theme.

With a group of average ability, this could take the form of a guided whole class activity with teacher input as required.

This is a nice resource which will make students think and question ideas and therefore help to consolidate their learning in this important area.

### Theory v Practice: Do they Compare?

This is an interesting activity which could be done at the end of the module to allow students an opportunity to revise and consolidate what they have learned.

Teams determine the enthalpy of reaction between calcium and water either experimentally or by constructing a Hess cycle, and the results are then compared.

The time allocation is quite significant but could be justified in terms of the practice students get with ideas and techniques. A practical approach is suggested, as is a solution to the theoretical approach, The idea of electron affinity of a hydroxyl radical will need explaining and its relevance to the theoretical scheme pointed out.

This is an interesting resource providing ample opportunity to practice many of the ideas presented in this module.

### Entropy and Equilibrium

The first part of this resource outlines an interesting context in which to practice calculations related to the Gibbs function and in particular, to investigate the way that stability changes with temperature.

The context is that of the stability of allotropes of tin and the effect that transitions between the two as a consequence of temperature changes can have. Tin pest (deterioration of tin due to cold) is briefly discussed within the context of deterioration of organ pipes, Captain Scott's failed Antarctic mission and the Napoleonic wars, giving interest to the activity.

This first part of the resource should be accessible to the majority of students at this level.

### What Makes It Go?

This is a problem solving exercise which encourages students to think about the driving forces behind chemical reactions.

Three reactions are presented and students asked to consider why they are feasible. It gives students the opportunity to apply ideas related to enthalpy change, entropy change, Gibbs function, lattice enthalpy and enthalpies of hydration to answer the question.

The resource suggests use as a group activity but it could equally well be used as extension material for able students who are comfortable with the ideas explored. It is best used towards the end of the module when ideas have already been practised in simple contexts.

Subject(s) | Science |
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Tags | n.a |

Age | 16-19 |

Last updated | 12 July 2016 |

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URL | https://www.stem.org.uk/lxpqd |

Comments | 0 |