The energetics module is the first of two modules in thermodynamics and is studied as part of the AS course or first year of A level.
Students have met the idea of enthalpy change at GCSE in the context of energy changes associated with chemical reactions and will be familiar with the concept of exothermic and endothermic reactions. Some students may not have called these changes enthalpy changes, but should be familar with constructing anergy level diagrams and using bond energies to calculate an enthalpy change for a reaction from GCSE.
At A level, enthalpy change needs to be rigorously defined and students need to be able to represent named enthalpy changes with equations showing the correct stoichiometry and state symbols. They must be able to define correctly standard enthalpy of formation and standard enthalpy of combustion, and use these, along with Hess' law, to calculate standard enthalpies of reaction.
Hess' law and Hess cycles are of fundamental importance since, once established, they are used in order to extend thermodynamics in the second year of the A level, for example when calculating enthalpies of solution. Born Haber cycles are in fact also complex Hess cycles.
First year thermodynamics also requires students to understand and be able to carry out experiments to measure the enthalpy change associated with a reaction. This includes measuring enthalpy changes associated with combustion reactions and also a wider range of reactions such as neutralisations and displacement reactions. Both of these require an understanding and application of the principles of calorimetry.
Students may be asked to suggest sources of error in experimental measurement of enthalpy changes in order to account for discrepancies and, unlike at GCSE, must take account the direction of the error in their suggestions.
This first encounter with thermodynamics lays the foundation for much of what follows in the second year of A level and so it is well worth taking the time to ensure that students are comfortable with the ideas met before moving on.
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Links and Resources
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.
Pages 15 to 18 cover the principles involved in measuring the enthalpy of reaction in aqueous solution, and include a practical outline showing clearly how to obtain and process data in order to determine the enthalpy change for the reaction. This method is often used by exam boards in their questions and practical activities.
Pages 25 to 26 provide a further experimental investigation combining the previous method of measuring and enthalpy change for an aqueous reaction with Hess' law in order to calculate the enthalpy of a reaction that cannot be carried out, indirectly.
This resource, from the Royal Society of Chemistry's Starter for Ten collection, provides useful worksheets covering those all important definitions: bond enthalpies, Hess' law and calorimetry. The material is useful to reinforce learning either in class or as homework exercises.
This is a very simple introduction to calorimetry where students measure the heat energy released in burning several different alcohols. The method heats a fixed mass of water by approximately 40oC then asks the students which alcohol releases the most energy per gram. Students are not told how to approach answering this question and therefore have to think about how to best use the data. Repetition helps to familiarise students with the method. The calculations can be easily extended in order to estimate the enthalpy of combustion of each alcohol from the data obtained.
This is a nice resource giving students an opportunity to apply their understanding of enthalpy changes in a real and novel context based on the construction of modern sewage tunnels in the City of London. The resource includes teaching notes and student worksheets for the activity. There are links to the video and presentation slides mentioned in the teaching notes in the rubric to the activity itself.
This is the video associated with the previous Tunnelworks resource.
The Thermodynamics and Equilibria Involved in the Solvay Process (post-16) *suitable for home teaching*
The first section of this resource allows students to apply their understanding of Hess' law to solving problems in the unfamiliar context of the Solvay Process for manufacture of sodium carbonate.
It reinforces calculation of the enthalpy change for a target reaction by adding together several other equations, the sum of which reproduces the desired target. If this is the case, then the enthalpy change for the overall reaction is given by the sum of the enthalpy changes for the individual component reactions. This is an almost essential principle to master in first year (AS) thermodynamics. Pages one and two lead students through this skill. Pages three and four may be used as extension material for more able students. The remainder of the resource focuses on acid base equilibria.