Something to think about

1. Problems with enthalpy diagrams

It is not difficult to find enthalpy level diagrams in books or on the Internet and most teachers will draw out simple ones themselves. The typical diagram below which actually gives the values for two specific reactions, one exothermic and one endothermic is from the website avogadro.co.uk.

What is perhaps not appreciated is the labelling of the y-axis. It is an enthalpy diagram so it should be labelled enthalpy as above - the problem is that no scale can ever be given as the enthalpy values for both the reactants and the products can never be known. This important fact is not obvious from these diagrams and there is a danger that they can give you a misleading message if it is not made clear. All that can ever be measured is the difference between the two values, i.e. the enthalpy change, ΔH. The x-axis also causes some problems. 'Progress of reaction' is often used when the activation energy or some intermediate is also shown on the diagram. All that a simple enthalpy level diagram shows is the initial and final states and they could quite legitimately be placed one above the other so that no x-axis is required (as we do for energy diagrams of energy levels within an atom).

2. What is the 'true' value?

One of the best practicals you can do is to determine the enthalpy of combustion of a flammable liquid using a spirit burner underneath a beaker containing a known mass of water (see right). The experiment is fraught with problems such as incomplete combustion, heat loss and mass loss through evaporation when finding the mass of the burner before and after the combustion. However the calculations involved in determining the result cover this sub-topic well and the experiment also lends itself to a thorough and meaningful evaluation and a discussion as to how the experimental method could be improved. To do this thoroughly the literature ('accepted') value needs to be known so that the percentage error can be determined. The problem is "What is the literature value?"

The IB seems to place great emphasis on you stating your experimental value together with the total uncertainties. Yet the various data books available never state an uncertainty in their values. One liquid that could be used in the spirit burner is methylbenzene (toluene). The following values can be found in the standard data books for the enthalpy of combustion of methylbenzene.

Literature values for the enthalpy of combustion, ΔH_c^⦵, of methylbenzene (left)¹

1. – 3908 kJ mol^-1
2. – 3909 kJ mol^-1
3. – 3907 kJ mol^-1
4. – 3910 kJ mol^-1

This emphasises several points. Firstly, that you should give the source of your 'literature value' as different sources give different values and secondly why is the IB so 'hung up' on students at this age calculating all their uncertainties when in the scientific world the recognized data books do not include them? It also strongly elicits the TOK question "Which one is the true value?"

It might also be worth pointing out that some Internet sources give other different values. For example, 5. – 3906 kJ mol^-1 and 6. – 3916 kJ mol^-1.

¹Sources:

1. Handbook of Chemistry and Physics, 72^nd Ed. 1992
2. Chemistry Data Book ,Stark and Wallace, 2^nd Ed. 1982 and IB Chemistry Data Booklet (pre-2009 version)
3. Binas, 1992
4. IB Chemistry Data Booklet (post 2009 version)
5. J.Chem.Eng.Data, 1969 14 (1), p 102-106 (Note this actually gives the value as -934.49 ± 0.12 kilocalories mol^-1 which is equivalent to -3906 ± 0.5 kJ mol^-1 so it does include uncertainties.)
6. Wikipedia (Note this gives the value as – 40.589 MJ kg^-1 equivalent to 3740 kJ mol^-1. However this is the Low Heating Value which means the enthalpy of vaporization of water has been subtracted. Adding this value for 4 mols of water gives an extra – 176 kJ of energy which brings the total to – 3916 kJ mol^-1.