Energy changes
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I'm confused about the alternate routes that can be taken for a Hess cycle if a particular reaction is too slow or dangerous to do, such as Mg and oxygen.
Ulex replies ...
Relax; don’t panic! I’m sure all will be well! There are three ways to cope with Hess triangles.
1. Before doing any calculating, check to see whether you can look up the information you want in the Nuffield Book of Data. For example, the instance you quote is one where all you need do is to look up the enthalpy change of formation of magnesium oxide and there it is, done.
2. If you can’t just look up the answer you need to inspect the information you are given or required to use. It may be that this is in the form of experimental results as, for example, in Q5.10 on page 111 of the Nuffield Chemistry Students’ Book. Here you work out the enthalpy changes for the reactions between calcium carbonate and hydrochloric acid and then calcium oxide and hydrochloric acid and combine them in a suitable Hess triangle. You should notice that the form of the triangle is given. Examiners don’t like to leave too much to the candidate because it makes the answers too difficult to mark when the candidate doesn’t get the right final answer! There is, however, a case in the examples given at the end of Topic 5 where you are asked to do precisely this – if you can cope with this, you can easily manage the ones which have been asked in Unit Test 1. Do Q 5.2 and 5.3 and then look at Q5.7 which invites you to put your answers together from 5.2 and 5.3.
3. If you are not given experimental results then you use enthalpy changes of formation. Q5.5 gives you a step-by-step lead-in to this and the various parts of Q5.6 give you practice in applying the same method.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 20 January 2004
