Q1) Unless I am mistaken, The Hill and Hunt A-level (edexcel endorsed) book refers to enthalpy of hydration as forming a 1 molar solution of (aq) ions. Yet other books refer to it as being “to infinite dilution” or until no further heat change occurs. The former will obvious be less exotheric than the latter. The Jan 2011 mark scheme U4 suggests the latter. A student who owned Hill and Hunt’s A2 book would be unfairly penalised in such a case. Please can you clarify the correct definition of enlthalpy of hydration (and the related enthalpy of solution).
Thanks.

Their response (Rod Beavon):

The definition of hydration enthalpy is indeed that of the gaseous ions to an aqueous solution at infinite dilution, so that is the answer.

Use of a particular book is a matter of choice and books differ in their degree of authority. It is in my view unwise to rely on a single book and it is not in the nature of scholarship to do so. Books might contain errors – but incorrect chemistry cannot be credited as a result. The way the universe actually works is what matters.

To which I replied:

Thank you for your reply and advice on this matter. It is appreciated.

Unfortunately however, it doesn’t just occur in one book – I just happened to mention only one of them. If you checked other A-level texts, you will come across other occurrences.

And I must say, to put it down to a position of ‘authority’ is rather surprising. Presumably, when edexcel endorsed the book in the first place – which one imagines involved some kind of mutual, projected financial benefit for both parties – edexcel thought their ‘authority’ was fine then. Hill and Hunt wrote the excellent and established ‘Chemistry in Context’ book. In many ways a groundbreaking book, cherished by some, and indeed, a quick Amazon search shows revels Mr. Graham Hill have a strong record of publishing scientific texts.

And on the subject of edexcel endorsed books, that the edexcel endorsed books no longer contain answers to their questions (something I first noticed with the ‘Nelson advanced series’) don’t reflect well on edexcel, or indeed on any of the other exam boards engaging in this practice.

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Q2) In the Edexcel endorsed book A2 Chemistry by George Facer, on page 214 regarding aqua ions, it says “The oxygen atom in a water molecule has a lone pair of electrons that forms a bond with an empty 3d or 4p orbital in the metal ion” No mention is made of the empty 4s orbirtal. Why? and if edexcel ask a question based on this in the future, what would be the accepted answer? If the students never involved the 4s orbital in accepting a pair of electrons for a dative bond, surely they could not be penaised. A question of this nature has been asked in past years papers (e.g. London board Jan 1993 involving [Cr(H2O)6]3+ and again in Edexcel June 2002 Unit 5 with hexaaqua chromium once more). Thanks.

Their response:

Dear Customer,
On page 214 of this book, George does, in fact, mention “3d-, 4s- and 4p-orbitals of the d-block ion” six lines further down the page to where he mentioned “… an empty 3d- or 4p- orbital in the metal ion” at the top of this page (line 5).

The sort of question to which you refer – from the Legacy Specification – in which the candidate has to assign ligand lone pair electrons into orbitals (along with the “valence” electrons of the metal ion) would no longer appear under the new Specification.

Yours sincerely,
Ask The Expert

To which I replied:

Thank you for the response.
The two ‘parts’ tell a different story however, and as to which one the student should choose, seems rather hit and miss.
The legacy specification did not specify assigning electrons into the 3d and higher orbitals/subshell either [under the "Topic 5.2: Transition metal chemistry"] aiding concern that reappearance on this type of question may occur.

Re: Coordination number of complexes, e.g. does the Cu(en)3 complex have a coordination number 3 or 6. According to Jim Clark at chemguide.co.uk coordination number is the number of dative bonds to the metal ion. Cu(en)3 is therefore still 6 coordinate, even though the complex only contains 3 ligand molecules. Note: en is ‘bidentate’. Sorry for not being able to remember on the spot earlier. See what lack of revision does to you?     {and old age of course}