A pedantic chemist
Wednesday 10 January 2024
Authoring this website, writing IB examination questions and my Study Guide etc. has made me become quite a pedantic chemist! Apologies for this, but it is often necessary to be pedantic in order to be able to communicate clearly and unambiguously. What I have also realised is that it is an important aspect of critical thinking. Much of the time pedantry can be annoying and not really important, but at other times it can be crucial to understanding. The two examples I've given involve the (mis)use of oxidation states and numbers and the imprecise naming of compounds.
Oxidation states and oxidation numbers
In the 2014 programme (last exam November 2024) much emphasis is put on using the terms oxidation number and oxidation state correctly. The 2014 guide states (my bold),
"Oxidation number and oxidation state are often used interchangeably, though IUPAC does formally distinguish between the two terms. Oxidation numbers are represented by Roman numerals according to IUPAC. Oxidation states should be represented with the sign given before the number, e.g. +2 not 2+."
This is actually the reverse of what chemistry teachers and students used before. In the 2009 syllabus it clearly stated under 9.1:
"oxidation numbers in names of compounds are represented by a sign (+ or -) and a number, for example, +7 for Mn in KMnO4, i.e. using Arabic not Roman numerals for oxidation numbers."
What matters really is the number (rather than how it is it expressed) but it is unnerving when the pre-2016 markschemes for questions penalised students for using Roman numerals for oxidation numbers whereas in post-2016 markschemes students are penalised for using Arabic numerals.
Examples of the changing answers to IB Questions on oxidation numbers and oxidation states over the years
Pre−2007 syllabus. (First exam May 2003)
M08TZ2P2 Question 8.(c)
Alcohols can be oxidized by refluxing with hot acidified potassium dichromate(VI). State the colour change occurring when this compound acts as an oxidizing agent and the oxidation number of the chromium compound formed. [2]
Markscheme
orange to green; [1]
+3; [1]
Accept 3+ and III but not 3 or Cr3+ .
Pre−2014 syllabus (First exam May 2009)
M10TZ2P2 Question 8.(i)
Determine the oxidation number of lead in Pb, PbO2 and PbSO4 . [1]
Markscheme
Pb: 0, PbO2 :+4, PbSO4 : +2 ; [1]
Need sign for mark.
Do not accept notations such as 4+, 2+, or IV, II.
2014 syllabus (First exam May 2016)
M17TZ1P2 Question 3(a)
Determine the oxidation state in each of the following species: V2O5 and VO2+. [2]
Markscheme
V2O5: +5 [1]
VO2+: +4 [1]
Do not penalize incorrect notation twice
2023 syllabus (First exam May 2025)
SL and HL Specimen Paper 1 Section B
Question 1.(a) Deduce the oxidation states of chlorine in the reactants and products.
Reactant: Cl2; Products: HOCl and HCl [2]
Markscheme
Reactant: Cl2 0. Products: HOCl +1; HCl -1 [2]
Award [2] for three correct.
Award [1] for any two correct.
No other instructions given
The whole system has an inbuilt flaw since Roman numerals do not have zero so how can the name for iron pentacarbonyl be written using Roman numerals to show the oxidation number of iron since the oxidation number of iron in Fe(CO)5 is zero!
Thankfully the IB seems to have realised this and in the new 2023 programme (first exam May 2025), although it still says that "Oxidation states are shown with a + or – sign followed by the Arabic symbol for the number, e.g. +2, –1.", it goes on to state "The terms “oxidation number” and “oxidation state” are often used interchangeably, and either term is acceptable in assessment".
However, the IB seems to have muddied the waters in the 2023 syllabus when it comes to oxyanions. Under Structure 2.1.2 it states:
"The following polyatomic ions should be known by name and formula:
ammonium NH4+, hydroxide OH–, nitrate NO3–, hydrogencarbonate HCO3–, carbonate CO32–, sulfate SO42–, phosphate PO43–."
but then under Structure 3.1.6 it states:
"Naming conventions for oxyanions use oxidation numbers shown with Roman numerals, but generic names persist and are acceptable. Examples include: NO3– nitrate, NO2– nitrite, SO42– sulfate, SO32– sulfite."
It does mean that when a student uses either sulfite or sulfate(IV) to describe salts containing the SO32- ion both are acceptable, but which convention will be used in questions or will both or neither be given?
Imprecise naming of compounds
Common or shortened names can be useful as non-chemists can more easily recognise substances. For example, consider the typical contents of a sample of mineral water.
Of course, mineral water does not contain the metals sodium, potassium and calcium etc. The list is mainly for non-chemists and is actually showing the values for Na+(aq), K+(aq) and Ca2+(aq) ions etc. Similarly many household chemicals are loosely named. For example, washing soda is often labelled as sodium carbonate so you would expect the formula for washing soda crystals to be Na2CO3.
In fact, washing soda is sodium carbonate decahydrate, so its formula is Na2CO3.10H2O. This is pedantic and not important if you are communicating with non-chemists, but consider Question 13 on the Paper 1 Section A specimen papers for the new syllabus which is for chemistry students. The question asks why copper(II) sulfate is blue and then gives four responses that all refer to the splitting of the d orbitals. This is chemical nonsense as copper(II) sulfate is white and the d orbitals are not split as copper(II) sulfate contains no ligands. Non-chemists might accept that copper(II) sulfate is blue but when communicating with chemists the chemistry needs to be precise. The question-setter should have made it clear that the compound is copper(II) sulfate pentahydrate, not copper(II) sulfate, by giving either the correct name or the correct formula or at least stating "an aqueous solution of copper`(II) sulfate".
Giving the precise formula can be particularly important both in food chemistry and in medicine. Limonene is a natural product present in the peel of citrus fruits and can be added to foodstuffs to give the flavour of oranges. However, limonene contains a chiral carbon atom so is optically active and the two enantiomers can be separated. Only the d− form gives the smell of oranges. If the l− form is used the smell will be of pine or turpentine.'Penicillin' is widely used as an antibiotic and the general formula for penicillin is given online and in many textbooks. However, this is not the general formula of substances prescribed by medical practitioners under the general term 'penicillin' as they are usually given as their hydrates or salts to increase their bioavailability. Similarly, the painkiller diamorphine is normally administered as its chloride salt rather than the neutral structure often given in textbooks and online.
