IBChemblog

Just read up the changes to the IA criteria for exams 2009. This will affect all students starting the IB course September 2007.

There are now only 5 criteria instead of the 8 previous. Personal skills have been condensed into 1 criteria to be assessed ONLY through the group IV project. This now puts greater emphasis on the students during group IV as 6 full marks are available (3% of the final grade). IMHO this seems rather disparate as the project dosn’t occupy 3% of the time spent on the course. For example, it is suggested that the students spend 60 hours (HL) doing practical work. After write-ups are included this is easily doubled and perhaps tripled. That makes more than 120 hours for 21% of the marks (taking off the personal skills component). This provides approximately 1% per 5-6 hours work in an intense situation. Having been party to many group IV projects I can’t see that they are worth more than the practical work that is carried out individually.

Question 1 was a kinetics question using the reaction between nitrogen II oxide and bromine in which the data given was rounded up.  The question is straightforward and involves some catalysis and enthalpy level diagrams.

Question 2 was an electrode potentials question again with no complications

Question 3 Was a redox question involving the reaction between tin 2+ ions and manganate VII ions. Balancing the equation using the half equations was required although the question did not ask for the half equations.

Question 4 was a moles quesiion in which the first part required use of the gas laws and the second part the empirical formula of crocetin.

Question 5 was an acids and bases question about Bronsted Lowry theory and simple pH determination.

In section B the question choices were:

• Equilibrium and energetics
• Bonding and structure
• Atomic theory and periodicity
• Organic chemistry

The only doubt I have about these questions came in the bonding question in which it was asked why fullerenes can conduct electricity. According to the bonding fullerenes are discrete molecules, not giant structures. Therefore, according to normal structural characteristics of covalent substances it would be expected that they don’t conduct electricity. Nothing is mentioned in the syllabus either to this effect or to the contrary and so I think that the question is misleading.

The periodicity question asked which of the reactions with iodide ions would be more vigorous, Cl2 or Br2. The wording here is hardly appropriate as anyone who has actually carried out the reactions in question will know. Vigorous suggests energy release of some kind. I have no doubt that there is an energy change but personally I have never observed it in either reaction. You are expected to know that chlorine reacts more readily than bromine so most students sould not have been thrown by the wording.

In summary, section A was very simple - probably more so than previous years and the section B questions were also straightforward.

So, that’s it for 2007 IB Chemistry exams in the northern hemisphere at least. What did you think of them? I’m going to kick off with a look at the paper 1 HL exam.

It is a constant source of amazement to me that an exam board can include basic errors in the content of a script. Question 3, for example, asks about a reaction between sulphuric acid and calcium carbonate producing a solution of calcium sulphate. Most chemistry students will know that calcium sulphate is one of the insoluble/slightly soluble substances exemplifying periodicity within group II. The sulphates show a decrease in solubility descending the group from MgSO4 to BaSO4.  Calcium sulphate has a solubility product of 2.45 x 10(-5), making the likelihood of sulphuric acid being able to react with calcium carbonate rather limited. Text books suggest that there is an initial reaction that subsides as a protective insoluble layer of CaSO4 forms on the CaCO3

Otherwise, the paper seems very well balanced although the reactions elicited from students seem to suggest that they feel that the questions had a distinctly unfamiliar feel this year.

Question 8: Oxidation states of cobalt in two isomeric complexes also caused problems. [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br - they are both in the III state.

Question 20: A kinetics question using the iodine clock reaction was a little more complex than similar questions as the time rather than the rate was given. However, the numbers were simple to manipulate.

Question 25: An acids question requiring the students to manipulate definitions of Kb and Kw to obtain a ratio for a given equation. I imagine that this caused some difficulties although well within the syllabus scope.

The organic section was particularly easy this year except for question 40, about mass spectrometry fragmentation of C3H6O, which was more than ambiguous (IMHO) as three out of the four possible answers are technically correct!