Paper 5 prediction 14/5/15
Looking at paper 42, It stands out like a sore thumb the almost total zero content of
c) transition metal chemistry.
So I very strongly suspect that these will form the basis of the questions you will face in paper 5. Please remember: I may be totally wrong, but I do think my guess is quite reasonable
It may be that redox and kinetics are combined in one question. So you may have to measure the rate of reaction of a redox reaction.
Typical redoxspecies involving kinetics:
H2O2 (hydrogen peroxide) oxidation
thiosulfate / iodine “clock” reactions – please note there are various oxo-sulfur compounds that can be used in such reactions, but thiosulfate (S2O3)2- ions turning into (S4O6)2- ions [‘di-thionate’] are the most common using starch as the indicator. It is very common to use these species in ‘clock’ reactions. A table of various volumes of reagents is made (with the same total volume) and then th time taken for the starch to turn blue/black is measured.
Also common is the “disappearing cross” experiments in which small crystals of sulfur are precipitated out over time and eventually cause a solution to become opaic so you can’t see the cross at the bottom of a beaker, recording the time taken to disappear.
H2O2 reactions can also be used in a similar fashion in clock reactions.
It is quite rare for manganate(VII) ions to be used in kinetics experiments as it usually reacts very fast, being such a strong oxidizing agent. BUT…. MnO4- is a very common reagent for quantitative analysis – for the same reason (i.e. it reacts quickly) in determining the amount of an oxidizable species present.
It is possible that you may have to plan to do a redox reaction use the ‘sampling’ method, i.e. withdraw a certain amount of the reaction mixture at measured (preferably approximately equally spaced out) time intervals, quench the reaction mixture, and then determine their concentration using something like KMnO4.
If you do get a kinetics question, you are very likely to have to draw a graph, probably to determine the order (they may may, somehow, give you sample data to plot) and if so my guess that you will have to measure the rate using the gradient of the conc vs. time graph, or use the half life method as the usually give you something that has first order kinetics.
The other question, and again I may be utterly wrong!!!, I predict will involve transition metals, possibly using precipitation methods to determine the type and quantity of ion. E.g. a mass x is precipitated and you may have to explain how to calculate the mass of the transition metal ion. so you may have to wash the ppt in a solvent that the ppt is insoluble in, dry it to constant mass.
I also suspect somewhere you will be asked about making a standard solution either to make up the concn of the solution used for the kinetics or to ‘keep control’ / ‘keep track’ of the how much substance was used in different volumes. So revise how to make a standard solution. (and what a ‘standard’ solution actually is!)
Other things to know/consider….
1) Independent and dependent variables. Know what these are and how to identify them.
2) Sources of experimental errors.
3) Identification of and possible explanations for anomalous points.
4) Improvements to be made to some experimental method.
(e.g. different ways to minimize heat loss, use larger masses so % error of reading is lower, use more accurate thermometer etc)
5) Errors in readings, e.g. burettes, thermometers, measuring cylinders, balances etc.
Please remember, this is just a guess based on what I’ve seen in the previous papers. I may be absolutely wrong in my predictions, and the more specific the prediction the more likely I am to be wrong.