Some extra tips for P3

Here are some extra tips:

• In case (just in case), the first part of the question is not about titration, don't panic. Usually, if not titration, it will be about rate of reaction. Unlike biology, here we need to use the data that we get in practical and make a graph out of it. Marks will mostly be counted in the quality of the graph, just like Physics. The good (OR bad) things about this kind of practical, instead of calculation and stoichiometry, you will be asked to comment on the experiment or suggest/improve a hypothesis. This is like the fusion of both Biology and Physics. And yes I do realize some of the questions asked has an extremely nerdy instructions (believe me, I took 15 minutes to fully grasp the concept on some questions) but don't worry. Usually for this kind of experiment, you will be asked to vary certain values (concentration/mass) while keeping the other values constant. O/N 2007 P3.2 is a good example (freeexampapers.com) (thx to Kenny Buntara). Here you are asked to determine the rate of reaction H2O2 and we need to vary the concentrations of H2O2. So those long and confusing instructions is just basically tell us on how to vary the concentrations. See, if we limit the volume of H2O2 to be only 40 cm3 and you add 20 cm3 of water and 20 cm3 of H2O2, it's just dilution, nothing special. How to make it more concentrated? Simple. Add less water but more H2O2, but make sure the total volume is still 40 cm3 (10 cm3 of water+30 cm3 of peroxide).
  

• Now about qualitative analysis. On some occasion, we are also required to determine unknown organic compound (holy s***!). But don't worry, on scope of our knowledge, we only need to know how to identify :
  
• Alkane
• Alkene
• Alcohol
• Halogenoalkane
• Aldehyde
• Ketone
• Organic acid (carboxylic acid)
• And we don't need to know what exactly it is (you don't need to know that it is pentane or pentanol, you just need to know whether it is alkane or alcohol)
  
• Now here's the catch. At our current level, it is impossible for us to handle combustible material such as alkane and alkene. Anyway, to identify alkane and alkene is to use liquid bromine and source of UV light. So if the question doesn't show such reagents in the question, you can easily forget about alkane and alkene. Halogenoalkane, to identify this is actually the same as identifying Cl-, Br- or I-. So that will leave us out with Alcohol, Carboxylic acid, Aldehyde and Ketone.
• Start with an easier one first, identifying carbonyl compound. The only way to identify them is to use DNPH (2,4-Dinitrophenylhydrazine) and it will produce this precipitate:

left picture is negative test, right pic. is positve test

• To identify between aldehyde or ketone, we will use Tollen's reagent (silver dissolved in ammonia) or Fehling's reagent (basically it looks like Benedict's reagent). Picture on the book at page 354 shows a good picture of positive Tollen's test.
  

The picture labelled 2 shows the positive result of Fehling's test.

• Take note though. Sometimes if our preparation is a little bit messed up, carboxylic acid will sometimes give positive result for DNPH. I suggest that you clean everything after usage. If the result is still the same, I suggest do the Tollen/Fehling test first to identify the aldehyde.
  
• Now identifying alcohol and carboxylic acid. The simplest way to identify them is to put carbonate/bicarbonate (magnesium/calcium is fine) inside the two unidentified liquid. The one that gives off bubble is the carboxylic acid. It's like adding baking powder with antiseptic alcohol and vinegar (ethanoic acid). The one that will react is the acid. Note that both of them will react with reactive metal (not salt) such as magnesium.
  
• If you are still not assured, usually in the test, they will ask us to make an ester (add carboxylic acid with alcohol, boil then add water). Remember it will gives sweet smelling odour (if you ever play those balloon with straw, then you can imagine that smell).
• And remember to put all of your evidence that indicates that those tests shows this compound is "this" or "that". Don't take the risk!
• Last form of the test is usually measuring of enthalpy change. Usually they will ask us to react something and record the temperature. The only tips that I can get here is that you need to be extremely, extremely, d*** accurate! Although the marks are not really high, but their marks for accuracy is really and impossibly strict. Let's say that the examiner get a value of 2.7 and their limit of accuracy is 0.05 while some of our equipments can't measure until that degree of accuracy. Even slightest difference will give us wrong answer, such as finding the average of 2.7 and 2.8. This accuracy limit is also applied during our calculation. So our only hope is to get the measurement right. Fortunately, their marks are not really high and they are quite fair, such that they can miraculously design the test that it is possible for us to get the marks in one part but not the other.

So this is the end of extra tips. Comments are much appreciated.