CIE A-Level Chemistry (9701) Paper 3 Notes
The Chemistry Paper 3 is 2 hours long. First check if you have 2 or 3 questions in your question paper. If 2, dedicate 1 hour for each question, if 3, you have about 40 minutes for each question.
Titration (Practical):
First make sure you read the question carefully and read the instructions well – you do not want to switch around chemicals or skip a step by mistake, then you have to do the full thing again.
Make sure to rinse your burette and pipette with the chemicals that should go into them.
After rinsing your burette, fill it up to a tad above the 0.00 mark and then open the tap over the sink to dislodge any air bubbles. After you make sure there are no air bubbles, top up the burette to 0.00 or a tad less if needed.
Make sure to remove the funnel as soon as you finish filling the burette, then take your initial volume.
The precision of a burette is +/- 0.05cm3 and the precision of a pipette is to one
decimal place (25.0cm3). Your burette readings in the table should reflect the 0.05 precision of the burette, by always ending in either .00 or .05
Be familiar with the indicator colours:
Acid
Endpoint
Methyl Orange
Base
Acid Endpoint Base
Bromophenol Blue
Phenolphthalein
When you feel you are getting close to the endpoint, tighten your burette knob in
order to control the drops well. Stir the conical flask well and continuously with your stronger hand, and use your weaker hand to operate the knob.
If you think you have reached the end-point but you are not sure, note down the
value you have on your burette, and then continue a bit further to see if the colour is more within the shade of the endpoint or acid/base (after you continue you should be able to distinguish clearly between the two).
If you think you have went a bit too far there is something called Back Titration.
Note down the value you have (A) and then grab a dropper pipette, and fill it with the same chemical as that in the conical flask (not the burette). Each drop is about 0.05cm3. Add just 1 drop and then shake well and wait long enough for the colour to fully settle. Repeat until you see the endpoint colour. Your reading V should be V=A0.05n, where n is the number of times you added a drop from the dropper.
Make sure to at least do 2 titrations. If they are concordant, you can use your rough as an accurate, and use that result + 1.00 cm3 as your rough titre in the rough table. If they were not, do another titration in order to get a concordant result. Tick your concordant results in the accurate results table.
Table Formats:
Rough
Final Volume/cm Initial Volume/cm3 Titre/cm3 3
Final Volume/cm Initial Volume/cm3 Titre/cm3 3
1
2
Titration (Calculations):
Give your answers to 3s.f. (unless otherwise indicated) If you feel like you are lost, or unsure about what to do, go back to the question briefing and instructions to grab some data you have forgot, or to refresh the experiment procedure in order to know how your calculations work together.
Make sure to put the mole ratio in mind when doing titration questions. When calculating %uncertainty consider if you are taking two readings (final and initial) or just a single reading. If you have two readings and you are given the precision for just one of the readings (like 0.05 for a burette), use (2xprecisionx100%)/value to calculate %uncertainty.
At the end of this section you may find some questions that regard how to improve the experimental procedure or %uncertainty. Take your time to think well, and return back to the briefing/instructions to analyse the experiment.
Qualitative Analysis:
Make sure to mention your observations (WHAT YOU SEE) in detail and clearly. Mention at what stage a change happens (for example when you add reagent in excess) and state any change in solubility and colour. Always use a boiling tube when heating is required. Make sure to notice when you are instructed to add a volume in cm3 and when you are told to add a volume in depth (1cm3 is not the same as 1cm depth) If you are asked for a reagent to be used for a test, be specific, do not say “strong dilute acid” but instead say “Dilute Hydrochloric Acid” for example. Distinguishing between Sulphate and Sulphite ions: To a small volume of each sample, add just one drop of Barium Chloride/ Barium Nitrate. A white precipitate should form. Fill the rest of the rest tube with the strong dilute acid available, with slight shaking, and you should be able to tell which one has dissolved. (Sulfite will dissolve while Sulphate will not). You can also bubble the gas produced by adding acid to each sample through Potassium Dichromate instead: Sulphite will turn it from orange to green, Sulphate will not affect it. Be prepared for limewater test and for NO2 brown gas test when adding acid to a sample that is not a Sulphate/Sulphite test. If you add Aqueous NaOH to a sample and there is no visible change, then you are asked to heat, be prepared for ammonia gas test. More often than not you will not have to actually do all the tests. As you are doing them you will be collecting information about their identity, and will be able to tell what the outcomes of some of the following tests will be.
Enthalpy Change:
Ensure that you stir well the plastic cup by swirling it with your hand, as you are
holding it from the top. Do not do it too fast otherwise you will spill some of the chemical out.
A greater change in temperature achieved over a shorter period of time indicates a
more accurate result (assuming no change in the amounts of chemical added). E=mcΔT is the equation for calculating the enthalpy change in the reaction (not per mol of substance). m will be equivalent to the total volume of solution in the plastic cup (in cm3). c will be usually given, and is the specific thermal capacity of water. ΔT is your change in temperature.
When you are asked to calculate the enthalpy change in kJmol-1 make sure to divide your value of E by 1000 if it was in Joules, and by the number of moles of chemical you have calculated as well.
