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Chemistry Paper 3 Notes
Enthalpy Change
Tips and Notes โ Try to minimise the beakers movement โ There must be at least .5 or .0 in your readings for temperature โ Tilt the beaker when wanting to measure the temperature; make sure the bulb of the thermometer is fully covered โ Keep the polystyrene cup in the beaker AT ALL TIMES โ Make sure your cup isn't leaking after you add the water before proceeding and adding your FAX solution โ Make sure the electronic balance is clean and is at 0.0g โ If the question is thermal decomposition, whilst waiting for the crucible to cool, move on to the next question. (15-20 minutes cooling time) โ If your Temperature is rising , it is an Exothermic reaction so you need to record the Maximum possible temperature. This is done by waiting (while looking at the scale) for the temperature values to start decreasing again, taking the maximum values โ If your Temperature is decreasing , it is an Endothermic reaction , so you need to record the Minimum possible temperature. This is done by waiting for the temperature values to start increasing again, taking the minimum value
Error Improvement
Heat loss to the surrounding โ Lid - to prevent convection or evaporation โ Insulation - Prevents conduction โ Use beaker - Provides Insulation
Thermometer โ Use thermometer with smaller scale, gives smaller % error โ Use digital thermometer
Small Temperature falls โ Use larger quantities of reacting substances and less volume of solution, this will result in a greater temperature change
Use of glass beaker โ Use a polystyrene cup, for lower heat capacity
Volume of water โ Use burette for more accurate volume
Table - Always write this
- Write the unit with the heading
- Mass: 2 d.p
- Temp: 1 d.p (.0 or .5)
- Do not round any values
Marks - Emf in calculation questions
Formulae - Unit of Q is J
Video: https://learning.cambridgeinternational.org/classroom/course/view.php?id= Drawing graphs โ Always draw a line of best, unless mentioned otherwise in the question โ The axis must be labelled with the quantity AND its unit โ If you suspect an anomalous point, you MUST circle it โ The graph should cover more than half of the provided graph paper along both axises
Titration
Tips and notes โ For the first titrate, always completely fill the burette โ Your actual titrate values should be less than the values of your rough titrate โ The smaller your titrate value, the more accurate your value is โ You should try to keep the colour in the flask as light as possible
โ Make sure your burette doesn't leak and can close properly before starting to get your actual titre values โ All burette readings must be given to the nearest 0.05 cm^3 (4s.f)
โ During your rough titre, don't drop the solution dropwise, it's a โroughโ titre, find an approximation โ If something is supposed to be in excess, the method (burette, pipette, etc..) of adding the volume doesn't matter โ Calculating moles from g dm^-3: (gdm^-3 / Mr) * (Volume in dm^-3)
Tables For Rough titre
For Actual titre
Marks
Video: https://learning.cambridgeinternational.org/classroom/course/view.php?id=
Sample Question
โ When writing any reagent or solution, you NEED to write its state e.g. aqueous ammonia, white precipitate , white solid โ NO2 decolourises KMnO4, NO3 doesn't Sample Question
Issues with testing for aluminium ion with NaOH โ The precipitate formed by Al3+ is very soluble and disappears very quickly. You may not be able to notice this precipitate and you would write โno changeโ โ use a very tiny quantity of NaOH at first and shake lightly, then add another drop and so on. A white precipitate will form floating on the surface of the solution.
Organic Analysis โ Tollens Reagent: Made by mixing AgNO3 and NH3. It gives a black precipitate with aldehyde which has a silvery mirror floating on top. Most of the times the mirror will be visible so, black precipitate is enough โ Fehling solution: Used to test aldehyde, by adding a few drops of fehling solution (Ammoniacal silver nitrate). Producing a red brown precipitate. โ 2,4 DNPH: This is an orange coloured solution, which has a strong acid. Used to test for carbonyl groups (aldehydes and ketones) forming a yellow-orange precipitate. Ignore the colour of the solution (cux dnph is yellow) and focus on the appearance of a coloured precipitate โ Carboxylic acid test: add NaCO3, observe vigorous effervescence of gas that turns limewater milky white โ Potassium DiChromate: turns from orange to green when added to alcohols and aldehydes, when added to solution you should gently heat it. Donโt heat it strongly or else the alcohol will evaporate (primary and secondary alcohols) โ Potassium Manganate (VII): When added to a solution you should warm it, if purple colour disappears, its either an aldehyde or an alcohol Organic Videos: https://learning.cambridgeinternational.org/classroom/course/view.php?id= https://learning.cambridgeinternational.org/classroom/course/view.php?id= Youtube link showing all the tests: https://youtube.com/@msshanti2011?si=ejpBbzN3wnDeUGBa
Reaction Rate
โ Appearance of product/change in concentration of product โ Disappearance of reactants/change in mass โ Unit: 1/time โ Finding gradient of a concentration-time graph โ The higher the gradient (the steeper the graph), the higher the rate of reaction โ The gradient of graph decreases with time, thus, rate is inversely proportional to time
General Table
Improvements โ Increase the concentration of a reactants โ Increase the temperature of the reactants โ Increase the surface area of a reactant โ Add a catalyst to the reaction
