P3 Markschemes Flashcards
(16 cards)
Why does carrying out a flame test on a mixture of potassium and strontium chloride not clearly show that two different metal ions are present
The red colour of the strontium ion will mask the lighter lilac colour of the potassium ion
Why is the wire used in flame test made of in chrome and not iron
-nichrome produces no colour whereas iron can produce a colour
-nichrome is inert but iron reacts with oxygen and HCl
Why is the nichrome wire in a flame test dipped in acid and heated before u add acid again and dip into the metal salt
To remove any residue of the previous sample being tested which cleans the wire
Why is fresh HCl used when re-applying HCl to the newly cleaned nichrome wire
Because the previous acid can become contaminated with residue from previous tests
Explain the formation of colour in a flame test made
Electrons absorb heat energy which promotes them to higher energy levels
Electrons then fall from this excited energy state to ground state, releasing energy as visible light as they do so
Why can’t u just remove the excess water added to a volumetric flask above the fill line with a pipette
Removal of excess solution will remove some of the dissolved compound so the exact concentration of it will be unknown
How to answer a question like “assess the suitability of this indicator for this titration” when given the titration curve
-state the pH range that the indicator changes colour in
-state the pH range of the vertical region of the titration curve
-state whether the pH range of the indicator lies completely within the vertical range of the titration curve (and thus if it’s suitable)
-if not suitable, state whether the colour change will be complete before or after the equivalence point is reached
Why is the enthalpy change for the reaction of one mole of copper sulphate with five moles of water to form hydrated copper sulphate not able to be measured directly
You cannot react exactly five moles of water with one mole of copper sulphate (because the hydration doesn’t happen in a controlled manner) and you cannot measure the temperature change of a solid
Why does the addition of HCl to an aqueous solution of sodium benzoate cause benzoic acid to precipitate out of solution
The benzoate ion gets protonated by the HCl to form benzoic acid which is less soluable in water than the sodium benzoate salt so it crystallises out of solution
How to dry apparatus for filtration under reduced pressure
You draw a conical looking flask with a flat funnel that has filter paper in it, then draw a bung to connect the flask and funnel and draw a side arm labelled “pump” out of the side of the flask
What is the expected observation when aromatic compounds undergo incomplete combustion
Black smoke
Why must the front window of a fume cupboard be below the safety line even if the exhaust fan is switched on
-window above the safety line means the exhaust system is not strong enough to draw in fumes so the toxic fumes will escape into the lab
Compare and contrast the preparation of Phenylamine from nitrobenzene with the preparation of butylamine from butanenitrile
-both are reduction reactions
-reagents for the preparation of Phenylamine from nitrobenzene are tin and concentrated HCl at 50c
-reagents for the preparation of butylamine from butanenitrile are hydrogen gas and a nickel catalyst at 150c
Compare and contrast the basicity of Phenylamine and butylamine
-both are basic because of the lone pair on the nitrogen atom which can accept a proton
-Phenylamine is a weaker base because the lone pair of electrons on the nitrogen atom gets incorporated into the delocalised ring of electrons and so the electron density of the N atom decreases so it is less able to accept a proton
How do u name an N-substituted amide
Identify three differences in the esterification reaction when ethanoyl chloride vs ethanoic acid react with ethanol
-with ethanoyl chloride the reaction is irreversible but with ethanoic acid it is reversible
-with ethanoyl chloride the by product is HCl vs H2O with ethanoic acid
-the reaction with ethanoyl chloride is very fast and occurs at room temp but an acid catalyst is needed with ethanoic acid
