Paper 3 Last Minute
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In Step 2 the reaction mixture from Step 1 is shaken with aqueous sodium carbonate. State the purpose of the sodium carbonate. Suggest a precaution that should be taken while this mixture is shaken in the separating funnel. Give a reason for your suggested precaution.
M1 To neutralise / remove / react with (excess) acid Remove stopper/bung OR tip the funnel upside down and open tap There will be a build up of pressure / gas / carbon dioxide Allow add stopper To prevent spillage
Purpose of anhydrous magnesium sulfate
Drying agent / to remove water
Observation when silver nitrate is added to 1-chloropropane
M2 No visible change White precipitate forms slowly
Enthalpy of combustion / kJ mol–1 Standard enthalpy of combustion ΔcHo298 = –4194 Value calculated from a calorimetry experiment –1100 Value calculated using mean bond enthalpies –3159 Suggest reasons for the differences between the values obtained by each of Methods 2 and 3, and the value obtained by Method 1 in Table 3.
M1 Value from calorimetry less exothermic / lower / smaller (than method 1 value) M2 (Calorimetry = ) because of heat / energy loss M3 (Calorimetry = ) incomplete combustion M4 (Calorimetry = ) some liquid hydrocarbon could have evaporated M5 Mean bond enthalpies values use enthalpies taken across a range of compounds M6 Value from bond enthalpy data ignores energy changes in vaporisation of the fuel or condensing the water
Entropy graph what does each bit of the equation represent
𝝙G = -𝝙SxT + 𝝙H y = m x + c
Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous magnesium chloride
(1a) Measures water with named appropriate apparatus (1b) Suitable volume/mass / volume/mass in range 10 – 200 cm3/g (1c) Into insulated container / polystyrene cup (NOT just ‘lid’) (1d) Add known mass of MgCl2(s) (1e) Use of ‘before and after’ weighing method. NOT ‘added with washings’ Stage 2 Measurements (could mark from diagram) (2a) Record initial temperature (min 2 measurements) (2b) Record T at regular timed intervals for 5+ mins / until trend seen (2c) Plot T vs time Stage 3 Use of Results (3a and 3b could come from diagram) (3a) Extrapolate lines to when solid added (to find initial and final T) (3b) Tfinal – Tinitial = ∆T / idea of finding ∆T from graph at point of addition (3c) q = mc∆T –3 (3d) amount = mass/Mr (0.80/95.3 = 8.39 x 10 (3e) ∆Hsoln = q/8.39 x 10–3 or in words mol) This could all be described in words without showing actual calculations but describing stages
1.1
Not possible to prevent some dissolving
Why is sodium benzoate soluble in cold water but benzoic acid is not?
Sodium benzoate soluble because it is ionic Benzoic acid insoluble because: despite the polarity of the COOH group / ability of COOH to form H-bonds, the benzene ring is non-polar.
3.8
Dissolve crude product in hot solvent/water of minimum volume Filter (hot to remove insoluble impurities) Cool to recrystallise Filter under reduced pressure / with Buchner/Hirsch apparatus wash (with cold solvent) and dry
Purpose of reflux
allows reactant vapours (of volatile organic compounds) to be returned to the reaction mixture / does not allow any reactant vapour to escape
What happens when nRT is constant?
P1V1 = P2V2
Explain why pH stays constant when NaOH is added to propanoic acid
OH- reacts with H+ Equilibrium shifts to the right to replace H+
Purpose of adding NaOH dropwise near the endpoint
Large, rapid pH change