Paper 3

Mistakessssss

  • Enthalpy effects

    • why calculated enthalpy is more exothermic than the experimental one

      • incomplete reaction e.g. combustions

      • heat loss to the surrounding

      • evaporation of eater from the calorimeter, with no lid

      • specific heat capacity is ignored, and if ask for not accurate, specfic heat capacity of water only an estimate

    • If reaction does not stop in on time, i.e. supposed to stop at 3 minutes but actual time was 4 minutes, there is a greater heat loss, so temperature change less than the actual value, so enthalpy change is less exothermic

  • Reduce percentage uncertainty

    • increase the value of the denominator, e.g. increases mass or volume

    • the apparatus to more decimal points

    • also mention taking repeats and mean though this normally won’t give the mark

    • repetitions when reducing percentage uncertainty may lead to a greater transfer loss

  • Distillation set up mistakes

    • water in should be at the bottom, out at the top to ensure a more efficient cooling effect as the whole condenser can be filled with water

    • should seal the top or sealed with a thermometer, so to prevent gases escaping (for reflux this should be opened)

    • but should leave the connection between the condenser and conical flask at the end open to prevent building up of pressure

    • add anti-bumping granule to provide a smooth surface for smaller bubbles to form - however, in a reflux set up, be aware that the connection between the condenser and round bottled flask should be secured

  • Equilibrium

    • When using titration graph to calculate Kc

      • read off value from half equivalence point

      • then use pH =pKa, Ka = 10^(-pKa)

  • Energetics

    • Why cannot measure the enthalpy change directly?

      • if involves solids, like anhydrous solids, cannot measure temperature of a solid

  • Practicals

  • After colour change in titration, why when you leave it for some times, the solution turns colourless/back to original colour - react with CO2 (acidic) in air, becomes acidic, equilibrium shifts to the left (as water is made when neutralising, so shifts to the left)

  • In experiment of measuring volume of gas produce, why might the volume of gas be less than actual

    • bung not replaced fast enough when putting solid in the test tube

    • so some gas is lost to the surrounding

  • If the conical flask has some water left, and solution is poured in, there is no effect on the mole of the solution.

  • In titrations, if burette is washed with distilled water but didnt dry and put solution in, this would dilute the solution and hence titre volume would increase

  • In flame test

    • first step dip in HCl to clean

    • then dip in fresh HCl to prevent using contaminated HCl; HCl is used in this step to form volatile chlorides and so metal ions can stick to the wire

    • If two metal ions tested together, one colour will mask the other - darker colour will mask the other

  • Why fume cupboard window has to be in a low position - to prevent toxic gases escape as fan might not be strong enough to prevent gas escaping

  • In standard cell, if one of the solutions needs to contain chromate(III) ion, use CrCl3

  • In experiment of carboxylic acid and alcohol to purify carboxylic acid, the organic layer in the separating funnel needs to be washed after the aqueous layer is removed because some of the carboxylate salt might dissolve in the organic layer instead of aqueous.

  • HCl is added after to protonate the carboxylate salt, carboxylic acid is less soluble than the salt, so forms precipitate

  • molar volume practical with magnesium reacting with HCl, gas produced, measured using inverted measuring cylinder - what can be improved

    • use a gas syringe instead with a bung on boiling tube

    • magnesium covered with oxides before weighing, clean with abrasive

    • mass of magnesium may be to small so need a high resolution mass

    • should use a small measuring cylinder to measure volume accurately - high resolution

  • Control variable in experiment of comparing thermal decomposition of solids

    • distance between flame and solid

    • setting of Bunsen burner e.g. blue/red flame

  • General

    • What is the formula for TMS, what is the benefit

      • tetramethylsilane - (CH3)4Si

      • unreactive/inert, small amound needed, only gives one peak

    • dipeptides means no repeating unit!

    • how to identify an end reaction - heat to a constant mass

    • hydrogen is flammable, acid is corrosive

    • grignard is a reducing agent also a nucleophile

    • naming amide with alkyl group - e.g. propanamide

    • in forming ester with carboxylic acid and alcohol, the loss of oxygen is from carboxylic acid not alcohol

    • For impure substances, melting point tends to be lower than pure substances and has a broader range

    • when asked about mean in titration, remember also to mention that the values that are not used are not concordant results, they are rough values