practical questions (1) Flashcards
what is the purpose of the concentrated sulfuric acid (1 mark)
- catalyst
- dehydrating agent
The reaction mixture is flammable. Suggest how the reaction mixture should be heated in step 1.
1. A mixture of ethanol, ethanoic acid, and concentrated sulfuric acid, with
anti-bumping granules, is heated under reflux for 10 minutes.
(1 mark)
- water bath
- electric heater
Figure 1 shows how a student set up the apparatus for reflux in step 1.
You should assume that the apparatus is clamped correctly.
Figure 1
Identify two mistakes the student made in setting up the apparatus.
State the problem caused by each mistake. (4 marks)
- bung/stopper in the end of the condenser (M1), meaning there’s pressure build up (M2)
- water goes the wrong way through the condenser (M3), meaning the condenser is not cool enough/does not fill the condenser enough (M4)
suggest why anhydrous calcium chloride is added
- to remove/absorb water
- drying agent
explain why there is a buildup of pressure in the separating funnel (2 marks)
M1 to neutralise/react with/remove the acid
M2 carbon dioxide / gas is produced
Give a reason why two layers form in the separating funnel.
Suggest why ethyl ethanoate forms the upper layer. (2 marks)
M1 ethyl ethanoate is insoluble in water (immiscible)
M2 ethyl ethanoate has a lower density (than
water)
Suggest a reason why the percentage yield is not 100%. (1 mark)
- reaction is an equilibrium/reversible
- losses during distillation/isolation/purification/transfer
- incomplete distillation/reaction
- impurities/contamination
The student analyses the resulting mixture by thin-layer chromatography, TLC. State a reagent and the conditions needed for the hydrolysis. (2 marks)
M1 (aqueous) HCl/hydrochloric acid
M2 reflux/heat
State why the amino acids separate on the TLC plate. (1 mark)
mobile phase
and attraction to/retention on stationary phase
- The crude product was dissolved in the **minimum quantity of hot water **and the hot solution was filtered through a hot filter funnel into a conical flask. This filtration removed any insoluble impurities.
- The flask was left to cool to room
temperature. The crystals formed were filtered off using a Buchner funnel and a clean cork was used to compress the crystals in the funnel. A little cold water was then poured through the crystals.
After a few minutes, the crystals were removed from the funnel and weighed. A small sample was then used to find the melting point.
Give reasons for each of the following practical steps (stuff in bold) [4 marks]
- **min. quantity of hot water **- to ensure the hot solution would be saturated / crystals would form on cooling
- left to cool at room temp - yield lower if warm / solubility higher if warm
- compress the crystals in the funnel - air passes through the sample not just round it
- cold water poured through the crystals - to wash away soluble impurities
the melting point of the sample (crystals) in the previous question was slightly lower than the data-book value.
suggest the most likely impurity to have caused this + how you could improve the method to get a more accurate value for the melting point.
water = impurity;
to solve -> press the sample of crystals between filter papers OR give the sample time to dry in air
how to determine enthalpy changes (of any kind) - 6 marks
- measure a fixed volume (e.g. 25-50cm3) of deionised water and add into a polystyrene cup
- place a thermometer into the water + use a glass rod to stir. measure initial temp for about 3 mins
- add known mass of solid (weighing boat method)
- continue measuring temp + stirring every min for 10 mins
- note final temp
- plot graph T vs time
- note temp change = initial temp - final temp
- calculate q = mcT -> find enthalpy change
After the solid benzoic acid has been filtered off, it can be purified.
Describe the method that the student should use to purify the benzoic acid. (6 marks)
- dissolve crude product in hot solvent e.g. ethanol (M1) of minimum volume (M2)
- filter to remove insoluble impurities (M3); cool to recrystallise (M4)
- filter under reduced pressure using a Buchner funnel (M5)
- wash + dry using cold, distilled water + filter paper (M6)
what happens at 1/2 the equivalence point
pH = pKa (inverse of Ka value)
describe the procedure to prepare a solid organic product (e.g. synthesis of aspirin from 2-hydroxybenzoic acid + ethanoic anhydride) [5 marks]
- M1 -> using a conical flask, place a specific volume of 2-hydroxybenzoic acid, ethanoic anhydride + a few drops of conc. H2SO4 (catalyst)
- M2 -> swirl the mixture + then place in a warm water bath for 20 minutes
- M3 -> cool the conical flask + add the contents to cold water in a beaker (allows aspirin to crystallise out)
- M4 -> filter under reduced pressure using a Buchner funnel
- M5 -> leave to dry
procedure to test the purity of a solid organic product
- place a finely powdered + dry sample of aspirin into a capillary tube
- heat sample in the melting point apparatus
- heat slowly around the melting point
TO ENSURE ACCURACY:
- product must be finely powdered + VERY dry
- heat gently
- repeat procedure to get a range of measurements
TLC procedure + diagram
DIAGRAM:
- base line drawn in pencil (pen is not used bc ink has its own chromatogram)
- lid to prevent the solvent from evaporating
- solvent below base line so the substances do not dissolve in the solvent
PROCEDURE:
- dissolve a small amount of crude product + recrystallised product in a solvent
- place a spot of the crude product on the baseline
- place TLC plate in a beaker w/ the solvent; cover beaker w/ a lid + allow the solvent to travel up
- draw another pencil line where the solvent reaches (solvent front)
- remove TLC plate + allow spot to dry
how to identify the spot on a TLC plate
- use UV light
- or spray with iodine
- for amino acids: ninhydrin
advantages of using TLC to analyse medicines
- quick + inexpensive
- only a small amount of substance needed
how to remove excess acid when purifying a liquid organic product
- react with NaHCO3 (base), which neutralises the excess acid
- release pressure at regular intervals; add until no more fizzing is produced
- filter under reduced using a Buchner funnel
how to remove excess water when purifying a liquid organic product
- add drying agent (MgSO4 or CaCl2)
- add until no more clumping is seen
- filter under reduced pressure using a Buchner funnel
