organic practical skills questions Flashcards
anti bumping granules are placed in the flask when refluxing. Suggest why these granules prevent bumping
prevent big bubbles from forming
state the purpose of the anti bumping granules
to prevent vigorous boiling
the student purified the crude solid product, N phenylethanamide, by recrystallisation.
Outline the method that the student should use for this recrystallisation.
- dissolve the product in the minimum volume of HOT solvent (in boiling tube/beaker)
- allow the solution to cool and allow crystals to form
- filter off pure product under reduced pressure (using Buchner funnel and side arm flask)
Outline how you would carry out a simple laboratory process to show that the recrystallised product is a pure sample of N phenylethanamide
- measure the melting point
- use of melting point apparatus
- sharp melting point / melting point matches data source value
Assume that the reaction goes to completion.
Suggest two practical reasons why the percentage yield for this reaction may not be 100%
- product left in beaker or glassware
- sample was still wet
- sample lost during recrystallisation
explain why the student might want to use ethanoic anhydride, instead of ethanoyl anhydride, even though it has a slower rate of reaction.
- reacts violently with water
- HCl gas, fumes released
you are provided with a small sample of pure aspirin in a melting point tube.
Describe briefly how you would determine an accurate value for the melting point of aspirin.
- heat melting point tube in melting point apparatus
- slowly near the melting point
suggest why a pure sample of aspirin may sometimes appear to melt at a temperature different from 135°C
- temperature on the thermometer not the same as sample
A student prepared a sample of aspirin (melting point 135°C) in the laboratory and attempted to purify it by recrystallisation. To check the purity of the aspirin the student determined its melting point.
State two observations, during this melting point determination, that would indicate that the sample is not pure.
OBSERVATION 1 -
melting range would be wide (not sharp)
OBSERVATION 2 -
melting point below the true melting point
The method for recrystallisation is outlined below.
Step 1: the sample is dissolved in a MINIMUM VOLUME of HOT WATER.
Step 2: the solution is FILTERED HOT
Step 3: the filtrate is COOLED IN ICE to form crystals.
Step 4: the crystals are collected by filtration, WASHED WITH COLD WATER and left to dry
Explain the purpose of each underlined point
MINIMUM VOLUME
increases yield
HOT WATER enable crystallisation (on cooling)
FILTERED HOT
removes insoluble impurities + to prevent crystals forming during filtration
COOLED IN ICE
increases the amount of crystals that are formed
WASHED WITH COLD WATER
removes soluble impurities
Give one reason why using a water bath is better than direct heating with a bunsen burner when oxidising a primary alcohol.
the alcohol is flammable
Explain why different amino acids have different Rf values
amino acids have different polarities
thus, have different retention on the stationary phase or different solubility in the developing solvent
parts of a practical procedure are given below. (TLC)
- WEARING PLASTIC GLOVES TO HOLD A TLC PLATE, draw a pencil line 1.5cm from the bottom of the plate.
- use a capillary tube to apply a very small drop of the solution of amino acids to the mid-point of the pencil line.
- Allow the spot to dry completely
- in the developing tank, add the developing solvent to a DEPTH OF NOT MORE THAN 1 CM.
- Place your TLC plate in the developing tank
- Allow the developing solvent to rise up the plate TO THE TOP
- Remove the plate and quickly mark the position of the solvent front with a pencil.
- Allow the plate to dry IN A FUME CUPBOARD
For each of the bold parts consider whether it is essential and justify your answer.
WEAR PLASTIC GLOVES
- essential = to prevent contamination
ADD DEVELOPING SOLVENT TO A DEPTH OF NOT MORE THAN 1cm3
- essential = if the solvent is too deep it will dissolve the mixture from the plate
ALLOW SOLVENT TO RISE UP THE PLATE TO THE TOP
not essential - the Rf value can be calculated if the solvent front does not reach the top of the plate
ALLOW THE PLATE TO DRY IN A FUME CUPBOARD
essential = the solvent is toxic
outline the steps needed to locate the positions of the amino acids on the TLC plate and to determine their Rf values.
Spray TLC plate with developing agent or use UV
Measure distances from initial pencil line to the spots (x)
measure distance from initial pencil line to solvent front line (y)
Rf value = x/y
describe what happens when a reaction mixture is refluxed and why it is necessary, in this case for complete oxidation of ethanoic acid.
a mixture of liquids are heated to boiling point for long time
vapour forms and escapes from liquid mixture, is changes back to liquid and returned to liquid mixture
any ethanal and ethanol that initially evaporates can then be oxidised (as it goes back into liquid form)
Ethanol BP = 78°C
Ethanal BP = 21°C
Ethanoic acid BP = 118°C
Use data to describe how you would obtain a sample of ethanal from a mixture of these 3 compounds. Include in our answer a description of the apparatus you would use and how you would minimise the loss of ethanal.
mixture heated heated in a suitable flask
with still heat containing a thermometer
water cooled condenser connected to the still head and suitable cooled collecting vessel.
collect sample at the bp of ethanal
cooled collection vessel necessary to reduce evaporation of ethanal
use knowledge of structure and bonding to suggest why it is is possible to separate ethanal in this way.
there is hydrogen bonding in ethanol and ethanoic acid
there is no hydrogen bonding in ethanal , only permanent diplole dipole forces
which are weaker than hydrogen bonding
describe how the student could use chemical tests to confirm the liquid contained ethanal and did not contain ethanoic acid
confirm ethanal is present:
Add Fehling’s solution
warm
red precipitate formed
confirm ethanoic acid is not present:
Add sodium hydrogencarbonate
no effervescence observed: so no acid present
what is the method to determine the yield of aspirin?
Step 4 is shown below.
- Reweigh the empty boat
what is the importance of step 4?
suggest a suitable piece of apparatus to measure out the ethanoic anhydride in step 5.
- Add 10cm3 of ethanoic anhydride to a conical flask and swirl.
identify a hazard of phoshoric acid in step 6.
- Add about 6g of salicylic acids to a weighing boat
- Place the weighing boat on a 2 decimal place balance and record the mass.
- Tip the salicylic acid into a 100cm3
- Reweigh the empty boat
- Add 10cm3 of ethanoic anhydride to a conical flask and swirl.
- Add 5 drops of conc. phosphoric acid.
- Warm for 20 minutes
- Add ice cold water to the reaction mixture and place the flask in an ice bath
- Filter off the crude aspirin mixture and leave it to dry.
- Weigh the crude aspirin and calculate the yield.
Justification: in order to calculate the exact mas s of salicylic acid added in the reaction mixture
Burette/ pipette
it is corrosive
describe two important precautions when heating the mixture of ethanol and crude aspirin
- ethanol is flammable so use a water bath and not a Bunsen burner
- Heat below boiling point so ethanol does not boil away.
Describe one difference in appearance you would expect to see between these two solid samples
pure products will have larger crystals
pure product will be lighter in colour
the paracetamol formed in this reaction is impure. It contains both soluble and insoluble impurities that must be removed by recrystallisation.
the propertied of a suitable solvent for recrystallisation are shown.
- the paracetamol should dissolve when the solvent is hot but be almost insoluble when the solvent is cold
the impurities - the impurities should either be insoluble in the solvent at all temperatures or soluble even in cold solvent.
- when hot saturated solution of the paracetamol is cooled, as much product as possible should crystallise out, leaving soluble impurities in the solution.
A solvent has been suggested for this recrystallisation. It is a flammable liquid with a boiling point of 80 degrees celcius.
Outline how you would carry out an investigation to show that this solvent is suitable for the recrystallisation of the impure paracetamol. You should include brief practical details of how you would carry out your investigation.
you are not required to describe the full recrystallisation process.
Explain how you would check that a recrystallisation process had been effective at producing a pure sample of paracetamol
Place a small amount of pure paracetamol in a test tube.
- Add small volume (0.5-1.0 cm3 ) of possible solvent (cold)
- Shake/stir.
- If paracetamol dissolves then solvent unsuitable.
- Heat (with suitable awareness of flammability and boiling point; eg hot - not boiling - water bath).
- If paracetamol does not dissolve = unsuitable.
- If paracetamol dissolves partially try adding more solvent.
- If/when completely dissolved place test-tube in ice water bath.
- If crystals form = suitable.
- Effectiveness determined by measuring melting point. - Purity indicated by melting point being sharp and close to 170 °C.
suggest reasons why, in an industrial situation, ethanoic anhydride would be preferred to ethanoyl chloride in the production of paracetamol
- less exothermic reaction
- easier to control
- dangerous gas not involved
- ethanoic anhydride is cheaper/ more easily recycled
Deduce two reasons why the student’s value for the standard enthalpy of
combustion of ethanol is different from a Data Book value of –1279 kJ mol–1
.
- incomplete combustion
- heat loss
- heat capacity of Cu not included (calorimeter)
- some ethanol lost by evaporation
