Alcohols Questions with Verified
Answers
The boiling points of the organic compounds in a reaction mixture are shown in the
following table.
Compound ethanol ethanal ethanoic acid
Boiling point / °C 78 21 118
Use these data to describe how you would obtain a sample of ethanal from a mixture of
these three compounds. Include in your answer a description of the apparatus you
would use and how you would minimise the loss of ethanal. Your description of the
apparatus can be either a description in words or a labelled sketch. - ANSWERSMixture
heated in a suitable flask
With still head containing a thermometer
1
Water cooled condenser connected to the still head and suitable
cooled collecting vessel
1
Collect sample at the boiling point of ethanal
1
Cooled collection vessel necessary to reduce evaporation of ethanal
Use your knowledge of structure and bonding to explain why it is possible to separate
ethanal in this way - ANSWERSHydrogen bonding in ethanol and ethanoic acid or no
hydrogen
bonding in ethanal
1
Intermolecular forces / dipole-dipole are weaker than hydrogen
bonding
A student obtained a sample of a liquid using the apparatus in part (c). Describe how
the student could use chemical tests to confirm that the liquid contained ethanal and did
not contain ethanoic acid. - ANSWERSReagent to confirm the presence of ethanal:
Add Tollens' reagent / ammoniacal silver nitrate / aqueous silver
nitrate followed by 1 drop of aqueous sodium hydroxide, then enough
aqueous ammonia to dissolve the precipitate formed
OR
Add Fehling's solution
1
Warm
M2 and M3 can only be awarded if M1 is given
correctly
1
Result with Tollen's reagent:
, Silver mirror / black precipitate
OR
Result with Fehling's solution:
Red precipitate / orange-red precipitate
1
Reagent to confirm the absence of ethanoic acid
Add sodium hydrogencarbonate or sodium carbonate
1
Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-
step
process.
ethanol to ethanal to ethanoic acid
(a) In order to ensure that the oxidation to ethanoic acid is complete, the reaction is
carried out under reflux.
Describe what happens when a reaction mixture is refluxed and why it is necessary, in
this case, for complete oxidation to ethanoic acid - ANSWERSA mixture of liquids is
heated to boiling point for a prolonged time
1
Vapour is formed which escapes from the liquid mixture, is changed
back into liquid and returned to the liquid mixture
1
Any ethanal and ethanol that initially evaporates can then be oxidised
1
State the purpose of the anti-bumping granules - ANSWERSTo prevent vigorous
boiling/uneven boiling/bubbling
vigorously + large bubbles from forming
Anti-bumping granules are placed in the flask when refluxing. Suggest why these
granules prevent bumping. - ANSWERSPrevents large bubbles
A sample of 2-methylpropan-2-ol was contaminated with butan-2-ol. The student
separated the two alcohols using chromatography. Identify a reagent or combination of
reagents that the student could use to distinguish between these alcohols. State what
would be observed for each alcohol.
Reagent(s)
_____________________________________________________________
Observation with 2-methylpropan-2-ol
________________________________________
______________________________________________________________________
_
______________________________________________________________________
_
Observation with butan-2-ol - ANSWERSReagent = acidified potassium dichromate
Observation with 2-methylpropan-2-ol = orange or no colour change
Answers
The boiling points of the organic compounds in a reaction mixture are shown in the
following table.
Compound ethanol ethanal ethanoic acid
Boiling point / °C 78 21 118
Use these data to describe how you would obtain a sample of ethanal from a mixture of
these three compounds. Include in your answer a description of the apparatus you
would use and how you would minimise the loss of ethanal. Your description of the
apparatus can be either a description in words or a labelled sketch. - ANSWERSMixture
heated in a suitable flask
With still head containing a thermometer
1
Water cooled condenser connected to the still head and suitable
cooled collecting vessel
1
Collect sample at the boiling point of ethanal
1
Cooled collection vessel necessary to reduce evaporation of ethanal
Use your knowledge of structure and bonding to explain why it is possible to separate
ethanal in this way - ANSWERSHydrogen bonding in ethanol and ethanoic acid or no
hydrogen
bonding in ethanal
1
Intermolecular forces / dipole-dipole are weaker than hydrogen
bonding
A student obtained a sample of a liquid using the apparatus in part (c). Describe how
the student could use chemical tests to confirm that the liquid contained ethanal and did
not contain ethanoic acid. - ANSWERSReagent to confirm the presence of ethanal:
Add Tollens' reagent / ammoniacal silver nitrate / aqueous silver
nitrate followed by 1 drop of aqueous sodium hydroxide, then enough
aqueous ammonia to dissolve the precipitate formed
OR
Add Fehling's solution
1
Warm
M2 and M3 can only be awarded if M1 is given
correctly
1
Result with Tollen's reagent:
, Silver mirror / black precipitate
OR
Result with Fehling's solution:
Red precipitate / orange-red precipitate
1
Reagent to confirm the absence of ethanoic acid
Add sodium hydrogencarbonate or sodium carbonate
1
Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-
step
process.
ethanol to ethanal to ethanoic acid
(a) In order to ensure that the oxidation to ethanoic acid is complete, the reaction is
carried out under reflux.
Describe what happens when a reaction mixture is refluxed and why it is necessary, in
this case, for complete oxidation to ethanoic acid - ANSWERSA mixture of liquids is
heated to boiling point for a prolonged time
1
Vapour is formed which escapes from the liquid mixture, is changed
back into liquid and returned to the liquid mixture
1
Any ethanal and ethanol that initially evaporates can then be oxidised
1
State the purpose of the anti-bumping granules - ANSWERSTo prevent vigorous
boiling/uneven boiling/bubbling
vigorously + large bubbles from forming
Anti-bumping granules are placed in the flask when refluxing. Suggest why these
granules prevent bumping. - ANSWERSPrevents large bubbles
A sample of 2-methylpropan-2-ol was contaminated with butan-2-ol. The student
separated the two alcohols using chromatography. Identify a reagent or combination of
reagents that the student could use to distinguish between these alcohols. State what
would be observed for each alcohol.
Reagent(s)
_____________________________________________________________
Observation with 2-methylpropan-2-ol
________________________________________
______________________________________________________________________
_
______________________________________________________________________
_
Observation with butan-2-ol - ANSWERSReagent = acidified potassium dichromate
Observation with 2-methylpropan-2-ol = orange or no colour change
