New 21st Century Chemistry
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
In-text activities
Checkpoint (page 64)
1 a) Any one of the following:
• Add aqueous bromine to each compound.
Propene turns the yellow-brown aqueous bromine colourless quickly but propan-1-ol
does not.
•
•
Add acidified dilute aqueous solution of potassium permanganate to each compound.
Propene turns the purple permanganate solution colourless quickly but propan-1-ol
does not.
Add phosphorus pentachloride to each compound.
Propan-1-ol gives steamy fumes but propene does not.
(Extension)
b) Treat each compound with the Lucas reagent.
Propan-2-ol gives cloudiness in 5 minutes.
Propan-1-ol shows no observable change.
(Extension)
c) Warm each compound with iodine and an aqueous solution of sodium hydroxide.
Pentan-2-ol gives a bright yellow precipitate but pentan-3-ol does not.
2
Warm each compound with acidified aqueous solution of potassium permanganate.
Pentan-1-ol is oxidized to pentanal and then pentanoic acid.
The purple permanganate ion is reduced to colourless manganese(II) ion.
Pentan-2-one resists oxidation.
Checkpoint (page 69)
1 a) Any one of the following:
• Treat each compound with 2,4-dinitrophenylhydrazine.
CH3CH2CHO gives a yellow to red precipitate but CH3CH2COOH does not.
• Treat each compound with the Tollens’ reagent.
CH3CH2CHO gives a silver mirror but CH3CH2COOH does not.
• Warm each compound with acidified aqueous solution of potassium dichromate.
CH3CH2CHO turns the orange dichromate solution green but CH3CH2COOH does not.
b) Any one of the following:
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
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New 21st Century Chemistry
•
Treat each compound with the Tollens’ reagent. CH3(CH2)3CHO gives a silver mirror
•
but C2H5COC2H5 does not.
Warm each compound with acidified aqueous solution of potassium dichromate.
CH3(CH2)3CHO turns the orange dichromate solution green but C2H5COC2H5 does not.
c) Any one of the following:
• Treat each compound with 2,4-dinitrophenylhydrazine.
C4H9COCH3 gives a yellow to red precipitate but C4H9CH(OH)CH3 does not.
• Warm each compound with acidified aqueous solution of potassium dichromate.
C4H9CH(OH)CH3 turns the orange dichromate solution green but C4H9COCH3 does
not.
2
a) A, B and C are either aldehydes or ketones because they give an orange precipitate with
2,4-dinitrophenylhydrazine.
B and C are aldehydes as they give a silver mirror with the Tollens’ reagent.
b)
A
B and C
Checkpoint (page 72)
a)
Test reagent
+
K2Cr2O7 / H3O
Test result
positive
Deduction
functional group(s) that may be present:
— OH, — CHO
2,4-dinitrophenylhydrazine
negative
functional group(s) absent:
— CHO,
C=O
Aqueous bromine
positive
functional group(s) that may be present:
— C=C, — CC —
b) Any three of the following:
H2C=CHCH2CH2OH
H3CCH=CHCH2OH
H2C=CHCH(OH)CH3
H3CCH=C(OH)CH3
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
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New 21st Century Chemistry
Problem Solving (page 72)
Treat each liquid with the Tollens’ reagent.
Only ethanal gives a silver mirror.
Treat the remaining four liquids with 2,4-dinitrophenylhydrazine.
Only propanone gives a yellow to red precipitate.
Mix the remaining three liquids with an aqueous solution of sodium hydrogencarbonate.
Only ethanoic acid gives effervescence.
Mix the remaining two liquids with phosphorus pentachloride.
Only ethanol gives steamy fumes.
The remaining liquid is pentyl ethanoate.
Checkpoint (page 82)
Solvent A
The solubility of X in A is high at high temperatures but low at low temperatures.
Checkpoint (page 86)
1 Substances 1, 2 and 4 are present.
2
a) Each amino acid distributes itself between the water in the paper fibres and the developing
solvent.
Amino acids that are more soluble in the developing solvent travel up more quickly. Thus,
the amino acids separate.
b) Spray a developing agent such as ninhydrin onto the paper. This converts the amino acids
into coloured spots on the chromatogram.
c) Rf value for B =
5.5 cm
7.0 cm
= 0.79
Checkpoint (page 91)
1 a) Carry out simple distillation. Collect the liquid boiling off between 70 °C and 90 °C.
b) Cyclohexanol and phosphoric acid.
c) Transfer the distillate to a separating funnel.
Add saturated sodium chloride solution and shake. Allow the two layers to separate. Run
off the lower aqueous layer.
Run the top cyclohexene layer into a small conical flask. Add some anhydrous calcium
chloride. Allow to stand until the cyclohexene becomes clear.
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
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Decant the cyclohexene into a clean flask and re-distil it, collecting the liquid which distils
off between 81 °C and 85 °C.
2
a) Pour the reaction mixture over crushed ice and stir until the product solidifies.
Collect the product by filtration under reduced pressure after all the ice melts.
Wash the product with a little ice-cold ethanol (keep this wash liquid for (c)).
b) To re-crystallize the product, dissolve it in the minimum amount of hot ethanol.
Filter the mixture while hot.
Allow the filtrate to cool. Collect the crystals by filtration under reduced pressure and dry
them.
c) Evaporate the wash liquid from (a) to 1 cm3.
Use a fine capillary table to put a spot of the solution on one end of a fine layer of silica
coated onto a glass plate.
After drying, place the glass plate in a jar lined with a piece of filter paper dipping in a
solvent (an ethoxyethane-hexane mixture).
Methyl 3-nitrobenzoate can be seen under ultraviolet light or by exposing the glass plate to
iodine vapour.
Methyl 2-nitrobenzoate, a minor product, may appear as a yellow spot on the silica layer.
STSE Connections (page 94)
1 Pros of using distillation for water purification
• Distillation is often used as the preferred water purification method in developing nations
or areas where the risk of waterborne disease is high, due to its unique capabilities to
remove bacteria and viruses from drinking water.
• Distilled water will not cause scales in water-using appliances like coffee maker, iron or
clothes steamer.
Cons of using distillation for water purification
• Distillation is expensive.
• Distillation removes every possible impurity from water, including minerals and salts. The
human body is incapable of surviving on distilled water for a prolonged period of time, as
it does not contain the much needed composition of minerals. These minerals are very
much needed by the body and a deficiency is bound to cause some ailments.
• The content of salts and minerals also give water a peculiar and welcoming 'taste'.
2
Two other water treatment methods:
• boiling;
• portable filters providing various degrees of protection.
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References
http://www.allaboutwater.org/distillation.html
http://www.buzzle.com/articles/distillation-of-water.html
http://campushealth.unc.edu/index.php?option=com_content&task=view&id=599&Itemid=65
Unit-end exercises (pages 98 – 107)
Answers for the HKCEE and HKALE questions are not provided.
1
Reagent
Observation with A
2,4-dinitrophenylhydrazine
a yellow to red precipitate
forms
a yellow to red precipitate
forms
Warm with Tollens’ reagent
silver mirror forms on the
wall of reaction vessel
no observable change
(Extension)
no observable change
a bright yellow precipitate
forms
Warm with iodine and an aqueous
solution of sodium hydroxide
2
Observation with B
Heat each compound with acidified aqueous solution of potassium dichromate under reflux.
Pentan-1-ol is oxidized to pentanal and then pentanoic acid.
Orange dichromate ion is reduced to green chromium(III) ion.
Pentan-2-one resists oxidation.
3
a) Warm each chemical with acidified aqueous solution of potassium permanganate.
CH3CHO is oxidized to CH3COOH. Purple permanganate ion is reduced to colourless
manganese(II) ion.
CH3CHO + [O]
CH3COOH
CH3COCH3 shows no observable change.
b) Add 2,4-dinitrophenylhydrazine to each chemical.
CH3CH2COCH3 gives a yellow to red precipitate but CH3CH2COOCH3 does not.
c) Mix each chemical with sodium hydrogencarbonate solution.
Effervescence occurs for HOOCCH2CH2CH3 but not for HCOOCH2CH2CH3.
+
HOOCCH2CH2CH3 + NaHCO3
Na–OOCCH2CH2CH3 + H2O + CO2
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New 21st Century Chemistry
d) Any one of the following:
• Add 2,4-dinitrophenylhydrazine to each chemical.
(CH3)2CHCHO gives a yellow to red precipitate but (CH3)2CHOH does not.
•
Add Tollens’ reagent to each chemical and warm.
(CH3)2CHCHO gives a silver mirror on the wall of the reaction vessel but
(CH3)2CHOH does not.
•
Add ethanoic acid to each chemical and warm.
(CH3)2CHOH gives a sweetish smell but (CH3)2CHCHO does not.
CH3COOH + (CH3)2CHOH
CH3COOCH(CH3)2 + H2O
ester
•
e) •
Add PCl5 to each chemical.
(CH3)2CHOH gives steamy fumes but (CH3)2CHCHO does not.
(CH3)2CHOH + PCl5
(CH3)2CHCl + POCl3 + HCl
steamy fumes
Any one of the following:
Add aqueous bromine to each chemical.
(CH3)2C=C(CH3)2 turns the yellow-brown aqueous bromine colourless quickly but
does not.
•
Add acidified dilute aqueous solution of potassium permanganate to each chemical.
(CH3)2C=CH(CH3)2 turns the purple permanganate solution colourless quickly but
does not.
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4
a) Paper chromatography
b) Sample A – one black spot
Sample B – three colour spots (yellow, green and blue)
5
First place some of the dry solids in a glass capillary tube.
Then tie the tube to a thermometer with a rubber band. The solids in the tube should be level
with the bulb of the thermometer.
Place this set-up together with a stirrer in an oil bath and heat gently with stirring.
6
a) CH3CH2CH2CH2OH
CH3CH2CH(OH)CH3
(CH3)2CHCH2OH
(Extension)
b) When treated with a solution of anhydrous zinc chloride in concentrated hydrochloric acid,
cloudiness appears in 1 minute for alcohol X.
Cloudiness appears in 5 minutes for the secondary alcohol CH3CH2CH(OH)CH3.
The reaction mixture of CH3CH2CH2CH2OH and (CH3)2CHCH2OH remains clear.
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7
a)
Reagent
Observation with
X
Observation with Y
Aqueous solution of sodium
hydrogencarbonate
Effervescence
occurs
no observable change
2,4-dinitrophenylhydrazine
no observable
change
a yellow to red precipitate forms
Acidified aqueous solution
of potassium dichromate
no observable
change
aqueous solution of potassium
dichromate turns from orange to
green
b) i)
ii)
8
a)
Test reagent
Test result
Deduction
+
K2Cr2O7 / H3O
positive
functional group(s) that may be present:
— OH, — CHO
Tollens’ reagent
negative
functional group(s) absent:
— CHO
Aqueous bromine
positive
functional group(s) that may be present:
C=C, CC
b)
9 a) As Y is an oxidation product of X and gives a negative result when warmed with Tollens’
reagent, it can be deduced that Y is an aromatic ketone.
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Topic 16 Unit 54
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New 21st Century Chemistry
Structure of Y:
b) i) Hydroxyl group
ii) As a ketone (Y) is obtained by the oxidation of X, thus X is a secondary alcohol.
Structure of X:
c)
10 a) Butanal gives a yellow to red precipitate when treated with 2,4-dinitrophenylhydrazine.
b)
Test
Deduction(s)
I
Treat with
2,4-dinitrophenylhydrazine
X and Y may either be aldehydes or ketones. Z
is neither an aldehyde nor a ketone.
II
Warm with Tollens’ reagent
Y is an aldehyde. X and Z are not aldehydes.
III
Shake with acidified aqueous
solution of potassium dichromate
Y and Z may be primary / secondary alcohols
or aldehydes. X is a ketone or tertiary alcohol.
From Tests I and III, it can be deduced that X is a ketone.
The structural formula of X is CH3CH2COCH3.
From Test II, it can be deduced that Y is an aldehyde.
The structural formula of Y is (CH3)2CHCHO.
From Tests I, II and III, it can be deduced that Z is a primary / secondary alcohol.
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Topic 16 Unit 54
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New 21st Century Chemistry
The structural formula of Z is H2C=CHCH2CH2OH or
or
or
11 a) A gives positive results when treated with the Tollens’ reagent. It can be deduced that A is
an aldehyde with the –CHO functional group.
A decolorizes aqueous bromine readily. It can be deduced that A contains either a –C=C–
bond or –C C– bond.
b) As the molecular formula of A is C9H8O, besides the aromatic ring (–C6H5 / –C6H4) and
the –CHO functional group, the formula of the other group should be C2H2 / C2H3. Thus, A
probably contains a –HC=CH–bond.
The structural formula of A is either
or
.
exhibits geometrical isomerism:
exhibits position isomerism:
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
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New 21st Century Chemistry
.
12 —
13 —
14 a) Isolate the crude 1-bromobutane from the reaction mixture by distillation.
b) The distillate collected has an organic and an aqueous layer. Use a teat pipette to remove
the aqueous upper layer.
Transfer the organic layer to a separating funnel and add concentrated hydrochloric acid.
Stopper and shake well. This removes the unreacted butan-1-ol as it is soluble in the acid.
Allow the two layers to separate. Run the organic layer into a conical flask.
Shake the crude 1-bromobutane with sodium hydrogencarbonate solution in another
separating funnel. This removes the excess hydrochloric acid and other acid impurities.
Allow the layers to separate and run the organic layer into a conical flask.
Add anhydrous sodium sulphate to dry the 1-bromobutane. Swirl the mixture until the
liquid is clear.
Decant the dried product into a pear-shaped flask. Carry out fractional distillation. Collect
the fraction with the boiling point range of 1-bromobutane. This removes any remaining
organic impurities.
15 a) Reflux condenser
b)
c) Add dilute sulphuric acid and filter.
d) Dissolve the crude sample in the minimum amount of hot water.
Filter the mixture while hot to remove the insoluble impurities.
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New 21st Century Chemistry
Allow the filtrate to cool. Collect the crystals by filtration and dry them.
e) Determine the melting point of the product to see whether the product has a sharp melting
point.
Compare the melting point obtained with data book value.
16 —
17 —
18 a) Each component in an orange drink sample distributes itself between the water in the paper
fibres and the developing solvent.
Components that are more soluble in the developing solvent travel up more quickly. Thus,
the components separate.
b) Only the spot from D travels the same distance and have the same colour as the spot from
A.
c) i) Similarity
Any one of the following:
• Spots are placed on a medium.
• Solvent is allowed to soak up the medium.
• Different components travel different distances.
• Separation is by differences in distribution between the mobile and stationary
phases.
Difference
Any one of the following:
• The medium is different.
• The medium is spread on a piece of glass.
ii) Permanent felt tip pen ink does not dissolve in water.
iii) Rf value for the felt tip pen ink =
44
96
= 0.46
Rf value for the prohibited dye =
45
98
= 0.46
iv) To be certain whether the spots are the same / different.
OR
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New 21st Century Chemistry
To allow more accurate comparison between the spots.
19 a) Band A
Each component in the mixture has its own equilibrium between adsorption onto the
surface of the adsorbent and solubility in the solvent.
The better adsorbed ones travel more slowly.
b) Band C
It is the component that is the most soluble in the water.
Thus, it moves the greatest distance from the origin relative to the solvent front.
20 —
21
Experiment title: Experiment A — Testing methanal with Tollens’ reagent
Hazardous chemical,
procedure or
equipment involved
Nature of hazard
Safety precautions
Source of
information
Methanal
flammable and
•
wear safety glasses and
Material
protective gloves
carry out the experiment
inside a fume cupboard
keep the reagent bottle
tightly closed
keep the reagent bottle well
away from naked flames
dispose of into appropriate
waste bottle inside the fume
Safety Data
Sheet (MSDS)
irritant
•
•
•
•
cupboard
Tollens’ reagent
irritant and
explosive
•
•
•
wear safety glasses and
protective gloves
Tollens’ reagent becomes
explosive on evaporation,
therefore wash away the
solution immediately after
use
dispose of into appropriate
Material
Safety Data
Sheet (MSDS)
and previous
instruction
waste bottle inside the fume
cupboard
Hot water
may cause burns
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
•
use water at 60 °C
13
previous
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New 21st Century Chemistry
if spilt
instruction
Experiment title: Experiment B — Preparing 2-chloro-2-methylpropane
Hazardous chemical,
procedure or
equipment involved
Nature of hazard
Safety precautions
Source of
information
Methylpropan-2-ol
flammable and
harmful
•
International
Chemical
Safety Cards
(ICSCs)
•
•
•
•
2-chloro-2methylpropane
flammable
wear safety glasses and
protective gloves
Material
Safety Data
•
keep the reagent bottle
tightly closed
keep away from naked
flames
dispose of into appropriate
waste bottle inside the fume
cupboard
Sheet (MSDS)
•
wear safety glasses and
protective gloves
Material
Safety Data
•
carry out this activity inside
a fume cupboard as
concentrated hydrochloric
acid gives off harmful fumes
wash affected areas with
plenty of water if spilt on the
skin or clothes; report the
accident to the teacher
wash the container of
Sheet (MSDS)
•
corrosive
keep the reagent bottle
tightly closed
keep away from naked
flames
dispose of into appropriate
waste bottle inside the fume
cupboard
•
•
Concentrated
hydrochloric acid
wear safety glasses and
protective gloves
avoid skin contact with the
chemical
•
•
concentrated hydrochloric
acid with great care; do not
add water to the acid directly
Suggested answers to in-text activities and unit-end exercises
Topic 16 Unit 54
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New 21st Century Chemistry
•
dispose of into appropriate
waste bottle inside the fume
cupboard
Separating funnel
contents of the
•
separating funnel
may spill if not
handled properly •
•
hold the stopper and tap
when inverting the funnel to
mix the contents
open the tap regularly to
release any pressure
when standing in the fume
cupboard, keep the funnel in
previous
instruction
an upright position safely
with stands and clamps
22 a)
Test
Procedure
Observation
Deduction
1
Add 2 cm3 of X to 10 cm3
of water.
X is soluble in water.
X may form hydrogen bonds
with water.
2
Add 1 cm3 of dilute
sulphuric acid to 2 cm3 of
The dichromate solution The dichromate ions are
changes from orange
reduced to chromium(III)
to green.
ions.
aqueous solution of
potassium dichromate.
Add 10 drops of X and
warm gently.
3
X is either a primary /
secondary alcohol, or an
aldehyde.
Mix 2 cm3 of X with
An orange precipitate
2,4-dinitrophenylhydrazine forms.
X is either an aldehyde or a
ketone.
b) Based on the results of tests 2 and 3, X is probably an aldehyde. Thus, X contains
group.
c)
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