suggested answers to in-text activities and...

New 21st Century Chemistry

Suggested answers to in-text activities and unit-end exercises 1 © Jing Kung. All rights reserved.

Topic 16 Unit 54

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.


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:



Topic 16 Unit 54

• Treat each compound with the Tollens’ reagent. CH3(CH2)3CHO gives a silver mirror

but C2H5COC2H5 does not.


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.


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


a)

Test reagent Test result Deduction

K2Cr2O7 / H3O+ positive 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, — CC —

b) Any three of the following:

H2C=CHCH2CH2OH

H3CCH=CHCH2OH

H2C=CHCH(OH)CH3

H3CCH=C(OH)CH3



Topic 16 Unit 54

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.


Solvent A

The solubility of X in A is high at high temperatures but low at low temperatures.


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 =

= 0.79


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.

5.5 cm

7.0 cm



Topic 16 Unit 54

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.



Topic 16 Unit 54

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 Observation with B

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)

Warm with iodine and an aqueous

solution of sodium hydroxide

no observable change a bright yellow precipitate

forms

2 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



Topic 16 Unit 54

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

• Add PCl5 to each chemical.

(CH3)2CHOH gives steamy fumes but (CH3)2CHCHO does not.

(CH3)2CHOH + PCl5 (CH3)2CHCl + POCl3 + HCl

steamy fumes

e) • 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.



Topic 16 Unit 54

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.



Topic 16 Unit 54

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, CC

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.



Topic 16 Unit 54

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.



Topic 16 Unit 54

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:



Topic 16 Unit 54

.

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.



Topic 16 Unit 54

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 = = 0.46

Rf value for the prohibited dye = = 0.46

iv) To be certain whether the spots are the same / different.

OR

45

98

44

96



Topic 16 Unit 54

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

irritant

• wear safety glasses and

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

cupboard

Material

Safety Data

Sheet (MSDS)

Tollens’ reagent irritant and

explosive


protective gloves

• Tollens’ reagent becomes

explosive on evaporation,

therefore wash away the

solution immediately after

use



cupboard

Material

Safety Data

Sheet (MSDS)

and previous

instruction

Hot water may cause burns • use water at 60 °C previous



Topic 16 Unit 54

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


protective gloves

• avoid skin contact with the

chemical


tightly closed

• keep away from naked

flames



cupboard

International

Chemical

Safety Cards

(ICSCs)

2-chloro-2-

methylpropane

flammable • wear safety glasses and

protective gloves


tightly closed

• keep away from naked

flames



cupboard

Material

Safety Data

Sheet (MSDS)

Concentrated

hydrochloric acid

corrosive • wear safety glasses and

protective gloves

• 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

concentrated hydrochloric

acid with great care; do not

add water to the acid directly

Material

Safety Data

Sheet (MSDS)



Topic 16 Unit 54



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

an upright position safely

with stands and clamps

previous

instruction

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

aqueous solution of

potassium dichromate.

Add 10 drops of X and

warm gently.

The dichromate solution

changes from orange

to green.

The dichromate ions are

reduced to chromium(III)

ions.

X is either a primary /

secondary alcohol, or an

aldehyde.

3 Mix 2 cm3 of X with

2,4-dinitrophenylhydrazine

An orange precipitate

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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