WB 2 Suggested Ans
Transcript of WB 2 Suggested Ans
Chemistry A Modern ViewExperiment Workbook 2 Suggested answers
Contents
PART V ACIDS AND ALKALIS
Chapter 15 Acids15.1 To investigate properties of dilute acids 3
15.2 To study the role of water in exhibiting characteristic properties of acid 6
15.3 To investigate the corrosive nature of concentrated acids 10
Chapter 16 Alkalis16.1 To investigate the action of dilute alkalis on ammonium compounds 11
16.2 To investigate the action of dilute alkalis on aqueous metal ions to form metal
hydroxide precipitates 12
16.3 Action of concentrated sodium hydroxide solution on meat (T) 14
Chapter 17 Indicators and pH17.1 To find pH values of some common substances 15
Chapter 18 Strength of acids and alkalis (Extension)18.1 To compare the relative strength of acids and of alkalis 17
Chapter 19 Neutralization and salts19.1 To investigate the temperature change associated with a neutralization
reaction 22
19.2 To prepare sodium sulphate crystals from an acid-alkali titration 23
Chapter 21 Simple volumetric work (Extension)
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21.1 To prepare standard ethanedioic acid solutions 24
21.2 To find the molarity of a given hydrochloric acid 25
21.3 To determine the concentration of ethanoic acid in commercial vinegar 26
Chapter 22 Rate of reaction (Extension)22.1 To investigate the effect of concentration on the rate of reaction 27
22.2 To investigate the effect of surface area on rate of reaction 28
22.3 Effect of temperature on the rate of reaction 33
PART VI CHEMICAL CELLS AND ELECTROLYSIS
Chapter 23 Chemical cells in daily life23.1 To compare the service life of a zinc-carbon cell with an alkaline manganese
cell of the same size (T) 38
Chapter 24 Simple chemical cells24.1 To make simple chemical cells and construct part of the Electrochemical
Series 39
Chapter 26 Cell reactions (Extension)26.1 To construct a simple chemical cell using inert electrodes 41
26.2 To examine and compare the internal structures of a well-used and a new zinc-
carbon cell 42
Chapter 27 Electrolysis (Extension)
27.1 Electrolysis of dilute sulphuric acid (T) 43
27.2 Effect of concentration on preferential discharge of ions 44
27.3 Effect of electrodes on products of electrolysis 47
27.4 Electroplating 48
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PART VII PRODUCTS FROM IMPORTANT PROCESSES
Chapter 28 Chlorine and hypochlorite28.1 To investigate the oxidizing property of chlorine water 51
28.2 To make chlorine bleach 52
28.3 Action of chlorine bleach on coloured substances 56
28.4 Action of acid on chlorine bleach 57
Chapter 29 Sulphuric acid and sulphur dioxide29.1 To investigate properties of concentrated sulphuric acid (S/T) 58
29.2 To dilute concentrated sulphuric acid (S/T ) 60
29.3 To prepare sulphur dioxide and test for its properties 61
29.4 To bleach coloured papers and flower petals with sulphur dioxide (S/T ) 65
MICROSCALE EXPERIMENT
M1 Electrolysis using a microscale Hoffman apparatus 66
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Experiment 15.1 To investigate properties of dilute acids
Test Observations
2 M hydrochloric acid 2 M ethanoic acid
1. ■ Test with a blue litmus paper. turns red turns red
2. ■ (a) Clean a 2 cm length of magnesium
ribbon with sandpaper. Add the
cleaned ribbon to the acid. Quickly
place the test tube upright in a test
tube rack. Immediately cover the
mouth of the tube with an inverted
rubber stopper (Figure 15.1).
Leave the tube to stand, until the
magnesium has dissolved
completely.
rapid
effervescence of a
colourless gas; the
magnesium dissolves
quickly to give a
colourless solution
effervescence of a
colourless gas; the
magnesium dissolves to
give a colourless
solution
■ (b) Remove the stopper and quickly
put a burning splint into the mouth
of the tube (Figure 15.2).
a ‘pop’ sound is heard a ‘pop’ sound is heard
(c) Name the gas evolved. hydrogen hydrogen
3. ■ (a) Repeat Step 2(a), using 2 calcium
granules instead of a magnesium
ribbon. (Caution! Handle calcium
with forceps, never with bare
fingers.)
rapid
effervescence of a
colourless gas; the
calcium granules
dissolve quickly to give
a colourless solution
effervescence of a
colourless gas; the
calcium granules
dissolve to give a
colourless solution
■ (b) Test any gas evolved with a
burning splint.
a ‘pop’ sound is heard a ‘pop’ sound is heard
(c) Name the gas evolved. hydrogen hydrogen
(d) Write a general word equation for
the reactions involved in Steps
2(a) and 3(a).
acid + metal → salt + hydrogen
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Test Observations
2 M hydrochloric acid 2 M ethanoic acid
4. ■ (a) Add 1/4 spatula measure of
copper(II) oxide to the acid. Warm
the mixture gently (Figure 15.3).
(Caution! Wear safety spectacles.)
the black copper(II)
oxide solid dissolves to
give a blue solution
the black copper(II)
oxide solid dissolves to
give a blue solution
(b) Write a general word equation for
the reactions involved here.
acid + metal oxide → salt + water
5. ■ (a) Add 1/4 spatula measure of
calcium hydroxide to the acid.
Warm the mixture gently.
(Caution! Wear safety spectacles.)
the calcium hydroxide
dissolves to give a
colourless solution
the calcium hydroxide
dissolves to give a
colourless solution
(b) Write a general word equation for
the reactions involved here.
acid + metal hydroxide → salt + water
6. ■ (a) Add 3 spatula measures of
anhydrous sodium carbonate to the
acid. Pass any gas evolved through
limewater (Figure 15.4).
rapid
effervescence of a
colourless gas; the
carbonate dissolves
quickly to form a
colourless solution;
limewater turns milky
effervescence of a
colourless gas; the
carbonate dissolves to
form a colourless
solution; limewater
turns milky
(b) Name the gas evolved. carbon dioxide carbon dioxide
(c) Write a general word equation for
the reactions involved here.
acid + carbonate → salt + carbon dioxide + water
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7. ■ (a) Repeat Step 6(a), using sodium
hydrogencarbonate instead of
sodium carbonate. Pass any gas
evolved through limewater.
rapid
effervescence of a
colourless gas; the
hydrogencarbonate
dissolves quickly to
form a colourless
solution; limewater
turns milky
effervescence of a
colourless gas; the
hydrogencarbonate
dissolves to form a
colourless solution;
limewater turns milky
(b) Name the gas evolved. carbon dioxide carbon dioxide
(c) Write a general word equation for
the reactions involved here.
acid + hydrogencarbonate
→ salt + carbon dioxide + water
8. Hydrogen gas.
Hydrogen ion, H+(aq).
Yes.
9. c. No.
d. No.
Hydrochloric acid reacts with the carbonate to give a salt, carbon dioxide and water. The
H+(aq) ions are removed as water. Thus the characteristic acidic properties are lost.
e. Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
10. a. red
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salt; hydrogen
salt; water
salt; carbon dioxide; water
b. hydrogen ions H+(aq)
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Experiment 15.2 sample laboratory report
Title: To study the role of water in exhibiting characteristic properties of acid
PurposeTo study the role of water in exhibiting characteristic properties of acid.
Apparatus and chemicals used• Watch glass (dry) • Magnesium ribbon (2 cm length)
• Dry test tubes (150 × 18 mm) in rack • Citric acid crystals (dry), 2 g
• Stopper that fits 24 mm diameter test tube • Deionized water
• Matches/lighter • Wooden splint
• Blue litmus paper (dry)
Chemical reactions involvedMg(s) + 2H+(aq) Mg2+(aq) + H2(g)
2H2(g) + O2(g) 2H2O(l) ('pop' sound test)
Procedure(A) To compare the action of solid citric acid and its aqueous solution on dry blue litmus
paper
1. (a) Half a spatula measure of dry solid citric acid was added to a dry watch glass.
(b) A dry blue litmus paper was dipped into the solid acid (Figure 1a)
(c) Any colour change of the dry blue litmus paper was recorded.
2. (a) 1 cm3 of water was added to the solid citric acid.
(b) A dry blue litmus paper was dipped into the aqueous solution (Figure 1b).
(c) Any colour change of the dry blue litmus paper was recorded.
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(B) To compare the action of solid citric acid and its aqueous solution on magnesium
3. (a) A piece of magnesium ribbon was put into a dry test tube (Figure 2a).
(b) 3 spatula measures of solid citric acid were added to the test tube (Figure 2b).
(c) Any observation occurred was recorded
4. (a) Water was added to the test tube (from Step 3) to a depth of 3 cm.
(b) The tube was shaken to dissolve the citric acid crystals. It was quickly placed upright in
a test tube rack. The mouth of the tube was immediately covered with an inverted
rubber stopper. The tube was allowed to stand for 5 minutes (Figure 3a).
(c) Any observation occurred was recorded.
(d) After 5 minutes, the stopper was removed and a burning splint was quickly put into the
mouth of the tube (Figure 3b).
(e) Any observation of the experiment was recorded.
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Observation1. (Reference to Step 1): No observable change.
2. (Reference to Step 2): The blue litmus paper changed to red.
3. (Reference to Step 3): No observable change.
4. (Reference to Step 4): The citric acid gradually dissolved to form a colourless solution. At the
same time, colourless gas bubbles were evolved from the surface of the magnesium ribbon.
The magnesium ribbon gradually became smaller. The bottom of the test tube became warm.
The gas gave out a 'pop' sound with a burning splint, which was identified as hydrogen.
Interpretation1. No observable change because there is no hydrogen ions in the absence of water. Thus, solid
citric acid does not show acidic property.
2. After the addition of water, hydrogen ions that can turn the blue litmus paper red, are formed.
3. No observable change because there is no hydrogen ions in the absence of water. Thus, solid
citric acid does not show any acidic property.
4. Citric acid ionizes in water to give H+(aq) ions. It reacts with magnesium to give hydrogen
gas.
Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)
It shows acidic property.
Discussion1. It is essential that the apparatus and chemicals used must be dry at the start, so that the effect
of adding water can be compared correctly.
2. It is not a good practice to cover the test tube with student 's thumb because there may be
some acid on the rim of the test tube. If a right-sized stopper is used to cover the test tube, do
not leave it unattended when there is effervescence in the test tube. Pressure will build up
inside the tube and the stopper may shoot out. This will accidentally hurt students' eyes. The
stopper used should have a bigger size (suitable for test tube with 24 mm diameter).
3. Unreacted magnesium should never be disposed of into the sink. It should be collected by the
teacher and the laboratory technician will take care of it.
4. Magnesium metal surface should be cleaned with sand paper to remove any oxide layer
together with the grease. It should be noted that holding the metal by hand would make the
metal greasy.
ConclusionWithout water, an acid does not show the usual acidic properties. Water must be present for an acid
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to give H+(aq) ions, which are responsible for the typical acidic properties.
Answers to questions for further thought1. It changes blue litmus paper to red colour.
It conducts electricity.
It reacts with metal oxides and hydroxides to give salts and water.
It reacts with carbonates and hydrogencarbonates to give salts, carbon dioxide and water.
2. (a) When it is dissolved in water, the solid acid ionizes to form H+(aq) ions, which react
with sodium hydrogencarbonate. There is effervescence, carbon dioxide gas being
given off.
(b) H+(aq) + HCO3−(aq) H2O(l) + CO2(g)
(c) It should be stored in a dry cool place.
3. In the presence of water, acidic gas will dissolve and ionize to form H+(aq) ions. Thus, it can
change the blue litmus paper to red colour.
4. (a) No colour change. It is because pure ethanoic acid liquid contains no water.
(b) From blue to red. In the presence of water, ethanoic acid ionizes to form H+(aq) ions.
Thus, it can show acidic properties.
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Experiment 15.3 To investigate the corrosive nature of
concentrated acids
1. d. A ‘pop’ sound is heard.
Hydrogen.
2. Yes.
Concentrated hydrochloric acid.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
3. c. No signs of reaction no matter the acid is cold or hot.
Yes. A dilute aqueous solution of a typical acid does not react with copper and those
metals below copper in the metal reactivity series.
4. Brown fumes are evolved quickly to fill the tube.
1; 4; 1; 2; 2
No. An aqueous solution of a typical acid reacts with those metals, which are above copper in
the reactivity series. In such reactions, the gas liberated is hydrogen.
5. a. zinc; the same; faster; acidic; acidity; corrosive; acidity
b. no; oxidizes; very corrosive; oxidizing
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Experiment 16.1 To investigate the action of dilute alkalis on
ammonium compounds
4. A characteristic pungent choking smell of ammonia.
Ammonia.
5. b. It turns from red to blue.
(NH4)2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2NH3(g) + 2H2O(l)
6. Ammonia; ammonium sulphate; sodium hydroxide; ammonia; pungent, choking; red; blue
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Experiment 16.2 To investigate the action of dilute alkalis on
aqueous metal ions to form metal hydroxide
precipitates
Test Observations
0.5 M NaOH(aq) 1 M NH3 (aq)
1. Add a piece of red litmus paper to
2 cm3 of an alkali solution in a test tube
(Figure 16.4).
turns blue turns blue
2. (a) Using a teat pipette, add 10 drops of
sodium hydroxide solution to 6 cm3
of water in a test tube (Figure 16.5).
Rub a little of the diluted solution
between your fingers.
(Caution! (1) Wash your hand
immediately afterwards with plenty
of water. (2) Omit this step if there
is a wound in your fingers. This is
to avoid possible irritation.)
(b) Rub a little of the 1 M ammonia
solution (undiluted) between your
fingers.
it has a slippery (soapy)
feel
it has a slippery (soapy)
feel
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6.
Metal ions Addition of a little
NaOH(aq)
Addition of excess
NaOH(aq)
Addition of a little
NH3(aq)
Colour of precipitate
formed (if any)
Does the precipitate
dissolve in excess
NaOH(aq)?
Colour of precipitate
formed (if any)
Pb2+(aq) white yes white
Cu2+(aq) blue no blue
Fe2+(aq) dirty green no dirty green
Fe3+(aq) reddish brown no reddish brown
K+(aq) (no precipitate) (not applicable) (no precipitate)
Zn2+(aq) white yes white
7. Yes.
Hydroxide ion, OH-(aq).
NaOH(s) + H2O(l) → Na+(aq) + OH−(aq)
NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)
8. a. Pb2+(aq), Cu2+(aq), Fe2+(aq), Fe3 +(aq), Zn2+(aq)
b. Pb2+(aq) + 2OH−(aq) → Pb(OH)2(s);
Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s);
Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s);
Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s);
Zn2+(aq) + 2OH−(aq) → Zn(OH)2(s)
9. Pb2+(aq), Zn2+(aq)
10. a. red; blue
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soapy
metal hydroxides
Pb(OH)2(s), Cu(OH)2(s), Fe(OH)2(s), Fe(OH)3(s), Zn(OH)2(s)
b. hydroxide ion OH−(aq)
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Experiment 16.3 Action of concentrated sodium hydroxide
solution on meat (T)
1. Pale pink or slightly yellow (answer varies).
Yes.
5. The chicken foot becomes smaller. It turns reddish yellow and somewhat ‘translucent’. The
solution becomes pale red.
The chicken foot becomes white. There is no other apparent change.
6. The chicken foot crumbles (breaks up) when stirred. A mixture of skin, meat and white oily
mass floats. Small pieces of bones sink to the bottom.
7. There is no apparent change.
8. Concentrated sodium hydroxide solution is very corrosive on meat.
9. He should wash the affected part with plenty of water (for at least a few minutes), then report
to the teacher.
10. corrosive; eat; chicken feet
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Experiment 17.1 To find pH values of some common substances
4. (1) 4
(2) 10
(3) 2
(4) 3
(5) 9
(6) 10
(7) 7
(8) answer varies
(9) 1
(10) 14
(11) 12
(12) 7
5. (a) distilled water, sodium chloride solution
(b) soft drink (7-up), vinegar, lemon juice, 1 M hydrochloric acid
(c) soap solution, Philips Milk of Magnesia/ window cleaner, limewater, 1 M sodium
hydroxide
12. pH scale
Indicator
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Methyl orange red orange yellow
Litmus red purple blue
Phenolphthalein Colourless very pale
pink
red
13. Red / yellow.
Not sure.
Acidic / not sure.
14. Red / blue.
Not sure.
Acidic / alkaline.
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15. Colourless / red.
Not sure.
Not sure / alkaline.
16. No, we could only get a rough idea of the pH, acidity or alkalinity of a solution.
17. By using universal indicator or pH meter.
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Experiment 18.1 sample laboratory report
Title: To compare the relative strength of acids and of alkalis
PurposeTo compare the relative strength of
(A) acids and
(B) alkalis.
Apparatus and chemicals usedApproach 1: To compare the relative strength of acids and of alkalis by measuring
electrical conductivities
(A) For comparing the relative strength of acids
• Beaker (100 cm3) • Hydrochloric acid (0.1 M)
• Power supply (0 − 24 V) / 6 V d.c. supply • Ethanoic acid (0.1 M)
• Electrode rod holder
• 2 carbon rods
• 3 connecting wires, with crocodile clips at both ends
• Digital multimeter (or milliammeter)
• Measuring cylinder (100 cm3)
(B) For comparing the relative strength of alkalis
• Same as in (A) • Sodium hydroxide solution (0.1 M)
• Ammonia solution (0.1 M)
Approach 2: To compare the relative strength of acids and of alkalis by measuring pH
values
(A) For comparing the relative strength of acids
• 2 test tubes (150 × 18 mm) in rack • Hydrochloric acid (0.1 M)
• Glass rod • Ethanoic acid (0.1 M)
• Measuring cylinder (100 cm3)
• pH paper (pH range 1 − 14) with colour chart
(B) For comparing the relative strength of alkalis
• Same as in (A) • Sodium hydroxide solution (0.1 M)
• Ammonia solution (0.1 M)
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Chemical reactions involvedHCl(aq) H+(aq) 1 Cl−(aq)
CH3COOH(aq) CH3COO−(aq) + H+(aq)
NaOH(s) + water Na+(aq) + OH−(aq)
NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)
Approach 1: To compare the relative strength of acids and of alkalis by measuring
electrical conductivities
Procedure(A) To compare the relative strength of acids by measuring electrical conductivities.
1. 80 cm3 of 0.1 M hydrochloric acid was put into a small beaker (100 cm3).
2. The apparatus was set-up as shown in Figure 1.
1 2
3. The range selector of the multimeter was adjusted to a suitable range for measuring d.c.
current. The reading was noted immediately.
4. The electrode rod holder was taken out of the beaker. The carbon rods were washed with
running tap water.
5. The beaker was emptied and washed well with water.
6. Steps 1 to 5 were repeated. 80 cm3 of 0.1 M ethanoic acid was used instead.
(B) To compare the relative strength of alkalis by measuring electrical conductivities.
7. Steps 1 to 5 were repeated. 80 cm3 of 0.1 M sodium hydroxide solution was used instead.
8. Steps 1 to 5 were repeated. 80 cm3 of 0.1 M ammonia solution was used instead.
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Approach 2: To compare the relative strength of acids and of alkalis by measuring pH
values
(A) To compare the relative strength of acids by measuring pH values.
9. (a) A glass rod was dipped into a little 0.1 M hydrochloric acid in a test tube to get a drop
of the solution (Figure 2a).
(b) The drop of acid was applied to a piece of pH paper (Figure 2b).
10. The colour of the pH paper was matched with the colour chart. The pH value was then
recorded.
11. Steps 9 to 10 were repeated using 0.1 M ethanoic acid instead.
(B) To compare the relative strength of alkalis by measuring pH values.
12. Steps 9 to 10 were repeated using 0.1 M sodium hydroxide solution instead.
13. Steps 9 to 10 were repeated using 0.1 M ammonia solution instead.Observation
Approach 1: To compare the relative strength of acids and of alkalis by measuring
electrical conductivities
1. (Reference to Steps 1 to 6): Equal concentration of hydrochloric acid (0.1 M) and ethanoic
acid
(0.1 M) show different electrical conductivities. Hydrochloric acid shows a higher electrical
conductivity than that of ethanoic acid.
2. (Reference to Steps 7 and 8): Equal concentration of sodium hydroxide solution (0.1 M) and
ammonia solution (0.1 M) show different electrical conductivities. Sodium hydroxide
solution shows a higher electrical conductivity than that of ammonia solution.
Approach 2: To compare the relative strength of acids and of alkalis by measuring pH
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values
1. (Reference to Steps 9 to 11): Equal concentration of hydrochloric acid (0.1 M) and ethanoic
acid (0.1 M) show different pH values. Hydrochloric acid shows a lower pH value than that
of ethanoic acid.
2. (Reference to Steps 12 and 13): Equal concentration of sodium hydroxide solution (0.1 M)
and ammonia solution (0.1 M) show different pH values. Sodium hydroxide solution shows a
higher pH value than that of ammonia solution.
InterpretationApproach 1: To compare the relative strength of acids and of alkalis by measuring
electrical conductivities
1. Equal concentration of hydrochloric acid and ethanoic acid means that they have the same
concentration of solute in the solution. However, hydrochloric acid is a strong acid, which
will ionize completely in aqueous solution. Ethanoic acid is a weak acid which will only
slightly/partially/incompletely ionize in aqueous solution. As a result, hydrochloric acid will
have a higher ionic concentration and conduct electricity better.
HCl(aq) H+(aq) + Cl−(aq) (complete ionization)
CH3COOH(aq) CH3COO2(aq) + H+(aq) (slight ionization)
2. Equal concentration of sodium hydroxide solution and ammonia solution means that they
have the same concentration of solute in the solution. However, sodium hydroxide solution is
a strong alkali which will dissociate completely in aqueous solution. Ammonia solution is a
weak alkali which will only slightly/partially/incompletely ionize in aqueous solution. As a
result, sodium hydroxide solution will have a higher ionic concentration and conduct
electricity better.
NaOH(s) + water Na+(aq) + OH−(aq) (complete dissociation)
NH3(aq) + H2O(l) NH4+(aq) + OH−(aq) (slight ionization)
Approach 2: To compare the relative strength of acids and of alkalis by measuring pH
values
3. Equal concentration of hydrochloric acid and ethanoic acid means that they have the same
concentration of solute in the solution. However, hydrochloric acid is a strong acid, which
will ionize, completely in aqueous solution. Ethanoic acid is a weak acid which will only
slightly/partially/ incompletely ionize in aqueous solution. As a result, hydrochloric acid will
have a higher concentration of hydrogen ions, and thus a lower pH value.
HCl(aq) H−(aq) + Cl−(aq) (complete ionization)
CH3COOH(aq) CH3COO−(aq) + H+(aq) (slight ionization)
4. Equal concentration of sodium hydroxide solution and ammonia solution means that they
have the same concentration of solute in the solution. However, sodium hydroxide solution is
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a strong alkali which will dissociate completely in aqueous solution. Ammonia solution is a
weak alkali which will only slightly/partially/incompletely ionize in aqueous solution. As a
result, sodium hydroxide solution will have a higher concentration of hydroxide ions, and
thus a higher pH value.
NaOH(s) + water Na+(aq) + OH−(aq) (complete dissociation)
NH3(aq) + H2O(l) NH4+(aq) + OH−(aq) (slight ionization)
Discussion1. For a fair comparison, the strong acid and weak acid used should be of equal concentration.
The difference in electrical conductivities is thus a result of the difference in the degree of
ionization of the two acids in aqueous solution.
2. The digital multimeter is an ideal instrument for measuring electric current here. The readings
are quite accurate and can be easily read on the LCD screen.
3. Always remember to connect the negative pole of the power supply to the negative (black)
terminal of the multimeter.
4. Electrolysis of solution will take place, liberating gases at electrodes. The gas bubbles on the
surface of electrodes increase the resistance of the circuit, causing the current reading to drop
with time.
Conclusion1. Hydrochloric acid is a stronger acid than ethanoic acid of the same concentrations?
2. Sodium hydroxide solution is a stronger alkali than ammonia solution of the same
concentrations?
Answers to questions for further thought1. Citric acid, ethanoic acid, ascorbic acid, etc.
2. No, it is because for a fair comparison, all factors such as temperature and concentration of
solute in aqueous solution should be the same, except that different acids or alkalis are used.
So when we are measuring the electrical conductivities, the difference is a direct reflection of
the degree of ionization (or dissociation) of the acids or alkalis. Thus, the strength of acids
and of alkalis can be compared.
3. (a) water, ammonia (the most plentiful), ammonium ions, hydroxide ions, hydrogen ions
(b) water, sodium ions, hydroxide ions (the most plentiful), hydrogen ions
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Experiment 19.1 To investigate the temperature change associated
with a neutralization reaction
4. Given out.
Exothermic
7. Roughly the same.
H+(aq); OH−(aq); Twice; given out; heat up; rises; roughly the same
11. The temperature rise is about half of that in Part B.
12. Exothermic.
13. Yes.
14. very little heat; bad; heat; heat; conduction; heat losses; bad; heat
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Experiment 19.2 To prepare sodium sulphate crystals from an
acid-alkali titration
5. Yellow.
9. To obtain the salt free from any indicator.
10. b. If all the water were driven away, the sodium sulphate obtained would be a powder, not
crystals.
12. Colourless crystals.
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Experiment 21.1 To prepare standard ethanedioic acid solutions
4. 9.51
(12.0 + 16.0 × 2 + 1.0) × 2 + 2 × (1.0 × 2 + 16.0)
126.0
250.0
9.51/126.0
250.0/1000
0.302
11. 0.302
25.0
250.0
Yes.
This is because its concentration (molarity) is accurately known.
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0.302 0.0302
Experiment 21.2 To find the molarity of a given hydrochloric acid
5. Yellow.
6. b. The end point has been reached.
9. 0.102
26.1
10. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
0.102
25.0
1000
1:1
26.1; 2.55 × 10−3
2.55 × 10−3
26.1/1000
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0.0977
0.102 2.55 × 10−3
Experiment 21.3 To determine the concentration of ethanoic
acid in commercial vinegar
2. c. Colourless.
6. 0.102
25.2
7. CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)
25.2; 0.102
25.2
1000
1:1
2.57 × 10−3
a. 2.57 × 10−3
25.0/1000
b. 250.0
25.0
©Aristo Educational Press Ltd. 2003 - 29 -
2.57 × 10−30.102
0.103
1.030.103
Experiment 22.1 To investigate the effect of concentration on
the rate of reaction
8. The curve is steep at first as the reaction rate is the fastest, but becomes less steep with time.
Finally, it becomes horizontal, indicating the finish of the reaction.
Quick and accurate results can be obtained by using data-logger. Besides, continuous
monitoring is possible.
However, using data-logger is more complicated than traditional methods and it is less easy
to be handled.
9. increase; increase
©Aristo Educational Press Ltd. 2003 - 30 -
Experiment 22.2 sample laboratory report
Title: To investigate the effect of surface area on rate of reaction
Purpose
To investigate the effect of surface area on rate of reaction by using data-logger.
Apparatus and chemicals used• Suction flask (250 cm3) • Dry lumps of calcium carbonate, 0.5 g
• Measuring cylinder (25 cm3) • Dry powdered calcium carbonate, 0.5 g
• Plastic sample bottle, large enough to hold • Dilute HCl(aq) (1 M)
about 25 cm3 of 1 M hydrochloric acid
• Stand, boss and clamp
• Data-logger connected to a computer with
pre-installed software, absolute pressure sensor
• Rubber tubing (8 inches) for connecting the
suction flask and the data logger
• Electronic balance (accurate to 0.01 g)
• Vacuum sealant and adhesive
Chemical reactions involvedCaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
Procedure1. 0.5 g of lumps of calcium carbonate was weighed out and put inside a dry suction flask.
(Figure 1)
©Aristo Educational Press Ltd. 2003 - 31 -
2. 25 cm3 of 1 M hydrochloric acid was measured by a measuring cylinder and transferred into a
plastic sample bottle (Figure 2).
3. The plastic sample bottle with hydrochloric acid was put into the suction flask carefully.
4. The suction flask was stoppered carefully and sealed to avoid air leakage. A piece of rubber
tubing was used to connect the data-logger and the suction flask for pressure measurement.
5. The data logging software was started and set properly for data collection.
6. The suction flask was inverted to mix the reactants thoroughly and the data collection process
was started immediately. See Figures 3 and 4.
pressure sensor data-loggerto computer
Figure 4
©Aristo Educational Press Ltd. 2003 - 32 -
7. When the reaction finished, the data collection process was stopped.
8. The results (data and graph) were saved and printed out to hand in together with the report.
9. Steps 1 − 8 were repeated, but 0.5 g of powdered calcium carbonate was used instead.
Observation1. (Reference to Step 6): Effervescence occurred in the reaction mixture.
2. (Reference to Step 6): The pressure inside the suction flask increased steadily and very
slowly with lumps of calcium carbonate added (more than 2.5 minutes) but very rapidly (less
than 0.5 minute) with powdered calcium carbonate added.
3. The results were shown below:
With lumps of calcium carbonate: (Specimen results)
©Aristo Educational Press Ltd. 2003 - 33 -
With powdered calcium carbonate: (Specimen results)
3
Pressure vs Time
Time (s)
Interpretation1. Effervescence occurred in the reaction mixtures, due to the formation of carbon dioxide gas.
CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
2. As more and more carbon dioxide was produced, the pressure inside the suction flask also
increased.
Discussion1. In case the calcium carbonate lumps are covered with powder, they should be washed
beforehand in dilute HCl and then in water to remove the powder. They should then be
blotted.
2. Since particle size of calcium carbonate solids would change as reaction proceeds, only the
initial rates of the two experiments should be compared.
3. Theoretically, the two graphs should become flat at the same value on the Y-axis, i.e. same
final pressure. But in practice, there may be some deviations as a result of experimental errors
©Aristo Educational Press Ltd. 2003 - 34 -
(e.g. masses used not the same). Actually, the final pressure cannot be reached as it takes
quite a long time for the calcium carbonate lumps to react completely.
ConclusionThe reaction rate increases with larger surface area of solid reactant.
Answers to questions for further thought1. Hydrochloric acid HCl. The hydrochloric acid should be in excess so that the concentration of
it does not change a lot.
2. Reaction rate was the greatest at the start and decreased gradually to zero. At the start, the
concentration of the reactants, were the highest (hence fastest reaction rate). As the reaction
proceeded, the concentrations of the reactants decreased (hence slower reaction rate), until
one of the reactants (calcium carbonate CaCO3 in this case) was used up. Reaction rate
became zero as the reaction stopped.
3. Chewing can break the food into small pieces, so that there can be a large increase in surface
area of the food for the digestive juices to work on, and thus facilitates digestion.
©Aristo Educational Press Ltd. 2003 - 35 -
Experiment 22.3 sample laboratory report
Title: Effect of temperature on the rate of reaction
PurposeTo investigate the effect of temperature on the rate of reaction.
Apparatus and chemicals used• Beakers (100 cm3) • Dilute HCl(aq) (2.0 M) (about 40 cm3)
• Measuring cylinder (50 cm3) • Na2S2O3 solution (~0.05 M) (about 80 cm3)
• Measuring cylinder (10 cm3) • Deionized water
• Stopwatch
• White tile
• Black/blue 'Wytebord' markers
• Bunsen burner
• Tripod and gauze
• Heat-resistant mat
• Thermometer (−0 110 °C, stirring rod type)
• Tissue paper or blotting paper
(to wipe beakers dry)
Chemical reaction involvedS2O32−(aq) + 2H+(aq) SO2(g) + S(s) + H2O(l)
Procedure1. A thick cross with the size just smaller than the base of a small beaker was marked with a
black/blue 'Wytebord' marker on a white tile placed on the bench.
2. (a) 5 cm3 of sodium thiosulphate solution was mixed with 45 cm3 of water in a 50 cm3
measuring cylinder.
(b) The mixture was added to the small beaker placed on the marked white tile.
(c) 5.0 cm3 of hydrochloric acid was poured into the beaker quickly. The stopwatch was
started at the same time (Figure 1a). The mixture was stirred gently with the
thermometer and the temperature of the reaction mixture was taken (Figure 1b).
©Aristo Educational Press Ltd. 2003 - 36 -
(d) The cross was observed by looking vertically down through the solution. The time
taken for the cross to be just 'blotted out' completely was recorded (Figure 1c).
3. Another 5 cm3 of thiosulphate solution was mixed with 45 cm3 of water in a 50 cm3
measuring cylinder. The mixture was added to another clean small beaker. The beaker was
heated until the temperature of the reaction mixture was just above 35°C (Figure 2a).
(a) The beaker was placed on the marked white tile. 5.0 cm3 of hydrochloric acid was
poured into the beaker quickly. The stopwatch was started at the same time. The
mixture was stirred gently with the thermometer and the temperature of the mixture was
taken once it became
©Aristo Educational Press Ltd. 2003 - 37 -
(b) The time taken for the cross to be just 'blotted out' completely was recorded.
4. Step 3 was repeated at about 45°C, 55°C, 60°C, 65°C and 70°C.
5. The results obtained in the experiment were tabulated and a graph of time
1 against
temperature was
plotted.
Observation1. At the start of the experiment, the cross was seen clearly. As reaction proceeded, sulphur
concentration continued to rise. The cross became fainter but could still be seen. After a
certain time, sulphur concentration became just sufficient to 'blot out' the cross completely.
2. The results were tabulated as below:
Temperature of reactionmixture (°C)
Time for cross to be just'blotted out' (s)
22 154.035 63.545 41.355 27.460 23.065 18.069 14.9
(Specimen results)
3. Time
1 was worked out in each case:
Temperature of reactionmixture (°C)
Time for cross to be just'blotted out' (s)
1/Time (s−1)
22 154.0 6.5 × 10−3
35 63.5 15.7 × 10−3
45 41.3 24.2 × 10−3
55 27.4 36.5 × 10−3
60 23.0 43.5 × 10−3
65 18.0 55.6 × 10−3
69 14.9 67.1 × 10−3
©Aristo Educational Press Ltd. 2003 - 38 -
A graph of Time
1 against temperature was plotted below:
0
10
20
30
40
50
60
70
10 20 30 40 50 60 70
Interpretation1. Sodium thiosulphate solution and hydrochloric acid reacted according to the equation:
S2O32−(aq) + 2H+(aq) SO2(g) + S(s) + H2O(l)
As sulphur formed, the mixture became cloudy white, and then cloudy yellow. The degree of
©Aristo Educational Press Ltd. 2003 - 39 -
cloudiness of the reaction mixture at any moment indicated the extent of the reaction. In this
experiment, the beaker containing the reaction mixture was placed over a cross marked on a
piece of white tile. The cross was observed directly from above. The time (t) taken for the
cross just to be 'blotted out' was recorded. This corresponded to the time needed for the
formation of a certain definite amount of sulphur.
2. As temperature of the reactants increased, the time to 'blot out' the cross became shorter.
Since reaction rate is inversely proportional to the time taken to 'blot out' the cross, a shorter
time means a faster reaction. From the graph, it could be seen that reaction rate increased
with an increase in temperature.
Discussion1. For a more accurate comparison, the same cross mark must be used throughout the
experiment. So it is necessary to keep the cross dry and clean. The same person is to observe
the cross at the same distance from above the cross. He should also be the timer.
2. Small amount of SO2 with choking smell is produced during the experiment. Although the
gas is poisonous, it does not matter much at such low concentration in a well-ventilated
laboratory.
3. The beaker inside should be cleaned and dried, otherwise, it will dilute the solution in the
beaker.
4. When heating Na2S2O3 solution in the beaker, the thermometer may be used as a stirring rod
(provided that care is taken). It is advisable to place the thermometer in the warm
thiosulphate solution before pouring the acid in. The temperature will only drop by 1 to
several degrees (depending on the starting temperature of the thiosulphate solution), and thus
the thermometer can quickly adjust to register the temperature after mixing. This practice is
particularly advantageous for experiments carried out at high temperatures (e.g. 70°C) the
reaction rate is so fast that we cannot wait too long for the mercury thread of thermometer to
go up from room temperature to the final high temperature.
ConclusionReaction rate increases with increasing temperature of the reaction mixture.
Answers to questions for further thought1. No. If the reaction rate is directly proportional to temperature, the shape of the graph should
be a straight line, which is not the case in this experiment. From the graph, it seems that
reaction rate rises exponentially with temperature. It is a rule of thumb that for every 10°C
rise in temperature, reaction rate will be doubled.
2. To investigate the effect of a certain factor (temperature in this case) on reaction rate, all other
factors should be made the same at the start.
©Aristo Educational Press Ltd. 2003 - 40 -
3. Fresh food can be kept fresh for a longer time in a refrigerator. Low temperatures would slow
down reactions that deteriorate the food.
©Aristo Educational Press Ltd. 2003 - 41 -
Experiment 23.1 To compare the service life of a zinc-carbon
cell with an alkaline manganese cell of the
same size (T)
2. c
3.
©Aristo Educational Press Ltd. 2003 - 42 -
4. (a) 13.5
52.5
(b) Alkaline manganese cell. About 4 times.
(c) Alkaline manganese cell.
©Aristo Educational Press Ltd. 2003 - 43 -
Experiment 24.1 To make simple chemical cells and construct
part of the Electrochemical Series
5.
Metal couple in cell Voltage of cell (volt)
Mg/Cu +1.41
Fe/Cu +0.20
Zn/Cu +0.79
Cu/Cu 0
Ag/Cu −0.22
8.
Metal couple in cell Voltage of cell (volt)
Mg/Cu +1.41
Fe/Cu +0.20
Zn/Cu +0.83
Cu/Cu 0
Ag/Cu −0.23
11.
Metal couple in cell Voltage of cell (volt)
Mg/Cu +1.84
Fe/Cu +0.49
Zn/Cu +0.92
Cu/Cu 0
Ag/Cu −0.17
©Aristo Educational Press Ltd. 2003 - 44 -
12. a. Mg/Cu cell, Fe/Cu cell, Zn/Cu cell.
b. From the other metal to copper.
c. The other metal.
d. Mg, Zn, Fe, Cu.
13. a. Ag/Cu cell.
b. From copper to silver.
c. Copper.
14. Mg, Zn, Fe, Cu, Ag.
15. a. Yes.
b. Metals react by losing electrons. The higher the tendency of a metal to form ions (the
higher its position in the E.C.S.), the more reactive it would be (the higher its position
in the metal reactivity series).
16. electrodes; electrolyte; positive ions; greater; voltage
©Aristo Educational Press Ltd. 2003 - 45 -
Experiment 26.1 To construct a simple chemical cell using
inert electrodes
2. b. 0.98
3. b. Yes.
Yellow.
Iodine.
c. 2I−(aq) → I2(aq) + 2e−
Oxidation.
4. a. MnO4−(aq) + 8H+(aq) + 5e− → Mn2+(aq) + 4H2O(l)
Reduction.
b. 2MnO4−(aq) + 16H+(aq) + 10I−(aq) → 2Mn2+(aq) + 5I2(aq) + 8H2O(l)
c. Y.
d. From X to Y.
X.
Y.
Cathode.
5. Zero volt.
To complete the circuit by allowing ions to move between the two half-cells.
©Aristo Educational Press Ltd. 2003 - 46 -
Experiment 26.2 To examine and compare the internal structures
of a well-used and a new zinc-carbon cell
1. a. The zinc cup of the used cell is thinner than that of the new cell. This shows that some
zinc metal has dissolved in the used cell.
b. A strong smell of ammonia is detected in the used cell, but not in the new cell. This
shows that some ammonia has formed in the used cell.
c. In the used cell, a large area around the graphite rod looks wet. In the new cell, the
ammonium chloride paste looks only very slightly wet. This shows that some liquid
(probably water) has formed in the used cell.
2. a. Zn(s) → Zn2+(aq) + 2e−
b. 2NH4+(aq) + 2e− → 2NH3(g) + H2(g)
2MnO2(s) + H2(g) → Mn2O3(s) + H2O(l)
3. (1) The metal casing makes the cell leakproof, preventing the electrolyte from leaking
out through the worn-out zinc cup. (Any electrolyte leaking out would damage the electrical
appliance in which the cell is put.)
(2) The cell with a metal casing has a longer life.
©Aristo Educational Press Ltd. 2003 - 47 -
Experiment 27.1 Electrolysis of dilute sulphuric acid (T)
3. b. Small colourless gas bubbles are evolved continuously.
Very small colourless gas bubbles are evolved continuously.
4. c. 2 : 1 (approx.)
6. The glowing splint is relighted.
Oxygen.
4OH−(aq) → O2(g) + 2H2O(l) + 4e−
7. A ‘pop’ sound is heard.
Hydrogen.
2H+(aq) + 2e− → H2(g)
8. 2H2O(l) → 2H2(g) + O2(g)
Water.
No. Distilled water is a very poor conductor of electricity. Although sulphuric acid itself is not
electrolysed, it can increase the electrical conductivity of water.
9. hydrogen H+(aq); hydrogen gas; hydroxide OH−(aq); oxygen gas; decrease;
remains unchanged; increases
©Aristo Educational Press Ltd. 2003 - 48 -
Experiment 27.2 Effect of concentration on preferential
discharge of ions
4. At cathode Colourless gas bubbles are evolved. The solution around the cathode turns purple
(in a few seconds).
At anode Colourless gas bubbles are evolved. The solution around the anode turns red (in a
few seconds).
5. c. A 'pop' sound is heard.
The gas is hydrogen.
8. At cathode Colourless gas bubbles are evolved. The solution around the cathode turns purple
(in a few seconds). The solution becomes bleached after some time.
At anode Colourless gas bubbles are evolved. The solution around the anode turns red (in a
few seconds). The solution becomes bleached within a short time (less than half a
minute).
9. A 'pop' sound is heard.
Hydrogen.
10. Bleaching solution or 'swimming pool'.
Chlorine.
©Aristo Educational Press Ltd. 2003 - 49 -
11.
0.1 M NaCl 2 M NaCl
C
A
T
H
O
D
E
Cations present Na+(aq), H+(aq) Na+(aq), H+(aq)
Is the solution around
the electrode acidic,
neutral or alkaline?
alkaline alkaline
Main product hydrogen hydrogen
Ionic half-equation 2H+(aq) + 2e− → H2(g) 2H+(aq) + 2e− → H2(g)
A
N
O
D
E
Anions present Cl−(aq), OH−(aq) Cl−(aq), OH−(aq)
Is the solution around
the electrode acidic,
neutral or alkaline?
acidic acidic
Main product oxygen chlorine
Ionic half-equation 4OH−(aq) → O2(g) + 2H2O(l) + 4e− 2Cl−(aq) → Cl2(g) + 2e−
12. hydrogen H+(aq); hydrogen gas; hydroxide OH−(aq); oxygen gas; chloride Cl−(aq) ;
concentration effect; chlorine gas
©Aristo Educational Press Ltd. 2003 - 50 -
15.
2 M NaBr 2 M NaI
C
A
T
H
O
D
E
Cations present Na+(aq), H+(aq) Na+(aq), H+(aq)
Is the solution around
the electrode acidic,
neutral or alkaline?
alkaline alkaline
Main product hydrogen hydrogen
Ionic half-equation 2H+(aq) + 2e− → H2(g) 2H+(aq) + 2e− → H2(g)
A
N
O
D
E
Anions present Br−(aq), OH−(aq) I−(aq), OH−(aq)
Colour of solution
around the electrode
brown colour
(bleached after some time)
deep brown colour
Main product bromine iodine
Ionic half-equation 2Br−(aq) → Br2(aq) + 2e− 2I−(aq) → I2(aq) + 2e−
16. hydrogen
hydroxide OH−(aq); reducing; halide; oxygen gas; halide; halide ions; hydroxide OH−(aq); halogen
©Aristo Educational Press Ltd. 2003 - 51 -
Experiment 27.3 Effect of electrodes on products of electrolysis
3. A reddish brown solid is deposited.
Cu2+(aq) + 2e− → Cu(s)
Colourless gas bubbles are evolved.
4OH−(aq) → O2(g) + 2H2O(l) + 4e−
4. The cathode is now electrode Y (graphite); the anode is electrode X (copper coated on
graphite).
5. Reddish brown copper is deposited.
Cu2+(aq) + 2e− → Cu(s)
The reddish brown solid (copper) dissolves gradually. (Some copper may fall to the bottom of
the beaker.) When there is little or no copper on the anode, colourless gas bubbles are
liberated there.
At first, Cu(s) → Cu2+(aq) + 2e−;
when there is little or no copper left,
4OH−(aq) → O2(g) + 2H2O(l) + 4e−.
6. copper; oxygen; copper dissolves
©Aristo Educational Press Ltd. 2003 - 52 -
Experiment 27.4 sample laboratory report
Title: Electroplating
PurposeTo design and perform an experiment to electroplate a metal object with nickel.
Apparatus and chemicals used• 6 V d.c. supply • Propanone (2 cm3), kept inside the fume
• 3 connecting wires with crocodile cupboard
clips at both ends • Nickel foil (5 cm × 2 cm)
• 6 V bulb in holder • Copper foil (5 cm × 2 cm) /brass key/
• Sand paper (5 cm × 5 cm), 2 pieces 50¢ coin
• Beaker (100 cm3) • Nickel plating solution (an aqueous
• Crucible tongs nickel(II) salt solution), 60 cm3
• Electrode foil holder • Tissue paper/ cotton wool
• Forceps (placed beside propanone,
kept in the fume cupboard)
Chemical reactions involvedAt the nickel anode: Ni(s) Ni2+(aq) + 2e−
At the cathode (object to be electroplated): Ni2+(aq) + 2e− Ni(s)
Procedure1. (a) A copper foil was cleaned with sandpaper (Figure 1a).
©Aristo Educational Press Ltd. 2003 - 53 -
(b) The foil was washed in running tap water (Figure 1b).
(c) The foil was degreased (grease was removed) by rubbing the foil with a piece of cotton
wool (soaked in propanone) held by forceps (Figure 1c).
(d) The foil was washed again under tap water (Figure 1d).
(e) The copper foil was put on a piece of tissue paper (Figure 1e) to keep the foil clean.
2. (a) 60 cm3 of the nickel plating solution was poured into a beaker.
(b) The circuit was connected as shown in Figure 2. The copper was made the cathode
(connected to the negative terminal of the battery); nickel was made the anode
(connected to the positive terminal).
(c) A current was allowed to flow for 10 minutes. When half the time had passed, the
cathode was taken out. The sides were reversed and it was put back into the plating
©Aristo Educational Press Ltd. 2003 - 54 -
solution. (This was to ensure even plating.)
3. (a) The cathode was removed (the copper foil was newly coated with nickel).
(b) It was washed in running tap water.
(c) It was allowed to dry on a tissue paper.
(d) The appearance of the electroplated copper was noted.
Observation1. (Reference to Step 1): The copper foil looked shiny and clean.
2. (Reference to Step 2): The electroplated copper became silvery grey in appearance.
3. (Reference to Step 3): A silvery grey coating was electroplated on the copper foil.
Interpretation1. Propanone is a very good solvent to dissolve grease, thus the copper metal looks shiny.
2. At the cathode, Ni 2+(aq) ions are discharged to form Ni(s) on the copper surface.
Ni2+(aq) + 2e− Ni(s)
Discussion1. Alternatively, the copper foil can be degreased by dipping it in concentrated detergent
solution for a minute, or rubbing with cotton wool soaked in dilute ammonia solution. Warm
dilute sodium hydroxide can also remove grease. However, the solution is corrosive.
2. Stirring of the plating solution during electroplating may result in a good coating. This can be
done by using the magnetic stirrer.
3. The light bulb indicated that an electric current was flowing and limited this to a small
current density suitable for electroplating. If the bulb is not used, a variable resistor and an
ammeter can be used to obtain good results.
4. The metal foils can be conveniently clamped by an electrode foil holder which consists of a
plastic strip fitted with two crocodile clips. The advantages are:
(a) The metal foils will not touch each other and the circuit will not be shorted.
(b) The foils can be kept at a fixed distance apart.
(c) The crocodile clips will not be wetted by the solution so easily. So that the clips will not
rust so easily.
ConclusionWhen an object is to be electroplated, it is made the cathode of an electrolytic cell. The plating
metal is usually made the anode. The plating solution (electrolyte) is a solution of one of the salts
©Aristo Educational Press Ltd. 2003 - 55 -
of the plating metal. Simple electroplating experiment can be done in the school laboratory
conveniently.
Answers to questions for further thought
1. (i) The object to be plated should be clean and free from grease.
(ii) Keep the electric current small.
(iii) The electrolyte should be maintained at a certain constant acidity or alkalinity.
2. Copper, silver, gold, tin or chromium. They are low in the Electrochemical Series.
3. These objects are first sprayed with a layer of powdered graphite or metal and then
electroplated in the usual way.
4. The nickel anode ionizes to form Ni2+(aq) ions, replacing those removed from the solution at
the cathode. Thus the concentration of the electrolyte can be kept constant at a desired level.
©Aristo Educational Press Ltd. 2003 - 56 -
Experiment 28.1 To investigate the oxidizing property of chlorine
water
1. d.
Halide (a) Colour of solution after
addition of chlorine water
(b) Colour of heptane
layer
Bromide yellow red-orange
Iodide brown purple
4. oxidizing; bromine; yellow; iodine; brown; heptane; red-orange; purple
5. Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq);
Cl2(aq) + 2I−(aq) → 2Cl−(aq) + I2(aq)
Redox (or displacement).
6. oxidizing
©Aristo Educational Press Ltd. 2003 - 57 -
Experiment 28.2 sample laboratory report
Title: To make chlorine bleach
PurposeTo make chlorine bleach in the school laboratory.
Apparatus and chemicals used• Electrolysis cell • Stand, boss and clamp
• Rubber bands • Dropper
• Rimless test tubes • Boiling tube (150 × 24 mm) in rack
• Stopper to fit small test tube • Stopper to fit 24 mm diameter test tube
• Wooden piece (longer than diameter • Red litmus paper
of wide glass tube) • Blue litmus paper
• Connecting wires fitted with crocodile clips • Brine (saturated NaCl solution)
• 6 V battery
Chemical reactions involvedAt the cathode: 2H+(aq) + 2e− H2(g)
At the anode: 2Cl− (aq) Cl2(g) + 2e−
Cl2(g) + 2NaOH(aq) NaOCl(aq) + NaCl(aq) + H2O(l)
sodium hypochlorite
ProcedureIn this experiment, brine (saturated solution of sodium chloride) was electrolysed. The chlorine gas
collected from the anode was allowed to react with the resulting alkaline solution.
1. Brine was added to an electrolysis cell, until the liquid level was slightly above the
electrodes. See Figure 1.
©Aristo Educational Press Ltd. 2003 - 58 -
2. (a) A dropper
was used
to fill two
small
rimless test
tubes with
brine
(Figure 2).
(b) The tubes
were
inverted
over the
electrodes
(Figure 3).
(Before the
tubes were
inverted,
an empty
beaker
should be
placed
under the
electrolysis
cell. This
would
prevent
possible
spillage of
liquid on
the bench.)
(c) The electrolysis cell was clamped.
(d) The electrodes were connected to a 6 V d.c. source.
3. Any observation around the
electrodes and in the small
test tubes A and B was
recorded.
4. (a) Electrolysis was allowed to continue. When the small
©Aristo Educational Press Ltd. 2003 - 59 -
tubeA
Figure 3
tube above the anode (tube B) was full of gas, the power supply was disconnected.
(b) The small tubes were taken out of the solution and tube B was quickly stoppered.
(c) The rubber bands were loosen to release tube B.
5. A piece of red litmus paper was used to test the resulting solution in the electrolysis cell. The
change in colour of the litmus paper was recorded.
6. (a) The resulting solution from the electrolysis cell was poured into a boiling tube to a
depth of
3 cm.
(b) Tube B was inverted. The stopper was quickly removed and tube B was dropped into
the boiling tube (Figure 4a).
(Caution! It must be very careful when tube B was dropped into the boiling tube. It is
afraid that the glass might be broken and the solution might be spilled out.)
(c) The boiling tube was stoppered tightly immediately (Figure 4b).
(d) The boiling tube was inverted gently and the contents were swirled (Figure 4c)
(e) The boiling tube was inverted back and it was swirled again. Steps (d) and (e) were
repeated for about 5 minutes.
7. A chlorine bleach had been prepared. It should contain sodium hypochlorite and show
bleaching action.
(a) A piece of blue litmus paper and a piece of red litmus paper were dropped into the
solution in the boiling tube.
(b) The changes in colours of the blue and the red litmus papers were recorded.
©Aristo Educational Press Ltd. 2003 - 60 -
Observation1. (Reference to Step 3): Colourless gas bubbles were evolved at cathode. The gas was collected
and water level in tube A dropped.
Greenish-yellow gas bubbles were evolved at anode. The gas was collected and water level in
tube B dropped.
2. (Reference to Step 5): The red litmus paper turned blue.
3. (Reference to Step 7): The blue litmus paper was bleached (from blue to white) immediately.
The red litmus paper was bleached (from red to white) immediately.
Interpretation1. Hydrogen was formed at the cathode and collected in tube A.
2H+(aq) + 2e− H2(g)
Chlorine was formed at the anode and collected in tube B.
2Cl−(aq) Cl2(g) + 2e−
2. The resulting solution was alkaline. It should contain sodium hydroxide.
3. The chlorine bleach formed bleached the dyes in the red and blue litmus paper.
Cl2(g) + 2NaOH(aq) NaOCl(aq) + NaCl(aq) + H2O(l)
OCl−(aq) + dye(aq) Cl2(aq) + (dye + O)
coloured colourlesss
Discussion1. The small tubes should not cover the electrodes completely, otherwise electrical resistance
would be increased and the rate of electrolysis would be reduced considerably.
2. The time needed to collect a full tube of hydrogen gas is shorter than that of chlorine.
This is because hydrogen is insoluble in water, while chlorine is fairly soluble.
3. The resulting solution is mainly a mixture of sodium hydroxide and sodium chloride, with a
little chlorine dissolved. It is alkaline. A longer electrolysis results in a higher alkalinity and
stronger corrosiveness of the resulting solution.
Conclusion1. Chlorine can be prepared by the electrolysis of brine.
2. Chlorine gas formed at the anode can react with the resulting sodium hydroxide solution in
the electrolysis cell. Chlorine bleach, containing sodium hypochlorite as the active ingredient,
is formed.
3. The chlorine bleach can bleach the dyes in the litmus paper.
Answers to questions for further thought1. The theoretical volume ratio of H2 to Cl2 should be 1:1. Smaller volume of chlorine gas is
collected because chlorine is fairly soluble in water.
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2. At room temperature, chlorine can be liquefied under pressure. It is stored and transported in
liquid state in steel cylinders.
3. (i) Chlorine bleach should be stored away from sunlight or excess heat. It is because
sunlight and excess heat can speed up the decomposition of chlorine bleach.
(ii) Chlorine bleach should be kept out of reach of children because it is toxic.
(iii) When chlorine bleach is used, the room must be well ventilated because a little chlorine
gas is given off, which is toxic.
(iv) When using chlorine bleach, wear plastic gloves because it is irritant to skin.
(v) Never mix chlorine bleach with acidic substances or other cleaners. It is because acidic
substances react with chlorine bleach to liberate the toxic chlorine gas.
OCl−(aq) + Cl− (aq) + 2H+(aq) Cl2(g) + H2O(l)
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Experiment 28.3 Action of chlorine bleach on coloured substances
1. Sodium hypochlorite (usually 5.25%).
3. A faint choking smell (like that of swimming pool).
4. b. It turns white (is bleached) immediately.
It turns white (is bleached) immediately.
5. i. The colour becomes paler.
ii. The stain is bleached.
iii. The stain is bleached.
iv. The stain is bleached.
6. sodium hypochlorite; oxidation
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Experiment 28.4 Action of acid on chlorine bleach
1. a. Colourless.
b. The solution changes from colourless to pale greenish yellow, a stream of small bubbles
being evolved.
2. The gas has a characteristic pungent, choking smell (like that of swimming pool).
Chlorine.
3. b. It becomes pink first (in about 5 seconds) and then white, i.e. bleached (in about 10
seconds).
Cl−(aq); OCl−(aq); 2; Cl2(g)
5. mineral acid; smell; blue litmus paper
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conc.
H2SO4
Experiment 29.1 To investigate properties of concentrated
sulphuric acid (S/T)
1. c. It turns from blue granular crystals to a pale blue powder.
CuSO4 5H‧ 2O(l) CuSO4(s) + 5H2O(l)
2. b. The sugar turns brown. A little white steam is given out.
d. The sugar chars (turns black). A lot of white steam is given out. A black spongy mass
rises up the beaker.
Sugar charcoal (carbon).
12C(s) + 11H2O(g)
3. d. All the words appear (black words on white paper).
On heating, water is driven away from dilute sulphuric acid. The dilute sulphuric acid
becomes concentrated. The concentrated sulphuric acid formed dehydrates the cellulose
in paper, leaving black carbon.
Dehydrating property.
4. c. No signs of reaction no matter the acid is cold or hot.
Yes. A dilute aqueous solution of a typical acid does not react with copper and those
metals below copper in the metal reactivity series.
5. a. No.
c. It changes from light orange to green.
Yes.
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e. Blue.
Cu2+(aq).
Cu(s) + 2H2SO4(l) → CuSO4(s) + SO2(g) + 2H2O(l)
No. A typical acid reacts with those metals which are above copper in the reactivity
series. In such cases, the gas liberated is hydrogen.
Redox reaction.
An oxidizing agent. Concentrated sulphuric acid is reduced, as the oxidation number of S
decreases from +6 in H2SO4(l) to +4 in SO2(g).
6. dehydrating; oxidizing
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Experiment 29.2 To dilute concentrated sulphuric acid (S/T)
7. lighter; heat; heating; boiling; steam; acid spray; heat; break
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Experiment 29.3 sample laboratory report
Title: To prepare sulphur dioxide and test for its properties
Purpose(A) To prepare sulphur dioxide gas in the school laboratory.
(B) To investigate the properties of sulphur dioxide gas.
Apparatus and chemicals used• Anti-bumping granules • 5 test tubes (4 fitted with rubber stoppers)
• Blue litmus paper/pH paper • Test tube rack
• Dilute sulphuric acid • Bunsen burner and matches
• Sodium sulphite solid • Heat-resistant mat
• Potassium dichromate solution • Stand, boss and clamp
• Boiling tube (fitted with a rubber stopper • Beaker (250 cm3)
carrying a bent delivery tube)
Chemical reactions involvedSO3
2−(aq) + 2H+(aq) SO2(g) + H2O(l)
SO2(aq) + H2O(l) H2SO3(aq)
sulphurous acid
3SO2(aq) + Cr2O72−(aq) + 2H+(aq) 3SO4
2−(aq) + 2Cr3+(aq) + H2O(l)
Procedure(A) To prepare sulphur dioxide gas in the school laboratory
1. (a) 3 spatula measures of sodium sulphite solid was put in a boiling tube.
(b) Dilute sulphuric acid was added to the boiling tube to a depth of 3 cm.
(c) A few anti-bumping granules were added to the boiling tube.
2. The boiling tube was clamped as shown in Figure 1.
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3. (a) The reaction mixture in the boiling tube was warmed gently. The flame was moved
about continuously to ensure uniform heating
(b) Record the observation when the reaction mixture was heated.
4. (a) The gas liberated was collected in a test tube by download delivery (upward
displacement of air). The test tube was stoppered once it was full of the gas. (As the gas
generated was misty, one could see when a tube was full of the gas.) The stoppered test
tube was put in a test tube rack.
(b) 3 more tubes of sulphur dioxide gas were collected. The tubes of gas would be used for
Part B of the experiment.
5. Heating was stopped. With a towel to protect the hand, the boiling tube was taken to the fume
cupboard, to be cleaned later after cooling.
(B) To investigate the properties of sulphur dioxide gas
6. Test for smell
(a) A tube of sulphur dioxide gas was taken. The stopper was lifted up slightly to leave a
small opening.
(b) The gas that escaped out from the tube was smelled carefully. This was done by
'fanning' a little of the gas towards the nose.
(c) The stopper was put in place immediately.
(d) The smell of sulphur dioxide gas was recorded.
7. Test for water solubility and acidic property
(a) A beaker was filled with water to half-full.
(b) A tube of sulphur dioxide was inverted and immersed under water in the beaker (Figure
2).
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(c) The stopper was removed under water.
(d) The water was stirred gently with the inverted tube for 4 minutes.
(e) A piece of blue litmus paper was dropped into the solution in the beaker.
(f) Any change in water level inside the tube and the colour change of the litmus paper
were recorded.
8. Test for bleaching property
(a) Inside the fume cupboard, the stopper of a tube of sulphur dioxide was removed. A
piece of moist blue litmus paper was quickly put into the tube.
(b) The stopper was immediately put in place again.
(c) The stoppered tube was taken back to the students' bench.
(d) The tube was placed in the test tube rack and allowed to stand for 10 minutes.
(e) Any colour change of the blue litmus paper was recorded.
9. Test for reducing property
(a) 1 cm3 of potassium dichromate solution and 1 cm3 of dilute sulphuric acid were put in a
test tube. (The resulting solution is called 'acidified potassium dichromate solution'.)
(b) Inside the fume cupboard, the acidified potassium dichromate solution was added
quickly to a tube of sulphur dioxide gas.
(c) The stopper of the tube was immediately replaced and the tube was shaken a few times.
(d) Any colour change of the acidified potassium dichromate solution was recorded.
Observation1. (Reference to Step 3): Effervescence occurred in the reaction mixture. A misty gas was
formed.
2. (Reference to Step 6): Sulphur dioxide gas had an irritating choking smell of burning sulphur.
3. (Reference to Step 7): Water level rose well up inside the test tube. The blue litmus paper
turned red.
4. (Reference to Step 8): The blue litmus paper turned red and then white (or very pale red).
5. (Reference to Step 9): The orange colour of acidified potassium dichromate solution changed
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to dark green.
Interpretation1. Effervescence occurred in the reaction mixture, due to sulphur dioxide gas formed.
SO32−(aq) + 2H+(aq) SO2(g) + H2O(l)
The sulphur dioxide gas liberated was misty because it contained moisture in it.
2. Water level rose well up inside the test tube. This shows that a lot of sulphur dioxide had
dissolved in water, hence the gas is very soluble in water.
The litmus paper turned red, showing that the solution was acidic. Sulphur dioxide
dissolves in water to form sulphurous acid:
SO2(aq) + H2O(l) H2SO3(aq)
sulphurous acid
3. The blue litmus paper turned red first. This shows that sulphur dioxide is an acidic gas. The
litmus paper then turned white (or very pale red), showing that sulphur dioxide is a bleaching
agent.
4. The orange colour of acidified potassium dichromate solution changed to dark green,
showing that sulphur dioxide gas is a reducing agent. The orange dichromate ions were
reduced to dark green chromium(III) ions:
3SO2 (aq) + Cr2O72−(aq) + 2H+(aq) SO4
2−(aq) + 2Cr3+(aq) + H2O(l)
orange dark green
Discussion1. Sulphur dioxide gas is much denser than air. Thus it can be collected by downward delivery,
as in this experiment. However, the gas collected would have air mixed with it.
2. Inevitably, some sulphur dioxide escaped into the laboratory in this experiment. It would be
better if the whole experiment had been carried out inside the fume cupboard.
3. The sulphur dioxide gas collected in this experiment (by downward delivery) was moist and
mixed with air. To prepare pure dry sulphur dioxide, the gas generated should be first passed
into concentrated sulphuric acid for drying; the dried gas can then be collected using a gas
syringe. Using a gas syringe can also minimize the sulphur dioxide gas escaped into the
laboratory.
Conclusion1. Sulphur dioxide can be prepared by warming a sulphite with a dilute acid.
2. Sulphur dioxide has an irritating choking smell of burning sulphur.
3. Sulphur dioxide is acidic.
4. Sulphur dioxide is a bleaching agent.
5. Sulphur dioxide is a reducing agent. It turns acidified potassium dichromate solution from
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orange to dark green.
Answers to questions for further thought1. Sulphur dioxide cannot be collected by displacement of water, as it is very soluble in water.
2. It is not advisable to test for a gas by its smell because the gas may be poisonous. A chemical
test would be preferred.
3. Sulphur dioxide emitted into the atmosphere is formed by the burning of sulphur-containing
fuels. Almost all of the gas comes from industrial sources ¾ electric power stations, factories
and incinerators. Sulphur dioxide dissolves in rainwater to form sulphurous acid, one of the
causes of acid rain:
SO2(aq) + H2O(l) H2SO3(aq)
sulphurous acid
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Experiment 29.4 To bleach coloured papers and flower petals with
sulphur dioxide (S/T)
1. A colourless gas (or a misty gas, due to presence of impurity).
3. d. The blue litmus paper becomes white (or very pale red).
The red litmus paper becomes white (or very pale red).
The blue (or red) flower petals become paler in colour.
4. Yes.
Reduction.
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Experiment M1 Electrolysis using a microscale Hoffman
apparatus
11. Small colourless bubbles are evolved continuously.
Very small colourless gas bubbles are evolved continuously
12. Test the gas evolved at the anode with a glowing splint, the glowing splint is relighted,
indicating that the gas is oxygen.
Test the gas evolved at the cathode with a burning splint, the burning splint gives a 'pop' sound,
indicating that the gas is hydrogen.
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