chemistryqb topic6c sq e
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ChemistryTRANSCRIPT
Structured questions1The diagram below shows a simplified set-up for the industrial preparation of chlorine.
a)i)Which electrode is the anode?
ii)Name a suitable material for making electrode A. Explain your choice.
b)Write ionic equations for reactions occurring at the mercury electrode and electrode Arespectively.
c)What will be observed if a piece of moist blue litmus paper is added to the chlorine gasobtained? Explain briefly.
d)Explain, with the help of an equation, how sodium hydroxide solution can be obtainedfrom the above experimental set-up.
e)i)Write a chemical equation for the reaction between chlorine and sodium hydroxidesolution.
ii)Suggest a major use of the product obtained in (i) in daily life.
f)Suggest ONE potential hazard in this experiment.(15 marks)Answer:1a)i)Electrode A
(1)
ii)Graphite (Do not accept platinum)
(1)
It has no reaction with the chlorine released / it is inert.
(1)
b)Mercury electrode: Na+(aq) + e-+ Hg(l) Na/Hg(l)
(1)
Electrode A: 2Cl-(aq) Cl2(g) + 2e-
(1)
c)The litmus paper turns red and then white (bleached).
(1)
Chlorine dissolves in water to give hydrochloric acid and hypochlorous acid
/ Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
(1)
The hydrogen ions turn the blue litmus paper red.
(1)
The hypochlorite ions bleach the litmus paper.
(1)
d)Sodium formed at the mercury electrode dissolves in the mercury (to form sodium
amalgam).
(1)
After the electrolysis, run the mercury layer into cold water. Sodium hydroxide solution
can be obtained.
(1)
2Na/Hg(l) + 2H2O(l) 2NaOH(aq) + H2(g) + 2Hg(l)
(1)
e)i)2NaOH(aq) + Cl2(g) NaCl(aq) + NaOCl(aq) + H2O(l)
(1)
ii)As a bleaching agent
(1)
f)Mercury vapour is toxic. / Chlorine gas is toxic.
(1)
2In industry, chlorine, hydrogen and sodium hydroxide solution can be obtained from theelectrolysis of brine.
a)Explain in terms of preferential discharge of ions, why chlorine is liberated at the anodein the electrolysis of brine.
b)Draw a labelled diagram to show the laboratory set-up for the electrolysis of brine and thecollection of the gaseous products.c)Suggest TWO problems of using the laboratory set-up shown in (b) for the industrialpreparation of chlorine.d)In a chemical plant manufacturing chlorine, chlorine is stored in metal cylinders.Draw a hazard warning label that should be displayed on the metal cylinders.e)Chlorine gas and sodium hydroxide solution react to produce bleach.
i)Write a chemical equation for the reaction between chlorine gas and sodiumhydroxide solution.
ii)The reaction is a redox reaction. State whether chlorine acts as an oxidizing agent, areducing agent, or both. Explain in terms of the change in oxidation number.(11 marks)Answer:2a)In the electrolysis of brine, chlorine ions and hydroxide ions are attracted to the anode.
(1)
Chloride ions are preferentially discharged (oxidized) because of the high concentration
of chloride ions in the brine.
(1)
b)
(3)
(1 mark for correct labels of electrodes; 1 mark for showing the collection of gaseous
products; 1 mark for correct labels of gaseous products; award 0 mark if the set-up is not
workable)
c)Chlorine and hydrogen gases may mix together. They may react to form hydrogen
chloride gas.
(1)
Chlorine may react with the sodium hydroxide solution directly in the container.(1)
d)
(1)
e)i)Cl2(g) + 2NaOH(aq) NaCl(aq) + NaOCl(aq) + H2O(l)
(1)
ii)The oxidation number of Cl in Cl2 is 0. The oxidation number of Cl in NaCl and
NaOCl is -1 and +1 respectively.
(1)
Therefore, chlorine is reduced and oxidized. It acts both as an oxidizing agent and
a reducing agent.
(1)
3Sodium hydroxide can be manufactured by the electrolysis of concentrated sodium chloride
solution (brine) using a diaphragm cell. The diaphragm allows sodium chloride solution topass through the compartments but does not allow gas X and hydrogen gas to pass through.
a)i)Name gas X.
ii)Give ONE industrial use of gas X.
b)Which electrode, A or B, is the cathode?
c)i)Write ionic equations for the formation of hydrogen and gas X respectively.
ii)Using the concept of preferential discharge of ions, explain the reaction at each
electrode and the formation of sodium hydroxide solution in the above electrolysis
process.
d)Suggest ONE advantage and ONE disadvantage for the production of sodium hydroxidesolution using a diaphragm cell compared with using a mercury electrolytic cell.
e)Domestic bleach can be made from the products obtained in the above method.
i)Write an ionic equation for the formation of domestic bleach.
ii)State the active ingredient in domestic bleach.(13 marks)Answer:3a)i)Chlorine
(1)
ii)Manufacture of bleach / manufacture of hydrochloric acid / manufacture of organic
solvents / sterilization of drinking water or water in swimming pools
(1)
b)Electrode A
(1)
c)i)Anode: 2Cl-(aq) Cl2(g) + 2e-
(1)
Cathode: 2H+(aq) + 2e- H2(g)
(1)
ii)Sodium ions and hydrogen ions move to the cathode. Chloride ions and hydroxide
ions move to the anode.
(1)
A hydrogen ion is a stronger oxidizing agent than a sodium ion. Therefore, hydrogen
ions are discharged preferentially at the cathode.
(1)
The concentration of chloride ion is higher than that of hydroxide ion. Therefore,
chloride ions are discharged preferentially at the anode.
(1)
Sodium ions and hydroxide ions are left as hydrogen ions and chloride ions are
discharged. Therefore, sodium hydroxide solution is left in the cell.
(1)
d)Advantage: mercury in a mercury electrolytic cell may cause mercury poisoning /
water pollution.
(1)
Disadvantage: the sodium hydroxide produced is not pure (mixed with sodium chloride).
(1)
e)i)Cl2(g) + 2OH-(aq) Cl-(aq) + OCl-(aq) + H2O(l)
(1)
ii)Hypochlorite ion / OCl-(aq)
(1)
4The following diagram shows a mercury electrolytic cell.
a)Identify the cathode of the cell.
b)i)Name products A and B respectively.
ii)With the aid of chemical equations, explain the formation of product B.
c)State TWO potential hazards associated with the electrolysis of brine using a mercury
electrolytic cell.
d)Explain why chlorine is used in water treatment plants.
e)Chlorine bleach can be prepared from products of the above process.
i)Write a chemical equation for the formation of chlorine bleach.
ii)What would be observed when the bleach is added to a solution of a coloured dye?
With the aid of an equation, explain the observation.
iii)Explain why small swimming pools usually use sodium hypochlorite solution rather
than chlorine gas for sterilization.
iv)The molarity of sodium hypochlorite in a sample of pool water is 0.15 mol dm-3.
Calculate the mass of sodium hypochlorite in 100 cm3 of pool water.
(Relative atomic masses: O = 16.0, Na = 23.0, Cl = 35.5)(17 marks)Answer:4a)Mercury electrode
(1)
b)i)A: hydrogen
(1)
B: sodium hydroxide solution
(1)
ii)During the electrolysis, sodium is produced at the mercury electrode.
(1)
Na+(aq) + e-+ Hg(l) Na/Hg(l)
(1)
sodium amalgam
The amalgam then flows into a second cell and reacts with water to form hydrogen
and sodium hydroxide solution.
(1)
2Na / Hg(l) + 2H2O(l) 2NaOH(aq) + H2(g) + 2Hg(l)
(1)
c)Any two of the following:
(1 ( 2)
Hydrogen gas is explosive.
Chlorine gas and mercury are highly toxic.
Sodium hydroxide solution is corrosive.
d)Chlorine can kill germs in water.
(1)
e)i)Cl2(g) + 2NaOH(aq) NaCl(aq) + NaOCl(aq) + H2O(l)
(1)
ii)The dye is decolourized.
(1)
The bleaching action is due to the oxidizing action of the hypochlorite ion. The
hypochlorite ion gives up the oxygen to the coloured dye, making the dye
colourless.
(1)
OCl-(aq) + dye Cl-(aq) + (O + dye)
(1)
colourless colouredcolourless colourless
iii)Sodium hypochlorite solution is easy to handle while chlorine is stored in heavy
steel cylinders that are difficult to transport.
(1)
iv)Number of moles of NaOCl in 100 cm3 of pool water
= 0.15 mol dm-3 ( (100 1 000) dm3
= 0.015 mol
(1)
Mass of NaOCl in 100 cm3 of pool water
= 0.015 mol ( (23.0 + 16.0 + 35.5) g mol-1
= 1.1 g
(1)
5During the laboratory preparation of chlorine and aqueous chlorine, dilute hydrochloric acid isadded to solution X.
a)i)What is solution X?
ii)Name apparatus A. What is the function of apparatus A?
b)Describe what you would observe in flask B.
c)Write an ionic equation for the reaction that occurs in flask B.
d)i)Name the method for collecting chlorine gas using gas jar C.
ii)Explain why this method is used.
iii)Suggest a test for the chlorine collected in gas jar C.
e)Suggest another method by which chlorine gas can be collected.
f)The chlorine gas collected above is not dry. With the help of the following apparatus,
draw a labelled diagram to show how the chlorine gas can be dried and collected.
g)To prepare aqueous chlorine, set-up D is connected to flask B instead of gas jar C.
i)Explain why an inverted funnel is used to dissolve chlorine in water.
ii)Write an equation for the reaction involved.(15 marks)Answer:5a)i)Chlorine bleach / bleaching solution
(1)
ii)Tap funnel
(1)
For controlled addition of acid / can close the tap to prevent gas escape.
(1)
b)Greenish-yellow gas is given off.
(1)
c)Cl-(aq) + OCl-(aq) + 2H+(aq) Cl2(g) + H2O(l)
(1)
d)i)Upward displacement of air / downward delivery
(1)
ii)Chlorine is denser than air.
(1)
iii)Put a piece of moist blue litmus paper at the opening of the gas jar.
(1)
The litmus paper turns red and then white.
(1)
e)Collected by a gas syringe / collected over brine
(1)
f)
(3)
(1 mark for showing the drying set-up and gas collection; 1 mark for suitable drying
agent; 1 mark for correct gas direction through the flask)
g)i)To prevent sucking back of water / increase the surface area for dissolving.
(1)
ii)Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
(1)
6a)Both chlorine gas and sodium hypochlorite solution can be used to sterilize pool water.
i)How can chlorine gas sterilize pool water?
ii)State ONE advantage of using sodium hypochlorite solution over chlorine gas for
sterilization.
b)When chlorine gas is dissolved in water, aqueous chlorine is obtained.
i)What is the colour of aqueous chlorine? Explain with an appropriate equation.
ii)What would be observed if a few drops of blue litmus solution are added to aqueous
chlorine? Explain with an appropriate equation.
c)Aqueous chlorine decomposes under sunlight. This can be illustrated by using the
following experimental set-up.
i)What would happen to the colour of the aqueous chlorine when it is exposed to
sunlight?
ii)Name gas A.
iii)Suggest a test for gas A.
iv)Write a chemical equation for the decomposition of aqueous chlorine.
d)In an experiment, aqueous chlorine is added to potassium iodide solution. Then 2 cm3 of
heptane is added to the mixture and shaken.
i)Describe what will be observed in the mixture.
ii)Name the type of the reaction involved.
iii)Write an ionic equation for the reaction.(17 marks)Answer:6a)i)To kill germs / bacteria / micro-organisms.
(1)
ii)Sodium hypochlorite solution can be handled more easily /
transportation of chlorine gas in heavy steel cylinders is difficult.
(1)
b)i)Greenish-yellow
(1)
Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
(1)
The reaction is reversible and thus greenish-yellow chlorine molecules are present
in aqueous chlorine.
(1)
ii)The solution turns red and then colourless.
(1)
The hydrogen ions turn the litmus solution red.
(1)
The litmus solution is then bleached by hypochlorite ion by oxidation.
(1)
OCl-(aq) + dye Cl-(aq) + (O + dye)
coloured colourless
(1)
c)i)The greenish-yellow colour of aqueous chlorine fades.
(1)
ii)Oxygen
(1)
iii)It relights a glowing splint.
(1)
iv)2HOCl(aq) O2(g) + 2HCl(aq)
(1)
d)i)The aqueous solution turns brown.
(1)
The heptane layer above the aqueous layer turns purple.
(1)
ii)Redox reaction / displacement reaction
(1)
iii)Cl2(aq) + 2I-(aq) 2Cl-(aq) + I2(aq)
(1)
7A student used the set-up shown below to conduct a microscale experiment to study some
reactions of chlorine.
The student added drops of the test solutions as shown above at the corners of the triangle.
Then he added two drops of bleach to the reaction vessel followed by three drops of dilute
hydrochloric acid. He immediately covered the petri dish with a lid.
a)Write an ionic equation for the reaction occurred in the reaction vessel.
b)Why did the student cover the petri dish with a lid quickly after the addition of dilute
hydrochloric acid to the bleach?
c)Why did the student leave a gap between the lid and the reaction vessel?
d)After a few minutes, what would be observed for the potassium bromide solution?
Explain your answer with the help of an equation.
e)What would be observed for the sodium chloride solution? Explain your answer.
f)i)What would be the observable change for the iron(II) sulphate solution?
ii)Write an ionic equation for the reaction.
iii)However, the student claimed that there seemed to be no observable change on the
drops of iron(II) sulphate solution. Suggest a reason for this.
g)After the experiment, the student added a few drops of sodium hydroxide solution to the
reaction vessel. Why did he do so?
h)In another experiment, the student wanted to study reactions of sulphur dioxide using the
above set-up.
i)What chemicals should he place in the reaction vessel?
ii)If the student wanted to study the reducing property of sulphur dioxide, state ONE
chemical that he should place at the corner of the triangle.
i)The use of microscale experiments in chemistry is becoming more popular nowadays.Suggest TWO advantages of carrying out experiments in microscale.(18 marks)Answer:7a)OCl-(aq) + Cl-(aq) + 2H+(aq) Cl2(g) + H2O(l)
(1)
b)To prevent the loss of chlorine gas.
(1)
c)To ensure that the chlorine gas can move freely within the petri dish so that chlorine can
react with the chemicals inside.
(1)
d)The colour changed from colourless to brown / orange.
(1)
It is because bromide ion was oxidized by chlorine to give brown / orange bromine.(1)
2Br-(aq) + Cl2(aq)2Cl-(aq) + Br2(aq)
(1)
e)No observable change
(1)
There is no reaction between chlorine and sodium chloride solution.
(1)
f)i)The colour of the iron(II) sulphate solution would change from green to yellow.(1)
ii)2Fe2+(aq) + Cl2(aq) 2Fe3+(aq) + 2Cl-(aq)
(1)
iii)It was because the concentration of iron(II) sulphate solution was not high enough
to give an obvious change.
(1)
g)To remove hydrogen ions (acid) in the reaction vessel
(1)
so as to stop the reaction.
(1)
h)i)Sodium sulphite and dilute hydrochloric acid
(1, 1)
ii)Acidified potassium dichromate solution / acidified potassium permanganate
solution / aqueous bromine
(1)
i)Any two of the following:
(1 ( 2)
Save chemicals / reduce the cost of chemicals used.
Reduce the chemical waste produced / disposal of chemicals after
experiment is easier.
Shorten the time for conducting the experiment.
Require less work space for carrying out an experiment.
8
In an experiment to determine the bleaching power of a bottle of bleach, a laboratory
technician poured 150 cm3 of the bleach to a conical flask and then added dilute hydrochloric
acid to the solution. Unexpectedly, a large amount of greenish-yellow gas was given off.
a)i)Name the gas given off.
ii)Write an equation for the formation of the gas.
b)Suggest what the technician should do immediately.
c)He inhaled some of this gas and felt discomfort with his respiratory tract.
Explain why this happened.
d)On the next day, he came back to the laboratory and found that the coloured papers near
to the beaker had turned white. Explain briefly with the help of equations.
e)His supervisor investigated this event and gave him several suggestions for improvement.
One was that the technician should put a warning statement on the bottle of bleach.
i)Write a warning statement that should be displayed on the bottle of bleach.
ii)State another suggestion that may be given by his supervisor.
f)Household bleach diluted by a volume ratio of 1:49 is commonly used as a domestic
disinfectant.
A 100.0cm3 sample of this brand of household bleach contains 5.00 g of sodiumhypochlorite. Calculate the concentration, in mol dm-3, of sodium hypochlorite in thediluted bleach.
(Relative atomic masses: O = 16.0, Na = 23.0, Cl = 35.5)(12 marks)Answer:8a)i)Chlorine
(1)
ii)OCl-(aq) + Cl-(aq) + 2H+(aq) Cl2(g) + H2O(l)
(1)
b)Leave the laboratory.
(1)
c)Chlorine is irritating.
(1)
d)Chlorine dissolved in moisture in the air to give hydrochloric acid and hypochlorous acid.
(1)
Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
(1)
Hypochlorite ions in the hypochlorous acid bleached the coloured papers.
(1)
OCl-(aq) + dye Cl-(aq) + (dye + O)
coloured
colourless
(1)
e)i)Keep away from acids.
(1)
ii)Dilute the bleach before performing experiment.
(1)
f)Molar mass of NaOCl = (23.0 + 16.0 + 35.5) g mol-1 = 74.5 g mol-1
Concentration of NaOCl in original bleach
= dm3
= 0.671 mol dm-3
(1)
Concentration of NaOCl in diluted bleach = 0.671 mol dm-3 50
= 0.0134 mol dm-3
(1)
9Bromine is below chlorine in the Periodic table. It is a liquid at room temperature andpressure. Its chemical properties are similar to those of chlorine. Bromine is commonlyextracted from sea water which contains a considerable amount of dissolved bromide. Theextraction involves the following four stages:
Stage 1:The sea water is made slightly acidic. Chlorine gas is then bubbled into the sea water
to liberate bromine.
Stage 2:Air is blown through the mixture obtained in Stage 1 to liberate the bromine vapour.
Stage 3:A mixture of sulphur dioxide and water, is allowed to react with the bromine vapour
to produce a hydrobromic acid solution.
Stage 4:Chlorine gas is added to the hydrobromic acid solution to displace bromine from thesolution. The bromine obtained is separated off and purified.
a)i)Write an ionic equation for the reaction between chlorine and bromide ions inStage 1.
ii)State the observable change in Stage 1.
iii)Suggest a reason why the reaction should be carried out under slightly acidiccondition.
b)State ONE physical property of bromine which helps bromine to be liberated from themixture in Stage 2.
c)i)Write a chemical equation for the formation of hydrobromic acid in Stage 3.
ii)State the type of reaction involved. Explain your answer.
d)Suggest ONE physical method that can be used to purify bromine in Stage 4.
e)Suppose that the concentration of bromide ions in sea water is 6.0 ( 10-4 mol dm-3.
Calculate the minimum volume of sea water needed to produce 4.0 g of liquid bromine.
(Relative atomic mass: Br = 80.0)(12 marks)Answer:9a)i)Cl2(g) + 2Br-(aq) 2Cl-(aq) + Br2(aq)
(1)
ii)The solution changes from colourless to orange.
(1)
iii)To minimize the extent of reaction between chlorine and water.
(1)
b)Volatile
(1)
c)i)SO2(g) + Br2(aq) + 2H2O(l) H2SO4(aq) + 2HBr(aq)
(1)
ii)Redox reaction
(1)
The oxidation number of S changes from +4 to +6
(1)
while the oxidation number of Br changes from 0 to -1.
(1)
d)Distillation / fractional distillation
(1)
e)Number of moles of bromine in 4.0 g = 4.0 g (2 ( 80.0) g mol-1
= 0.025 mol
(1)
1 mole of Br2 is produced from 2 moles of Br-.
Volume of sea water required = 2 ( 0.025 mol 6.0 ( 10-4 mol dm-3(1)
= 83 dm3
(1)
10 The following pie chart shows the uses of sulphuric acid in our daily lives.
a)Name the industrial process for the production of sulphuric acid.
b)i)One of the major uses of sulphuric acid is to make fertilizers. Explain why the
demand for fertilizers is so high.
ii)Name one common fertilizer which is made from sulphuric acid. Write a chemical
equation for the formation of the fertilizer.
c)White paint additives can be prepared by adding sulphuric acid to calcium nitrate solution.
i)Write an ionic equation for the reaction between sulphuric acid and calcium nitrate
solution.
ii)Name another white paint additive that can be prepared from sulphuric acid.
iii)Suggest ONE reason why paint additives containing lead are seldom used nowadays.
d)What is the function of sulphuric acid in cleaning metals for electroplating?
e)Suggest TWO reasons to explain why there is no industrial plant for making sulphuric
acid in Hong Kong.(11 marks)Answer:10a)Contact process
(1)
b)i)To increase agricultural yield
(1)
in order to meet the growth of population.
(1)
ii)Ammonium sulphate or any other suitable fertilizers
(1)
2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)
(1)
c)i)Ca2+(aq) + SO42-(aq) CaSO4(s)
(1)
ii)Barium sulphate
(1)
iii)Lead compounds are toxic.
(1)
d)Sulphuric acid is used to remove the metal oxides on metal surfaces
(metal oxide + acid salt + water).
(1)
e)Any two of the following:
(1 ( 2)
Hong Kong lacks the raw material (sulphur or sulphide ores) for the production of
sulphuric acid.
Hong Kong is an over-crowded city. Any leakage of raw materials or products may cause
serious pollution.
Land in Hong Kong is insufficient and expensive.
(Any other suitable answers)
11The following flow diagram shows the Contact process of the production of sulphuric acid:
a)Suggest a method for the production of sulphur dioxide. Write an equation for the
reaction.
b)Why should the sulphur dioxide and air be purified before passing into the catalyst
chamber?
c)i)What are the reaction conditions in the catalyst chamber?
ii)Assuming the conversion of sulphur dioxide to sulphur trioxide in the catalyst
chamber is 100%, calculate the volume of sulphur dioxide, measured at room
temperature and pressure, required for producing 16 kg of sulphur trioxide.
iii)Explain why the actual conversion of sulphur dioxide to sulphur trioxide is less than
100%.
d)Sulphur trioxide obtained is absorbed by 98% sulphuric acid and then followed by
dilution with water to produce sulphuric acid.
i)Give TWO advantages of this method over dissolving sulphur trioxide directly in
water to form sulphuric acid.
ii)Write equations for the absorption and dilution processes respectively.
e)The sulphuric acid obtained by the Contact process contains 98% by mass of sulphuric
acid and has a density of 1.84 g cm-3. Calculate the volume of acid required for dilution
to prepare 100.0 cm3 of 1.0 M sulphuric acid.
(Relative atomic masses: H = 1.0, O =16.0, S = 32.1; molar volume of any gas at room
temperature and pressure = 24.0 dm3 mol-1)(14 marks)Answer:11a)Burning of sulphur in air
(1)
S(s) + O2(g) SO2(g)
(1)
or, roasting iron pyrite
4FeS2(s) + 11O2(g) 2Fe2O3(s) + 8SO2(g)
or, roasting zinc blende
2ZnS(s) + 3O2(g) 2ZnO(s) + 2SO2(g)
b)To prevent the catalyst being poisoned by impurities in the reactants.
(1)
c)i)In the presence of V2O5(s)
(1)
450, 1 atm.
(1)
ii)2SO2(g) + O2(g) 2SO3(g)
Number of moles of SO3 = 16 000 g / (32.1 + 3 ( 16.0) g mol-1
= 200 mol
According to the equation, 1 mole of SO2 is required to produce 1 mole of SO3.
Hence, number of moles of SO2 required = 200 mol
(1)
Volume of SO2 required = 200 mol ( 24.0 dm3 mol-1 = 4 800 dm3
(1)
iii)The conversion of sulphur dioxide to sulphur trioxide is reversible.
(1)
d)i)Dissolving sulphur trioxide in water is highly exothermic and would lead to acid
mist formation. / Dissolving sulphur trioxide in 98% sulphuric acid would not lead
to the formation of acid mist.
(1)
Sulphur trioxide is more soluble in liquid sulphuric acid than it is in water /
concentration of sulphuric acid can be controlled.
(1)
ii)SO3(g) + H2SO4(l) H2S2O7(l)
(1)
H2S2O7(l) + H2O(l) 2H2SO4(l)
(1)
e)Number of moles of H2SO4 required = 1.0 mol dm-3 ( (100.0 1 000) dm3
= 0.10 mol
(1)
Molecular mass of H2SO4 = (2 ( 1.0 + 32.1 + 4 ( 16.0) g mol-1
= 98.1 g mol-1
Mass of H2SO4 required = 0.10 mol ( 98.1 g mol-1 = 9.8 g
Volume of 98% H2SO4 required= 9.8 g (1.84 ( 0.98) g cm-3
(1)
= 5.4 cm312The industrial process used to produce sulphuric acid can be represented by the following flowdiagram:
a)Name the industrial process used to produce sulphuric acid.b)Suppose the conversion of sulphur to sulphuric acid is 100%. If 52 kg of sulphuric acid isproduced, calculate the mass of sulphur consumed in the process.
c)i)Which of the above steps requires a catalyst?
ii)State a suitable catalyst for this step.
iii)What should be done on the reactants before this step proceeds? What is theimportance of this action?
d)i)Name one common fertilizer which is made from sulphuric acid.
ii)Why is it important for plant growth?
iii)Explain why the demand for fertilizers is increasing annually.
e)Sulphuric acid is used in the paint industry. Name one paint additive that is produced
from sulphuric acid.
f)Suggest TWO other uses of sulphuric acid.
(Relative atomic masses: H = 1.0, O = 16.0, S = 32.1)(14 marks)Answer:12a)Contact process
(1)
b)According to the flow diagram, 1 mole of sulphur produces 1 mole of sulphuric acid.
(1)
Molar mass of H2SO4 = (2 ( 1.0 + 32.1 + 4 ( 16.0) g mol-1 = 98.1 g mol-1
Sulphur consumed= 52 kg (
(1)
= 17 kg
(1)
c)i)SO2(g) SO3(g)
(1)
ii)Platinum / vanadium(V) oxide / Pt / V2O5
(1)
iii)Purify the gases / remove impurities before passing into the reactor.
(1)
Avoid poisoning of the catalyst by impurities.
(1)
d)i)Ammonium sulphate
(1)
ii)It contains nitrogen which is essential for the healthy growth of plants.
(1)
iii)Increase crop yields to meet the rapid growth of the worlds population
(1)
e)Barium sulphate / calcium sulphate
(1)
f)Any two of the following:
(1 ( 2)
Making plastics / making dyestuff / as an electrolyte in lead-acid accumulators /
cleaning metals in electroplating industry
13a)Concentrated sulphuric acid is added dropwise to a beaker containing some cane sugar.
i)What would be observed in the above reaction?
ii)Write a chemical equation for the reaction involved.
iii)Which property of concentrated sulphuric acid is responsible for the reaction?
iv)Give another example that illustrates the property of concentrated sulphuric acid
stated in (iii). Write a chemical equation for the reaction.
b)When the reaction is completed, excess concentrated sulphuric acid is added to the
reaction mixture. The resulting solution is then heated.
i)What would be observed in the above reaction?
ii)Write a chemical equation for the reaction involved.
iii)Which property of concentrated sulphuric acid is responsible for the reaction?
Explain briefly.
iv)Give another example that illustrates the property of concentrated sulphuric acid
stated in (iii). Write a chemical equation for the reaction.(13 marks)Answer:13a)i)Sugar turns black / brown.
(1)
Misty fumes are released / heat is released.
(1)
ii)C12H22O11(s) 12C(s) + 11H2O(g)
(1)
iii)Dehydrating
(1)
iv)Add concentrated sulphuric acid to hydrated copper(II) sulphate crystals.
(1)
CuSO4(5H2O(s) CuSO4(s) + 5H2O(l)
(1)
b)i)Gas bubbles are given off.
(1)
The black solid dissolves.
(1)
ii)C(s) + 2H2SO4(l) CO2(g) + 2SO2(g) + 2H2O(l)
(1)
iii)Oxidizing
(1)
The oxidation number of C changes from 0 to +4 / the oxidation number of S
changes from +6 to +4 in the reaction.
(1)
iv)Copper + concentrated sulphuric acid
(1)
Cu(s) + 2H2SO4(l) CuSO4(aq) + SO2(g) + 2H2O(l)
(1)
or, iron(II) sulphate solution + concentrated sulphuric acid
2Fe2+(aq) + 4H+(aq) + SO42-(aq) 2Fe3+(aq) + SO2(g) + 2H2O(l)
14A student writes notes for his revision. However, the following INCORRECT statements are
made by him.
Statements
a)Chlorine bleach is used to bleach delicate materials, e.g. silk and wool.
b)When a piece of blue litmus paper is added to sodium hypochlorite solution, the paper
turns red and then white.
c)In diluting concentrated sulphuric acid, water should be added to the acid while stirring.
d)Copper reacts with sulphuric acid of any concentration to give sulphur dioxide.
e)When a few drops of concentrated sulphuric acid is spilt onto the skin, the skin should be
washed with dilute sodium hydrogencarbonate solution immediately.
f)Concentrated sulphuric acid can be used to dry ammonia gas.
Explain why each statement is INCORRECT. Make amendment in each case and write an
equation where appropriate.(16 marks)Answer:14a)Chlorine bleach can damage delicate materials.
(1)
Amendment: Sulphur dioxide (aqueous sulphur dioxide) is used to bleach delicate
materials, e.g. silk and wool.
(1)
b)Aqueous sodium hypochlorite is not acidic and thus the blue litmus paper would not
turn red.
(1)
Amendment: When a piece blue litmus paper is added to sodium hypochlorite solution,
the paper turns white.
(1)
OCl-(aq) + dye Cl-(aq) + (dye + O)
coloured colourless
(1)
c)If water is added to concentrated sulphuric acid, the reaction will be highly exothermic
(1)
and the acid may splash out.
(1)
Amendment: In diluting concentrated sulphuric acid, the acid should be added to water
while stirring.
(1)
d)Copper does not react with dilute sulphuric acid.
(1)
Amendment: Copper reacts with concentrated sulphuric acid to give sulphur dioxide.
(1)
Cu(s) + 2H2SO4(l) CuSO4(aq) + 2H2O(l) + SO2(g)
(1)
e)If the skin is washed with dilute sodium hydrogencarbonate solution, a large amount of
heat would be released
(1)
and this can burn the skin.
(1)
Amendment: When a few drops of concentrated sulphuric acid is spilt onto the skin,
the skin should be washed with a large amount of water immediately.
(1)
f)Concentrated sulphuric acid will neutralize (react with) ammonia gas.
(1)
Amendment: Concentrated sulphuric acid should not be used to dry ammonia gas. /
Calcium oxide can be used to dry ammonia gas.
(1)
14In an experiment to investigate some reactions of sulphur dioxide, a mixture of yellow solid X
and concentrated sulphuric acid was heated in test tube as shown in the diagram below.
a)i)Suggest what the yellow solid X may be.
ii)Write an equation for the reaction between X and concentrated sulphuric acid.
b)i)State the observable change in test tube B.
ii)Write an equation for the reaction involved.
iii)Which property of sulphur dioxide does this reaction show?
c)i)State the observable change in test tube C.
ii)Explain the change with the help of an equation.
iii)What is the function of sulphur dioxide in the reaction?
d)i)State the observable change on the filter paper.
ii)Write an equation for the reaction.
e)After removing test tubes B and C, the delivery tube can be connected to a suitable set-up
for collecting the sulphur dioxide produced.
i)Explain why set-ups 1 and 2 shown below are NOT suitable for the collection.
ii)Draw a suitable set-up for collecting the sulphur dioxide produced.(14 marks)Answer:15a)i)Sulphur
(1)
ii)S(s) + 2H2SO4(l) 3SO2(g) + 2H2O(l)
(1)
b)i)The purple colour of the solution fades / turns colourless.
(1)
ii)5SO32-(aq) + 2MnO4-(aq) + 6H+(aq) 5SO42-(aq) + 2Mn2+(aq) + 3H2O(l)(1)
iii)Reducing property
(1)
c)i)The red litmus solution turns from red to colourless.
(1)
ii)Sulphur dioxide dissolves in water to give sulphite ions (SO32-(aq)), which remove
oxygen from the coloured dye, making the dye colourless.
(1)
SO32-(aq) + dye SO42-(aq) + (dye O)
coloured
colourless
(1)
iii)As a bleaching agent
(1)
d)i)Brown colour of iodine fades / turns colourless.
(1)
ii)SO32-(aq) + H2O(l) + I2(aq) SO42-(aq) + 2I-(aq) + 2H+(aq)
(1)
e)i)Set-up 1: SO2 is soluble in water.
(1)
Set-up 2: SO2 is denser than air.
(1)
ii)
(1)
16The diagram below shows the set-up used in an experiment to study the reaction of carbon and
concentrated sulphuric acid.
a)Write a chemical equaton for the reaction that occurred in test tube A.
b)What property of concentrated sulphuric acid is responsible for the reaction in test tube A?
c)Consider test tube B containing iron(III) sulphate solution.
i)State and explain the observable change in test tube B.
ii)Write an equation for the reaction occurred.
d)State and explain the observable change in test tube C.
e)Draw a hazard warning label that should be displayed on a bottle of concentrated
sulphuric acid.
f)State ONE safety precaution that should be taken in the experiment. Explain your answer.(10 marks)Answer:16a)C(s) + 2H2SO4(l) CO2(g) + 2SO2(g) + 2H2O(l)
(1)
b)Oxidizing property
(1)
c)i)The iron(III) sulphate solution turns from yellow to green.
(1)
Sulphite ions are formed when sulphur dioxide produced in test tube A dissolves in
water. Sulphite ions reduce the iron(III) ions to iron(II) ions.
(1)
ii)SO32-(aq) + H2O(l) + 2Fe3+(aq) SO42-(aq) + 2H+(aq) + 2Fe2+(aq)
(1)
d)The carbon dioxide gas produced in test tube A turns the limewater milky.
(1)
This is because the gas reacts with calcium hydroxide solution to form insoluble white
calcium carbonate.
(1)
e)
(1)
f)Any one of the following:
Conduct the experiment in fume cupboard because
(1)
the sulphur dioxide given off in the reaction is toxic.
(1)
Wear safety glasses and protective gloves
because concentrated sulphuric acid is corrosive.
17Sulphur dioxide and sulphur trioxide are the two oxides of sulphur.
a)Give ONE use of sulphur dioxide that depends on each of the following properties of
sulphur dioxide.
i)Ability to kill bacteria
ii)Bleaching property
b)Sulphur trioxide can be made from sulphur dioxide.
2SO2(g) + O2(g) 2SO3(g)
i)Why is this reaction important industrially?
ii)What does the sign in the equation stand for?
iii)What are the conditions for this reaction in industry?
c)Consider the following three experiments for aqueous sulphur dioxide.
i)What colour change would be observed when an excess of aqueous sulphur dioxide
is added to a reddish purple vegetable dye. Explain your observation.
ii)What colour change would be observed when an excess of aqueous sulphur dioxide
is added to acidified potassium dichromate solution?
Write an equation for the reaction involved.
iii)To aqueous sulphur dioxide, acidified barium chloride solution is added. The
mixture remains clear. When aqueous bromine is added, a thick white precipitate
forms. What is the white precipitate? Explain why it forms.
d)Sulphur dioxide reacts with chlorine to form sulphuryl chloride according to the
following equation.
SO2(g) + Cl2(g) SO2Cl2(l)
3 000 cm3 of sulphur dioxide were mixed with 4 800 cm3 of chlorine, measured at room
temperature and pressure.
Calculate the mass of sulphuryl chloride formed by this mixture.
(Relative atomic masses: O = 16.0, S = 32.1, Cl = 35.5; molar volume of any gas at room
temperature and pressure = 24.0 dm3 mol-1)(17 marks)Answer:17a)i)Preserve food / sterilizing.
(1)
ii)Bleach wood pulp for making newspaper.
(1)
b)i)To produce sulphuric acid.
(1)
ii)The reaction is reversible.
(1)
iii)Any two of the following:
(1 ( 2)
Vanadium(V) oxide as catalyst
450
Atmospheric pressure
c)i)The dye is decolourized.
(1)
The dye is reduced to a colourless product.
(1)
ii)The acidified potassium dichromate solution changes from orange to green.(1)
3SO32-(aq) + Cr2O72-(aq) + 8H+(aq) 3SO42-(aq) + 2Cr3+(aq) + 4H2O(l)
(1)
iii)Barium sulphate
(1)
Aqueous bromine oxidizes sulphur dioxide to form sulphate ions.
(1)
Barium ions and sulphate ions combine to give barium sulphate precipitate.(1)
d)Number of moles of SO2 present = 3 000 cm3 24 000 cm3 mol-1
= 0.125 mol
(1)
Number of moles of Cl2 present = 4 800 cm3 24 000 cm3 mol-1
= 0.200 mol
(1)
According to the equation, 1 mole of SO2 reacts with 1 mole of Cl2 to produce 1 mole of
SO2Cl2. Therefore, Cl2 is in excess. The amount of SO2 limits the amount of SO2Cl2
formed.
Number of moles of SO2Cl2 formed = 0.125 mol
(1)
Molar mass of SO2Cl2 = (32.1 + 2 ( 16.0 + 2 ( 35.5) g mol-1 = 135.1 g mol-1
Mass of SO2Cl2 formed = 0.125 mol ( 135.1 g mol-1 = 16.9 g
(1)
18Both aqueous sodium hypochlorite and aqueous sulphur dioxide can be used as bleaching solutions.
a)Give an advantage of using each of the bleaching solution.
b)i)A 200 cm3 sample of chlorine bleach contains 12 g of sodium hypochlorite.
Calculate the concentration, in mol dm-3, of sodium hypochlorite in the bleach.
ii)A 500 cm3 sample of sulphur dioxide bleach contains 3.0 mg of sulphur dioxide.
Calculate the volume, in cm3, of sulphur dioxide gas dissolved in 500 cm3 of the
solution at room temperature and pressure.
c)Draw a labelled diagram for the laboratory preparation of aqueous sulphur dioxide.
Specify the reagents required on the diagram.
d)What would be observed if a few drops of blue litmus solution were added to aqueous
sulphur dioxide? Explain this observation with appropriate equations.
e)The following label is displayed on a container of chlorine bleach:STRONG BleachNew formula with super bleaching action
Warning: Keep out of reach of children
Keep.......................Store in a cool place
i)Why chlorine bleach should be kept away from children?
ii)Complete the second warning statement, beginning with the word Keep.
Explain your answer.
iii)Explain why the bleach should be stored in a cool place.
(Relative atomic masses: O = 16.0, Na = 23.0, S = 32.1, Cl = 35.5; molar volume of any
gas at room temperature and pressure = 24.0 dm3 mol-1)(19 marks)Answer:18a)Aqueous sodium hypochlorite: bleaching action is fast / bleaching action is permanent
(1)
Aqueous sulphur dioxide: it causes less damage to the material to be bleached
(1)
b)i)Molar mass of NaOCl = (23.0 + 16.0 + 35.5) g mol-1 = 74.5 g mol-1
Concentration of sodium hypochlorite = dm3
(1)
= 0.81 mol dm-3
(1)
ii)Molar mass of SO2 = (32.1 + 2 ( 16.0) g mol-1 = 64.1 g mol-1
Volume of sulphur dioxide = ( 24 000 cm3 mol-1
(1)
= 1.1 cm3
(1)
c)
(3)
(1 mark for the correct set-up; 1 mark for the correct reagents; 1 mark for showing how to
dissolve SO2 in water)
d)The solution turns red and then colourless.
(1)
Sulphur dioxide dissolves in water to give hydrogen ions and sulphite ions.
(1)
SO2(g) + H2O(l) H2SO3(aq) 2H+(aq) + SO32-(aq)
(1)
Hydrogen ions turn the blue litmus solution red.
(1)
Sulphite ions remove oxygen from the coloured dye, making the dye colourless.(1)
SO32-(aq) + dye SO42-(aq) + (dye O)
coloured
colourless
(1)
e)i)It is irritating / toxic / corrosive.
(1)
ii)Keep away from acids
(1)
because chlorine bleach will react with acid to give chlorine which is toxic / the
bleach will deteriorate (loses its function).
(1)
iii)The rate of decomposition of chlorine bleach is slower at low temperatures.(1)
19A laboratory technician tried to prepare sulphur dioxide using the following experimentalset-up.
a)i)Write a chemical equation for the reaction between copper strips and 18.0 M
sulphuric acid.
ii)Is the reaction a redox reaction? Explain your answer in terms of oxidation number.
b)50.0 cm3 of 18.0 M sulphuric acid are added to 0.95 g of copper strips. Calculate the
volume of sulphur dioxide collected, measured at room temperature and pressure.
c)State the function of the filter paper soaked with acidified potassium dichromate solution.
Explain briefly with the help of an equation.
d)Aqueous sulphur dioxide can be used as a bleaching agent.
i)Suggest a simple experiment that can demonstrate the bleaching action of aqueous
sulphur dioxide.
ii)State ONE difference between the bleaching actions of sulphur dioxide bleach and
chlorine bleach.
e)Suggest another method of preparing sulphur dioxide in the laboratory. Write a chemical
equation for the reaction involved.
(Relative atomic mass: Cu = 63.5; molar volume of any gas at room temperature and
pressure = 24.0 dm3 mol-1)(15 marks)Answer:19a)i)Cu(s) + 2H2SO4(l) CuSO4(aq) + SO2(g) + 2H2O(l)
(1)
ii)Yes, because the O.N. of copper increases
(1)
from 0 to +2.
(1)
or,Yes, because the O.N. of sulphur decreases from +6 to +4.
b)No. of moles of H2SO4 = 18.0 mol dm-3 ( (50.0 1 000) dm3
= 0.900 mol
(1)
No. of moles of copper = 0.95 g 63.5 g mol-1
(1)
= 0.015 mol
According to the equation, 1 mole of Cu reacts with 2 moles of H2SO4.
i.e. 0.015 mole of Cu requires 0.030 mole of H2SO4 for complete reaction.
(H2SO4 is in excess and Cu is the limiting reactant.
No. of moles of SO2 produced = 0.015 mol
(1)
Volume of SO2 collected = 0.015 mol ( 24.0 dm3 mol-1
= 0.36 dm3
(1)
c)The filter paper is used to test whether the jar is full of sulphur dioxide.
(1)
When the gas jar is full of sulphur dioxide, sulphur dioxide at the opening of the jar
reduces orange dichromate ions to green chromium(III) ions.
(1)
3SO32-(aq) + Cr2O72-(aq) + 8H+(aq) 2Cr3+(aq) + 3SO42-(aq) + 4H2O(l)
(1)
d)i)Put a flower pedal into a gas jar of aqueous sulphur dioxide. The pedal will be
bleached.
(1)
ii)Any one of the following:
(2)
Sulphur dioxide bleach bleaches by reduction while chlorine bleach bleaches by
oxidation.
The bleaching effect of sulphur dioxide bleach is not permanent while that of chlorine
bleach is permanent.
e)Any one of the following:
(1, 1)
Heat sulphur in air.
S(s) + O2(g) SO2(g)
Add dilute hydrochloric acid to sodium sulphite.
Na2SO3(s) + 2HCl(aq) 2NaCl(aq) + SO2(g) + H2O(l)
20A company proposes to build an industrial plant for the production of sulphuric acid in town X. There are two possible locations, A and B. The following map shows the environment of
town X.
a)What is the raw material for the production of sulphuric acid?
b)State TWO factors that should be considered when the industrial plant is to be set up.
c)State TWO advantages of choosing
i)site A
ii)site B
to build the plant respectively.
d)If a plant for production of sulphuric acid is to be set up in Hong Kong, suggest a suitable
site.(8 marks)Answer:20a)Sulphur / iron pyrite / zinc blende
(1)
b)Any two of the following:
(1 ( 2)
Ease of availability of raw materials / ease of transportation / environmental impacts /
demand for sulphuric acid.
c)i)Any two of the following:
(1 ( 2)
Near to the industrial area where the demand for the acid should be greater
Close to the power station and thus there is enough supply of electricity for the
production process
Far away from the residential area and thus there is less environmental impact on the
residents
ii)Any two of the following:
(1 ( 2)
Close to the sulphur deposit and thus it is easy to obtain the raw material for acid
production
Close to the main road and thus the transportation for raw material, product and
labour is easier
Close to the residential area where labour is readily available
d)Tai Po industrial estate (or any sensible answer)
(1)
21The table below shows some information of two atoms, hydrogen H and deuterium D.AtomNumber of protonsNumber of neutrons
Hydrogen H10
Deuterium D11
a)Suggest a term to indicate the relationship between a hydrogen atom and adeuterium
atom.
b)State the mass number of deuterium.
c)Is it possible to separate a mixture of hydrogen and deuterium by chemical methods?
Explain your answer.
d)Deuterium reacts with oxygen in the same way as hydrogen.
2D2(g) + O2(g) 2D2O(l)
(H< 0
The product of the reaction is known as heavy water?
i)What is meant by (H< 0 ?
ii)80.0 cm3 of deuterium and 60.0 cm3 of oxygen, both measured at room temperature
and pressure, are allowed to react. Assuming that the two gases undergo complete
reaction, calculate
(1)the mass of heavy water produced.
(2)the volume of the remaining gas, measured at room temperature and pressure,
at the end of the reaction.
(Relative atomic mass: O = 16.0; molar volume of any gas at room temperature and
pressure = 24.0 dm3 mol-1)(9 marks)Answer:21a)Isotopes
(1)
b)2
(1)
c)No, because hydrogen and deuterium have the same chemical properties.
(1)
d)i)The reaction is exothermic.
(1)
ii)2D2(g) + O2(g) 2D2O(l)
No. of moles of D2 = 80.0 cm3 24 000 cm3 mol-1 = 0.00333 mol
(1)
No. of moles of O2 = 60.0 cm3 24 000 cm3 mol-1 = 0.00250 mol
(1)
According to the equation, 2 moles of D2 react with 1 mole of O2 to produce 2 moles
of D2O. Therefore, O2 is in excess. The amount of D2 limits the amount of D2O
produced.
Number of moles of D2O produced = 0.00333 mol
(1)
Molar mass of D2O = (2 ( 2.0 + 16.0) g mol-1 = 20.0 g mol-1
Mass of D2O produced= 0.00333 mol ( 20.0 g mol-1
= 0.0666 g
(1)
iii)80.0 cm3 of D2 react with 40.0 cm3 of O2
Volume of remaining gas = (60.0 40.0) cm3
= 20.0 cm3
(1)
22The problem is to determine the exact concentration of a solution of sodium carbonate, which
is known to have a concentration of about 0.1 mol dm-3. Two techniques are to be used. Read
the instructions below for each method and answer the questions which follow.
a)React 25.0 cm3 of the sodium carbonate solution with excess dilute hydrochloric acid.
Collect and measure the volume of the carbon dioxide gas evolved.
i)Write an ionic equation for the reaction of carbonate ions with hydrogen ions from
the dilute hydrochloric acid.
ii)Explain why excess acid is used.
iii)In the above experiment, 64.8 cm3 of carbon dioxide, measured at room temperature
and pressure, were evolved. Calculate
(1)the number of moles of carbon dioxide evolved.
(2)the concentration of the sodium carbonate solution.
b)React 50.0 cm3 of the sodium carbonate solution with excess calcium chloride solution.
Remove the precipitated calcium carbonate by filtration. Wash the precipitate well with
water and then heat to dryness.
i)Write an ionic equation for the reaction of carbonate ions with calcium ions.
ii)Explain why it was necessary to wash the precipitate.
iii)Explain why the precipitate was dried.
iv)In the above experiment, 0.540 g of calcium carbonate were obtained from the
50.0 cm3 of solution. Calculate
(1)the number of moles of calcium carbonate formed.
(2)the concentration of the sodium carbonate solution.
c)State one further different method which could be used to determine the concentration of
the sodium carbonate solution. Specify any reagent required. (Your method should NOT
involve evaporation to dryness.)
(Relative atomic masses: C = 12.0, O = 16.0, Ca = 40.1; molar volume of any gas at room
temperature and pressure = 24.0 dm3 mol-1)(13 marks)Answer:22a)i)CO32-(aq) + 2H+(aq) H2O(l) + CO2(g)
(1)
ii)To ensure that all the carbonate ions have been reacted.
(1)
iii)(1)No. of moles of CO2 = 64.8 cm3 24 000 cm3 mol-1
= 0.00270 mol
(1)
(2)No. of moles of carbonate in 25.0 cm3 solution = 0.00270 mol
(1)
Concentration of Na2CO3 solution= 0.00270 mol dm3
= 0.108 mol dm-3
(1)
b)i)Ca2+(aq) + CO32-(aq) CaCO3(s)
(1)
ii)To remove any soluble impurities that may adhere to the precipitate.
(1)
iii)To remove any water that may adhere to the precipitate.
(1)
iv)(1)No. of moles of CaCO3 = 0.540 g 100.1 g mol-1 = 0.00539 mol
(1)
(2)No. of moles of Na2CO3 in 50.0 cm3 = 0.00539 mol
(1)
Concentration of Na2CO3 solution = 0.00539 mol dm3
= 0.108 mol dm-3
(1)
c)Titration
(1)
Titrate 25.0 cm3 of sodium carbonate solution against standard hydrochloric acid /
sulphuric acid.
(1)23Compare the bleaching actions of chlorine bleach and sulphur dioxide bleach. Write chemical
equations where appropriate.
(You are required to give a paragraph-length answer. In this question, 6 marks will be awarded for chemical knowledge and 3 marks for effective communication.)(9 marks)Answer:
23Chlorine bleach bleaches by oxidation while sulphur dioxide bleach bleaches by reduction.
(1)
Chlorine bleach:
+1 -1
OCl-(aq) + dye Cl-(aq) + (dye + O)
colourlesscoloured colourlesscolourless
(1)
Sulphur dioxide bleach:
+4 +6
SO32-(aq) + dye SO42-(aq) + (dye O)
coloured
colourless
(1)
The bleaching effect of chlorine bleach is fast and vigorous while that of sulphur dioxide
bleach is slow and mild.
(1)
The bleaching effect of chlorine bleach is permanent while that of sulphur dioxide bleach is
not permanent and the original colour will be restored later.
(1)
Sulphur dioxide bleach is suitable for bleaching delicate materials such as wool and silk while
chlorine bleach can damage them.
(1)
(3 marks for effective communication)
24Concentrated sulphuric acid and concentrated nitric acid show some different chemical
properties when compared with their dilute acids respectively. Discuss the differences. Illustrate your answer with appropriate examples and equations.
(You are required to give a paragraph-length answer. In this question, 6 marks will be awarded for chemical knowledge and 3 marks for effective communication.)(9 marks)Answer:24Concentrated sulphuric acid is a stronger oxidizing agent than dilute sulphuric acid.
(1)
Concentrated sulphuric acid can oxidize non-metals, e.g. carbon or sulphur, but dilute sulphuric acid cannot.
C(s) + 2H2SO4(l) CO2(g) + 2SO2(g) + 2H2O(l) / S(s) + 2H2SO4(l) 3SO2(g) + 2H2O(l)
(1)
Concentrated sulphuric acid is a dehydrating agent but dilute sulphuric acid is not.
(1)
When concentrated sulphuric acid is added to blue copper(II) sulphate crystals, the crystals turn white.
(1)
CuSO4(5H2O(s) CuSO4(s) + 5H2O(l)
/ When concentrated sulphuric acid is added to cane sugar, the sugar chars.
C12H22O11(s) 12C(s) + 11H2O(g)
When concentrated nitric acid is added to a metal (e.g. copper), nitrogen dioxide is given off. However, when dilute nitric acid is added to copper, nitrogen
monoxide is given off.
(1)Cu(s) + 2NO3-(aq) + 4H+(aq) Cu2+(aq) + 2NO2(g) + 2H2O(l)
(1)
3Cu(s) + 2NO3-(aq) + 8H+(aq) 3Cu2+(aq) + 2NO(g) + 4H2O(l)
(3 marks for effective communication)