Section A: HKCEE Paper I Running Answer Text 1995 Chemistry
Section A: HKCEE Paper I Running Answer Text 1995 Chemistry
Section A: HKCEE Paper I Running Answer Text 1995 Chemistry
Section A
1. (a)
Rubidium is more reactive than potassium because rubidium can donate electrons more easily.
(b)
The chemical equation for the reaction between rubidium and water: 2 Rb(s) + 2 H2O(l) 2 RbOH(aq) + H2(g)
(c)
In the laboratory, rubidium should be stored under oil. One precaution of this experiment is to wear gloves.
2. (a)
(b)
(c)
(d)
Nitrogen is different form the others because it cannot burn in air. Monosodium glutamate (MSGis used as flavour enhancer. Benzoic acid is used as preservative. Sunset yellowE110is used for colouring.
3. (a)
4.
Atoms can attain the noble gas electronic structures either by sharing their outermost electrons or by the formation of ionic border. atoms of non-metal combine, they tend to share When their
(valence/outermost) electrons to form molecules. The following diagram shows the electronic structure of a molecule e.g. hydrogen chloride.
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Cl
When metal and non-metal combine, atoms of metal donate electrons (to atoms of non-metal) to form cations (positive ions/metallic ions), while atoms of non-metal accept electrons to form anions (negative ion/non-metallic ions). The following diagram shows the electronic structure of an ionic compound e.g. sodium chloride.
Na Cl
5.
A school laboratory can prepare large crystals of ammonium sulphate through the following reaction between an aqueous solution of ammonia and sulphuric acid:
Ammonia is titrated with (dilute) sulphuric acid until the end-point is reached. Then the same volumes of (dilute) sulphuric acid and ammonia are added (in correct mole ratio (of 1:2)).
The resulting solution is evaporated to dryness (heat to obtain a saturated/more concentrated solution). The solution is then cooled down slowly to obtain large crystals of ammonium sulphate (A small crystal of ammonium sulphate is put into the saturated solution to obtain large crystals of ammonium sulphate) (A small crystal of ammonium sulphate in the saturated solution as seed to obtain large crystal).
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Section B
6. (a)
(i)
The toilet cleaner should not be mixed with bleaches because the
hydrogen ions in the toilet cleaner will react with the bleachClO
(ii)
(1) (2)
Metals such as iron should not be mixed with toilet cleaner. If iron and the toilet cleaner are mixed together, effervescence will occur. Chemical equation: Fe(s) + 2H+(aq) Fe2+(aq)+ H2(g)
(iii)
It is necessary to handle the toilet cleaner with care because hydrochloric acid is corrosive.
(iv)
(1) (2)
Plastic is used because it is inert. Polyethene is suitable for making the bottle.
(b)
(i)
Gold was discovered a long time age because it can be found free in nature.
(ii)
Copper is the most suitable one because it does not corrode easily and has a high metallic strength.
(iii)
(1)
It is because that aluminium reacts with oxygen in air to form a thin layer of oxide which is not permeable to oxygen and prevent the metal from further corrosion.
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The strength of aluminium can be improved by alloying with other metals (e.g. copper).
(iv)
(1) (2)
The price depends on its abundance in the earths crust. The price also depends on the cost of extract.
7. (a)
(i)
Effervescent occurs because citric acid, when dissolving in water, gives hydrogen ions which reacts with calcium carbonate to give carbon dioxide gas bubbles. Ionic equation: CaCO3 + H+ Ca2++ CO2 + H2O = 40 + 12 +16 3 =100
(ii)
(1)
1 mole of CaCO3 gives 1 mole of CO2 No. of moles of CaCO3 present 625 10 3 100
6.25 10 3 (mol)
Theoretical volume of gas = 6.25 103 24 dm3 = 150 cm3 (0.15 dm3)
(2)
The volume of gas collected becomes less because some of the carbon dioxide produced dissolved in water.
(iii)
The warning statement: Keep out of moisture. Reason: The active ingredients of the tablet will react in the presence of water.
(b)
(i)
(ii)
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CH2=CH2 + Br2
CH2BrCH2Br
(iii)
(1)
Y is ethanoic acid.
(2)
Water out
Water In
Ethanol + acidified potassium dichromate (Heat) (iv) The breath of the driver can be passed into acidified potassium dichromate solution. The colour of the solution will change from orange to green.
8. (a)
(i)
Z is sulphur dioxide. Z attacks respiratory system. The amount of Z can be reduced by installation of scrubbers.
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(ii)
One other pollutant is carbon monoxide. Carbon monoxide is formed from the incomplete combustion of fuel. Carbon monoxide is poisonous. To reduce the amount of carbon monoxide, it should be ensure that there is sufficient supply of air during combustion of fuel.
(iii)
(b)
(i)
(ii)
The hypochlorite ions can be tested with litmus paper. The colour of the indicator will be bleached.
(iii)
= 23 + 16 + 35.5 = 74.5
(iv)
(v)
9. (a)
(i)
Heat vegetable oil with sodium hydroxide solution. Add concentrated sodium chloride solution . Separate the soap from the solution.
(ii)
(1)
(2)
Advantage: The soapless detergent can be used in hard water. Disadvantage: The soapless detergent is non-biodegradable. MnO4 + 8 H+ + 5 e Mn2+ + 4H2O
(b)
(i)
(ii)
I2
+ 2 e
(iii)
(iv)
(v)
(1)
The salt bridge is used to allow migration of ions between the two beakers.
(2)
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