2002 Hkcee Chemistry Paper I Marking Scheme

2002 HKCEE Chemistry paper I marking scheme Marks (1)

(a)

Nitrogen is required for the formation of amino acids / proteins / chlorophyll /

1

chloroplast.

(b)

Nitrogen, hydrogen, phosphorus and oxygen

1/0

or N, H, P and O (c)(i)

Formula mass of (NH4)2SO4 = (14+4) × 2 + 32 + 16×4 = 132

% by mass of N =

(ii)

14 × 2 132

1

× 100% = 21.2 % (Accept 21, 21.2 and 21.21 )

Calcium hydroxide / calcium oxide / calcium carbonate

1

1

( slaked lime / lime water / quick lime / limestone / marble / calcite ) Calcium hydroxide / calcium oxide / calcium carbonate reacts with H+ in soil

1

( neutralizes acid in soil ) or Ca(OH)2 / CaO / CaCO3 is not very soluble in water. It will not make the soil too alkaline. or Ammonia solution / NH3(aq)

(1)

+

Ammonia reacts with H in soil ( neutralizes acid in soil )

(1)

or Ammonia is a weak alkali. It will not increase the soil pH to a great extent.

6 marks (2)

(a) Magnesium burns with a brilliant flame / white or bright light.

1

A white solid (MgO) or a yellow solid (Mg 3N2) is formed.

(1)

2 Mg

+

O2

à

2 MgO ( white )

1

or 3 Mg

+

N2

à

Mg3N2 ( yellow )

(1)

(b) The colour of the potassium permanganate solution changes from purple to yellow

5 Fe2+

+

MnO4−

+

8 H+

à

5 Fe3+

+

Mn2+

+

1

1

4 H2O

10 FeSO4 + 2 KMnO4 + 8 H2SO4 à 5 Fe2(SO4)3 + 2 MnSO4 + K2SO4 + 8 H2O

(1)

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(c) The solution changes from colourless to brown / orange / yellow ( NOT GAS )

1

Cl2

+

2 Br−

à

2 Cl−

+

Br2

1

or Cl2

+

2 NaBr

à

2 NaCl +

Br2

(1)

à

H2O

or 2 H+

+

OCl−

+

2 Br−

+

Cl−

+

(1)

Br2

6 marks

(3) (a) Sodium nitrite

1

It is toxic to micro-organisms / bacteria

1

(b) Manganese(IV) oxide

1

It acts as an oxidizing agent / It reacts with ( removes ) H 2(g) produced at the cathode.

(c) Sodium dichromate

1

1

It changes from orange to green

1

6 marks Chemical Knowledge ( 6 marks ) + Effective Communication ( 3 marks )

4.

• Position of ion in the electrochemical series

1

( 1 mark for stating the factor and 1 mark for explanation in terms of comparison of two discharging processes. ) − If carbon / platinum / copper ( any appropriate material ) is used as the cathode, Cu2+ ions instead of H+ ions will be discharged because Cu2+ ion occupies a lower position on the 2+

1

+

e.c.s. ( Cu ion is more easily to be reduced / has a stronger oxidizing power than H ) OR − In the electrolysis of dilute CuCl2(aq) using carbon / platinum as anode ( any appropriate material ), OH− ions instead of Cl− ions will be discharged because OH − occupies a higher

(1)

position in the e.c.s. ( OH− ion is more easily to be oxidized / has a stronger reducing power than Cl − )

• Concentration of ion

1

( 1 mark for stating the factor and 1 mark for explanation in terms of comparison of two discharging processes. ) − In the electrolysis of very dilute / dilute CuCl 2(aq) using carbon / platinum as anode, OH − ions will be discharged / O2 is liberated at the anode.

2002 - CE-CHEM p.2

1

ions will be discharged / O2 is liberated at the anode. −

− If concentrated CuCl2(aq) is used, Cl ions will be discharged / chlorine gas will be liberated instead. • Nature of electrode

1

( 1 mark for stating the factor and 1 mark for explanation in terms of comparison of two discharging processes. ) − If carbon / platinum is used as the anode, Cl 2 / O2 ( conc. / dilute ) will be liberated at the

1

anode. − If copper is used as the anode, the anode will dissolve / oxidize. 6 marks Chemical Knowledge ( 6 marks ) + Effective Communication ( 3 marks )

5.

( Award 2 marks for characteristic related to structure / formula ; 2 marks for physical property ; 2 marks for chemical property )

2 marks for characteristic related to structure / formula − Members of a homologous series can be represented by the same general formula

1

− General formula of alkenes : CnH2n or (CH2)n

1

OR − Successive members of a homologous series differ in their structure by one CH 2 unit

(1)

− Molecular Formulae :

(1)

ethene ( C2H4 ) , propene ( C 3H6 ) etc.

OR − Members of a homologous series have the same functional group

(1)

− Functional group of alkenes : C=C

(1)

OR − Alkenes possess the C=C functionality. − Structures ( structural formula ) of alkenes

(1) : ethene ( CH2=CH2 ) ; propene ( CH 3CH=CH2 )

(1)

2 marks for physical property − Their physical properties change gradually from one member to the next.

1

− The melting point / boiling point of alkene increases with increase in relative molecular mass

1

OR − The volatility / flammability of alkenes decreases with increase in relative molecular mass. (1) 2 marks for chemical property − Members of a homologous series have similar ( BUT NOT SAME ) chemical properties. − ONE example of the reaction of alkenes which is characteristic of unsaturated

1

hydrocarbons

1 2002 - CE-CHEM p.3

( accept a description of the reaction or a chemical equation. ) e.g. CH2 CH2

+

CH2 CH2

Cl2

Cl CH2 CH2

+ +

KMnO4 / H

Cl

+

H2O

CH2 CH2

( O )

OH

(1)

OH

(1)

( Accept other reactions : e.g. addition polymerization )

6 marks Calcium hydroxide / Ca(OH)2

6(a)(i)

(ii) Mg(OH)2 (s) + 2 HCl (g)

1 à

MgCl2 (s) + 2 H2O (g/ l )

1+1

( 1 mark for a balanced equation ; 1 mark for state symbols if the formulae are correct )

(iii) molten magnesium chlorides contains mobile ions / ( movable ions ) / ( freely moving ions )

1

BUT NOT ACCEPT free ions )

(iv) duralumin

1

5 marks 6(b)(i)

Isotopes are atoms with the same atomic number but different mass number /

1

the same number of protons but different number of neutrons.

(ii) relative atomic mass

= 24 × 0.786 + 25 × 0.101 + 26 × 0.113

1

= 24.3 ( or 24.33 or 24.327 ) ( NO g unit !)

1

(iii) No, because isotopes of an element have the same chemical properties ( NOT similar )

1

4 marks 6(c) (i)

concentrated sulphuric acid / conc. H 2SO4 / H2SO4 (l)

(ii)

Diagram :

1

2 2002 - CE-CHEM p.4

open water out

water in

mixture of ethanol, ethanoic acid and conc. H2SO4

heat

( 1 mark for safe and appropriate set-up diagram , e.g. open but not close system ) ( 1 mark for direction of water flow in the condenser ) ( 0 mark for wrong set−up : e.g. closed system or distillation ) (iii) • Iodine has a simple molecular structure / Attraction between I 2 molecule is weak van der

1

Waals’ forces. • Sodium iodide has an ionic structure / Attraction between Na + and I− ions is strong ionic

1

bonds. • Strength of inter-particle attraction in ethyl ethanoate is comparable to that in iodine.

1

( Indication of an understanding of the idea of ‘like dissolve like’ in terms of the strength of attraction between particles. )

(iv)

Flammable / inflammable / C

7(a)(i) CaCO3

+

2 HNO3

+

2 H+

à

1

Ca(NO3)2

+

H2O

+

H2O

+

CO2

CO2

1

OR CaCO3

à

Ca2+

+

Evolution of CO2 stops ; Test the pH of the solution using pH paper, the pH should be less than 7.

(1)

1 (1)

(ii) Diagram :

2

2002 - CE-CHEM p.5

filter paper

filter funnel

( 1 mark for the diagram ; 1 mark for labeling the funnel and filter paper )

(iii) Ca2+

+

SO42−

à

1

CaSO4

(iv) To remove any soluble impurities ( or appropriate example : Na 2SO4 ; NaNO3 ; HNO3 )

1

(v) (1)

1

no. of mole of CaSO4

= 10.52 / ( 40 + 32 + 16× 4 ) = 0.0774

(1) = 0.077 × ( 40 + 12 + 16 × 3 )

mass of CaCO3 in the sample of calcite

1*

= 7.74 g = ( 7.74 / 7.98 ) × 100

% by mass of CaCO3

= 97.0 %

1

( Also accept answers from 96.5 to 97.0 ) The sample does not contain ions which form insoluble sulphate, e.g. Ba 2+ , Sr2+, Pb2+

(2)

2+

1

OR

There is no loss of Ca ions during the experiment

(1)

OR

CaCO3 is the only calcium−containing compound present in the sample.

(1)

10 marks 7(b)(i)

Cl2

−

à

+

2 OH

+

2 NaOH à

−

OCl

−

+

Cl

+

1

H2O

OR Cl2

NaOCl

+

NaCl

+

H2O

(1)

(ii) both oxidizing agent and reducing agent.

1

O.N. of Cl decreases from 0 to −1 ( Cl2 acts as an oxidizing agent )

1

O.N. of Cl increases from 0 to +1 ( Cl 2 acts as a reducing agent )

1

(iii) Sterilizing agent / making solvents ( CHCl=CCl2 ) / production of insecticides /

1

production of vinyl chloride ( CH2=CHCl ) / used as chemical weapons / making HCl(aq)

2002 - CE-CHEM p.6

7(c)(i)

3 Cl2

+

8 NH3

à

6 NH4Cl

+

N2

1+1

( 1 mark for the correct formulae ; 1 mark for balancing the equation )

(ii) ammonium chloride / NH4Cl

1 8 marks

8(a)(i) no. of moles of SO2 released = no. of mole of S = ( 1000 × 0.015 ) / 32 = 15 / 32

1

( or 0.46875 )

= (15 / 32 ) × 24

volume of SO2 released

= 11.25 dm

1*

3

1 3

( Accept 11 to 11.3 dm )

1 (ii)

acid rain / high incidence of respiratory illnesses / corrosion of buildings / lower crop yield ( NOT ACCEPT : affect … cause cancer )

(iii)

installation of scrubbers / installation of desulphurization system /

1

use coal of lower sulphur content.

(iv)

(1) high incidence of respiration illnesses / causing cancer / darkening of building walls /

1

reduce visibility / smog.

(2) installation of electrostatic precipitator.

1

7 marks 8(b)(i) O

C

O

1

(ii) Carbon dioxide is denser than air It can exclude air (oxygen ) from the fuel / can blanket the fire from air.

1 1

2002 - CE-CHEM p.7

(iii) Silicon dioxide has a covalent network structure / giant covalent structure .

1

Attraction between CO2 molecules is van der Waals’ forces.

1

The attraction between CO2 molecules is weak while the attraction between atoms in SiO2 is

1

strong covalent bond. ( 1 mark for the structure of SiO 2 ; 1 mark for the attraction between CO 2 molecules ; 1 mark for comparing the strength of inter−particle attractions. )

(iv) (1) or

(2)

SiO2

+

2C

à

Si

+

2 CO

1

SiO2

+

C

à

Si

+

CO2

(1)

1

making computer chips / electronic parts / alloys / semi−conductors / silicone ( accept other uses of silicon : Integrated Circuit )

8 marks (v)

Any ONE of the following : 1+1 O

O H

C

O

CH2

H

CH3

C

CH2

CH3 O

CH3

CH CH3

methylethyl methanoate ( iso-propyl methanoate )

propyl methanoate

CH2

C

O

CH3

O

methyl propanoate

( 1 mark for structure ; 1 mark for name ) 9 marks 8 (c)(i) A reaction in which monomer molecules join together to form polymer molecules, with the

1

elimination of small molecules.

(ii) Cl

HO

CH3 O

O

O

C

C

O

O

C

C

O

O

C

C

Cl

OH

O

HOCH2CH2OH

1+1

CH3

2002 - CE-CHEM p.8

3 marks +

9 (a) (i) (1)

+

NH4

(2)

H2O

à

+

+

NH4

OH

−

1

Oils react with alkalis ( undergoes hydrolysis ) to give soaps ( soapy detergent ) /

1

water soluble substances.

(ii) The glass cleaner should be used in a well−ventilated environment because ammonia has a pungent smell / is toxic.

1 1

OR Wear gloves because alkaline solutions can attack skin / cause skin allergy / it is corrosive.

(1) + (1)

OR Wear safety spectacles / safety glasses / goggles because ammonia solution attack eyes / it is

(1) + (1)

corrosive. 9(b)(i) (1)

distilled water / deionized water

1

(2)

distilled water / deionized water

1

(ii) pipette (iii) NH3

1 +

à

HCl

NH4+

No. of moles of HCl(aq) used

+

Cl− = 0.23 × 28.7 × 10−3 = 6.60 × 10

Concentration of NH3 in glass cleaner

−3

1

( 0.00660 )

= 6.60 × 10−3 / ( 25×10−3 ) × ( 250 / 25 )

1*

= 2.64 mol dm−3

1

( Also accept 2.6, 2.64, 2.640 and 2.6404 mol dm

−3

)

6 marks (9)(c) (1) violet / purple / blue

1

(i) 2 H+

2 e−

à

H2

1

∴ OH concentration increases around carbon rod A /

1

+

−

concentration of OH −(aq) is higher than that of H +(aq).

(2) oxygen / O2

1

OH− is discharged at carbon rod B ( anode ) / +ve pole OR

(ii)

4 OH

−

(1)

à

O2

pencils / zinc−carbon cells.

+

2 H2O

+

4e

−

1 (1)

1 2002 - CE-CHEM p.9

(iii)

Any TWO of the following : • save chemicals / reduce the cost of chemicals ( or laboratory equipment ) used

2

• reduce chance of chemical hazards • reduce chemical wastes produced / environmental problems • shorten the time required for conducting an experiment • require less working space for carrying out an experiment • easy to set−up / convenient to set up

8 marks

2002 - CE-CHEM p.10