2008 AL Chemistry Paper 1A
CE Paper I Section A
1 / (b) / Common mistakeCO32- + 2H+ CO2 + H2O
(c) / (ii) / Common mistakes
CO2 can act as fire-proofing additive.
Limestone is not flammable.
2 / (a) / (ii)(2) /
Common mistake
MgN / MgNO3 / Mg(NO3)2
Wrong electronic diagram – wrong charge, wrong no. of ions, wrong no. of electrons, covalent bond
(b) / (i) / Common mistakes
Burn carbon powder and copper(II) oxide.
“Gas is given out” as an observation
(ii) / Common mistake
Mg is more reactive than Cu. / Mg is reactive.
*ACCEPT Mg reactivity is far higher.
*ACCEPT Mg has higher reactivity and need electrolysis
3 / (a) / Common mistake
Iron powder reacts with air.
(c) / Common mistake
Act as catalyst
4 / (b) / (ii) / Common mistakes
Repeating unit is drawn instead of structural formula.
Most student miss out either one of the points in answering b(ii).
(c) / Common mistake
Does not x1000 above or x1000 below
5 / (d) / (ii) / Common mistake
Only a few students can answer part (d) correctly.
6 / (a) / Common mistakes
Chemical equation for the dehydration of sucrose (C12H22O11) is stated.
C12H22O11 12C + 11H2O
Concentrated sulphuric acid is included as reactant in the equation.
C6H12O6 + H2SO4 6C + 6H2O
(b) / (ii) / Common mistakes
No cooling before disposal.
Alkali such as ammonia is used to neutralize the sulphuric acid
7 / (a) / Common mistakes
Forget to add water.
Fail to show that calcium sulphate is an insoluble solid.
(b) / Common mistake
Many students do not show that silver nitrate is in aqueous state.
(c) / Common mistake
Many students spell the word ‘chromatography’ wrongly.
8 / (a) / Common mistake
A number of students wrongly use conc. H2SO4 as an oxidizing agent.
(c) / (ii) / Common mistakes
The ethanol / oxidizing agent / ethanoic acid cannot evaporate.
9 / Electrical conductivity:
Common mistakes
Many students wrongly state that there are free electrons or mobile ionsin metal.
Many students cannot give complete explanation for the conductivity of Cl2.
Melting point:
Common mistakes
Strong electrostatic attraction between metal ions in metals / Ionic bonding between molecules (or atoms) / Giant ionic bond/ Giant covalent bond
m.p.: Na > NaCl
CE Paper I Section B
10 / (a) / (i) / Common mistakes:as O.A. to remove H2 produced (Zn- C cell?)
(ii) / Common mistakes:
decomposition of H2O2 gives water and oxygen
decomposition of H2O2 gives O2 … volume can be traced…
by increasing pressure, rate of the decomposition can be increased
(iii) / 2 H2O2 2 H2O + O2
Common mistakes:
H2O2 H2 + O2
Oxidation number of hydrogen remains unchanged.
Oxidation number of oxygen (increases) from -1 to 0, and (decreases) from 1 to 2.
Common mistakes:
ON of H changes from +2 to +1.
ON of O changes from +2 to 0 and 2 to 0.
ON of O increases from -1 to 0. (answer not complete)
ON of both H and O remain unchanged.
ON of O2 / H2 / H+ …
(b) / (i) / At A: The rate of reaction is high because the concentration of H2O2 is high.
At B: The rate ofreaction decreases because the concentration of H2O2 decreases during reaction.
At C: The reaction stops because all the H2O2 has been used up.
Common mistakes:
- At A, rate is high because of large amount of H2O2 present.
- At A, rate is lowpressure is low at that time … At C, rate is the highest pressure is the highest
(ii) /
11 / (a) / Al(OH)3 + 3HCl AlCl3 + 3H2O OR
Al(OH)3 + 3H+ Al3+ + 3H2O
Common mistakes:
Al(OH)3 + 3HCl Al(Cl)3 + 3H2O
(b) / Pour all the solution obtained from Step I to a (250 cm3) volumetric flask.
Rinse all the solution left in the beaker by distilled water and transfer thewashing to the volumetric flask.
(Zero marks for the above two points if pipette is used)
Add distilled water to the mark of the volumetric flask and shake thevolumetricflask thoroughly.
Common mistakes:
- Pipette 25.0 cm3 / 50.0 cm3 of the solution …(>70% candidates made this mistake!)
- Pour the solution into a conical flask
- miss the procedure of ‘rinsing’, or ‘rinse’ without ‘transfer washing’
(c) / methyl orange: from red to orange / yellow OR
phenolphthalein:from colourless to pink / red
(purple not accepted)
litmus not accepted
(d) / (i) / No. of moles of excess hydrochloric acid
= No. of moles of NaOH = 0.20 (20.8 / 1000)
= 4.16103 (OR 4.2103 )
Common mistakes:
No. of moles of HCl= No. of moles of NaOH = 0.20 (20.8 / 1000)
= 4.16103
no. of moles of excess HCl
= 0.05 1.0 4.16103
(ii) / The amount of excess hydrochloric acid in the solution of Step I:
4.16103 (250 / 25) = 0.0416 mol (Step mark: OR)
No. of moles of hydrochloric acid reacts with aluminium hydroxide:
0.05 1.0 0.0416 = 0.0084
No. of moles of aluminium hydroxide in the tablet:
0.0084 / 3
= 2.8103 (OR 2.67103 )
very few candidates got the correct answer!
Common mistakes:
- No. of moles of HCl reacts with Al(OH)3 = 0.05 1.0 4.16103
- no. of moles of Al(OH)3 = nHCl / 3 = 4.16103 / 3
12 / (a) / saponification† / alkalinehydrolysis†
Common mistakes:
soaponification
(b) / concentrated sodium chloride(solution) / conc. NaCl(aq) /brine
Common mistakes:
NaCl solution
(d) /
Structure of white solid consists of hydrocarbon tail and anionichead.
Tail is hydrophobic. / dissolves in oil. Head is hydrophilic / dissolves in water.
Negatively (Same) charged oil drops repel from each otherforming an emulsion(turbid /oil droplets).
(e) / Preparation of soap/detergent OR Hydrolysis of caster oil
AND
Testing the emulsifying property of the product / cleaning action
Common mistakes:
Testing the function of soap. (what function?)
(f) / (White) precipitate (Scum) would be observed. (Accept: milky / turbid)
Common mistakes:
- No observable change
- Gas evolved
13 / A description of electroplating of iron:
a. / The protective layer plated on iron can be a metal such as nickel / chromium / copper / silver.(Accept Zn / Au / Sn) / 1
b. / Electrolyte used is an aqueous salt solution of the metal. Example: nickel(II) sulphate (solution)whole name or whole formulawith correct valency
If Au is chosen, accept any anion & valency (e.g. AuCl, Au2SO4 , …) / 1
c. / The metal (e.g. Ni) should be made anode (positive electrode / connected to positive pole of power supply). / 1
d. / The iron object should be made cathode (negative electrode / connected to negative pole of power supply). / 1
e. / The metal (e.g. Ni) (anode) is oxidised / loses electrons to form ions.
(Accept half equation: Ni Ni2+ + 2e-) / 1
f. / The metal ions (e.g. Ni2+) are reduced/gain electrons on iron (cathode) surface to form metal (e.g. Ni).
(Accept half equation: Ni2+ + 2e- Ni) / 1
Common mistakes:
-anodization:CK = 0 M
connect Fe as anode, OH are discharged to form O2, forming a thicker Fe2O3 layer.
-chemical cell:CK = 0 M
connect Fe as cathode and dip it into FeSO4, … connect with a salt bridge…
-wrong choice of metal (e.g. K, Na, Ca, Fe) and electrolyte: CK = 0 M
K is made as anode with KCl as electrolyte…
-inappropriate choice of metal (e.g. Mg, Al): deduct 1 mark for choice of ‘metal’
Mg is made as anode with MgSO4 as electrolyte…
-purification of Fe:
Fe is connected to the +ve terminal of d.c. power supply and another piece of Fe to the –ve terminal…
At anode: Fe Fe2+ + 2e; At cathode: Fe2++ 2e Fe
-sacrificial protection:
Zn is connected to Fe, Zn is more reactive and lose e to Fe, prevent Fe from losing e…
-other minor mistakes:
connect Fe to the cathode and Zn to the anode, molten zinc(II) sulphate as the electrolyte