Studying three experiments:

1
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??????????? ?????????????? ????????????? Studying
three experiments Cracking of paraffin/cracking
of ethanol/ oxidation of ethanol under reflux
2
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CH3CH2OH ? CH2CH2 H2O (Al2O3 as catalyst)
O
reflux
3

• Y belongs to group II.
• X(s) 2HCl(g) ? XCl2(aq) H2(g)
• a) sulphur dioxide, SO2 , which will turn wet
  orange dichromate paper green. or use
  KMnO4/H(aq)
• b) Blue solution turns to pale blue, brown solid
  forms and metal X dissolves.
• Polyethene (or polythene)
• a) Condensation polymeriation.
• b)

4

• a) (25 cm3) Pipette.
• b) H2SO4 2NaOH ? Na2SO4 2H2O
• no. of mol of NaOH used 0.0182 x 0.10
  0.00182 mol
• in 25 cm3 diluted solution, no. of mol of H2SO4
  0.00182/2 0.00091 mol
• in 500 cm3 diluted solution, no. of mol of H2SO4
  0.00091 x 20 0.0182 mol
• no. of mol of H2SO4 in 1.0 cm3 of domestic drain
  cleaner 0.0182 mol
• molarity of H2SO4 in Rainbow 0.0182 / 0.001
  18.2 M

5

• C7H16 (7 4)O2 ? 7CO2 8H2O
• C7H16 11O2 ? 7CO2 8H2O
• 8. Heat from combustion engine turns N2 and O2 in
  air to NO and NO is further oxidized by O2 to
  NO2.
• No. of mol of Cu No. of mol of O
• 7.62 / 63.5 (8.58 7.62) / 16 0.2 0.06
  2 1
• Empirical formula be Cu2O.
• i.e. Cu2O H2 ? 2Cu H2O or
• Cu2O CO ? 2Cu CO2

6
10.
Deduct mark for No indication of heat Closed
system Incorrect direction of water flow
7

• a) At R, Cu2 2e ? Cu. Therefore, electrons
  flow from Q to R in external circuit. (or from S
  to P / Y to X through the ammeter)
• b) i)Gas bubbles evolved at S.(4OH- ? O2 2H2O
  4 e )
• ii)Blue copper(II) sulphate solution turns to
  pale blue.
• c) Set-up X acts as an electrochemical cell.

8

• Solid lead contains mobile electrons (delocalized
  electrons) while ions in solid lead(II) bromide
  are not mobile (delocalized). Thus solid lead
  conducts electricity while solid lead(II) bromide
  doesnt.
• 2H2O2 ? 2H2O O2 (g)
• no. of mol of O2 produced 0.082 / 24 0.0035
  mol
• no. of mol of H2O2 2 x 0.0035 0.007
• molarity of H2O2 0.007 / 0.05 0.14 M

9
Marker reports
Wrong !
10
Marker report
Q.6 Many candidates forgot to include the
dilution factor in calculation. Q.9 Even though
many candidates were able to deduce correctly the
mole ratio of Cu to O, but quite a number of them
were not confident enough to put down the
empirical formula as Cu2O. Q.11 Set-up X some
just said that was used to complete the
circuit. Q.12 It is recommended that free
electrons should be replaced by more proper
terms such as delocalized electrons.