Covalent Bonding &
Shapes of Molecules
Mark Scheme 1
Level
International A Level
Subject
Chemistry
Exam Board
CIE
Topic
Chemical Bonding
Sub-Topic
Covalent Bonding & Shapes of Molecules
Paper Type
Theory
Booklet
Mark Scheme 1
Time Allowed:
74 minutes
Score:
/61
Percentage:
/100
Grade Boundaries:
A*
A
B
C
D
E
U
>85%
777.5%
70%
62.5%
57.5%
45%
<45%
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Question
uestion
1
(a)
(b) (i)
point
Marks
oxygen: (1s2) 2s22p4
fluorine: (1s2) 2s22p5
1
F2O / OF2
1
(ii)
1
F
O
F
bent or non-linear
1
Eo values: F2 / F– = 2.87 V and Cl2 / Cl – = 1.36 V
1
fluorine (has the more positive Eo so) is more oxidising
1
(ii)
redox
1
(iii)
Cl F + 2KBr → KCl + KF + Br2
1
(iii)
(c) (i)
[Total: 8]
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2
(a)
(b) (i)
(ii)
(c) (i)
(ii)
Scheme
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Question
uesti
Mark
ark
The amount of energy required / energy change / enthalpy change when one electron is
removed
from each atom / (cat)ion in one mol
of gaseous atoms / (cat)ions
OR energy change when 1 mole of electrons is removed from one mole of gaseous
atoms / ions
X(g) → X+(g) + e– gains 2 marks
1
Group V / 5 / 15
1
Big difference between fifth and sixth ionisation energies
1
1s2 2s2 2p3
ecf from (b)(i) if period 2
1
(Weighted) mean / average mass of an atom(s) (of an element)
1
Relative to 1 / 12th of (the mass of an atom of) carbon-12 OR
relative to carbon-12 which is (exactly) 12 (units)
allow as an expression
1
Z
31.13
Ar
So
1
1
3
2
2
Cl
68.87
= 1: 2
35.5
68.87/35.5
=2
31.13/Ar
2 × 31.13 × 35.5
= 32.0923 = 32.1 to 3s.f.
68.87
Allow alternative correct methods
Ar =
T
1
1
2
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(d) (i)
Scheme
NaCl (+ aq ) NaCl + H2O SiCl 4 + 2H2O SiCl 4 + 4H2O SiCl 4 + 4H2O www.acealevel.com
Question
uesti
Mark
ark
Na+ + Cl –
Na+ + Cl – + H2O
1
SiO2 + 4HCl
Si(OH)4 + 4HCl
SiO2.2H2O + 4HCl
1
2
Allow correct equation with other molar amounts of water
(ii)
(e)
T
NaCl is ionic AND giant / lattice
NaCl dissolves / does not react
SiCl 4 is covalent AND molecular / simple
SiCl 4 is hydrolysed / reacts
1
1
1
1
4
shape of SF6 = Octahedral
bond angle = 90°
1
1
2
18
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3
(a
(i)
S
C
S
S atom has 6 and C atom has 4 electrons
(1)
S=C double bonds (4 electrons) clearly shown
(1)
(ii) linear and 180°
(b)
b) (
(1)
1)
CS2 + 3O2 → CO2 + 2SO2
(1)
(ii) enthalpy change when 1 mol of a substance
(1)
is burnt in an excess of oxygen/air
or is completely combusted
under standard conditions
(1)
[3]
(c)
CS2 + 3O2 →
+
CO2
∆Hf⦵/kJ mol–1 x
–395
∆Hreaction = –395 + 2(–298) – x = –1110 kJ mol–1
gives x = –395 + (–596) + 1110 = +119 kJ mol–1
(d)
d) (
2SO2
2(–298)
(1)
(1)
(1) [3]
CS2 + 2NO → CO2 + 2S + N2
or
CS2 + 2NO → CO + 2S + N2O
correct products (1)
correct equation (1)
(ii) from –2 to 0
both required (1)
[3]
[Total: 12]
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4
(a)
NH3
CH4
pyramidal
tetrahedral
both ‘dot-and-cross’ diagrams correct
NH3 is pyramidal or trigonal pyramidal
CH4 is tetrahedral
(b)
b) (
nitrogen and hydrogen have different electronegativities
N–H bond has a dipole or
Nδ–  Hδ+ or
bonding pair is unequally shared
(1)
(1)
(1)
[3]
(1)
(1)
(ii) molecule is not symmetrical or
dipoles do not cancel out
(1)
(iii) NH3 has higher boiling point than expected from Mr value or
has higher boiling point than methane
or NH3 is soluble in water
(1)
[4]
(1)
(1)
(1)
[3]
(c) three covalent N–H bonds
one co-ordinate (dative covalent) N–H bond
one ionic bond between NH4+ and Cl –
[Total: 10]
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5
(a) fewer electrons in Cl 2 than in Br2 (1)
smaller van der Waals’ forces in Cl 2 or stronger van der Waals’ forces in Br2 (1)
[2]
(b) CO has a permanent dipole or N2 does not (1)
permanent dipole-permanent dipole interactions are stronger than those from induced
dipoles (1)
[2]
(c) (i) a co-ordinate bond (1)
(ii) a covalent bond (1)
or
(iii) a lone pair (1)
or
penalise any groups of 3 or 4 electrons that are circled
[3]
(d) CO and HCN both have a dipole or N2 does not have a dipole (1)
[1]
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(e) (i)
H H
 
HCCOH
 
H C≡N
C≡N must be shown (1)
(ii) nucleophilic addition (1)
(iii)
δ+ δ–
CH3C=O

H CN–
H
|
CH3CO–
|
CN
HCN
H

CH3COH + CN–

CN
C=O dipole correctly shown or correct curly arrow on C=O (1)
attack on Cδ+ by C of CN– (1)
correct intermediate (1)
CN– regenerated (1)
[5 max]
[Total: 13]
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