Group II Chemistry - Suggested Solutions
Topic 9.2: Group II Chemistry
ACJC/2009/P2/Q2a-c
1
ACJC/2009/P3/Q2b
2
149
Group II Chemistry - Suggested Solutions
AJC/2009/P2/Q3a
3
CJC/2009/P3/Q1a-b
4(a)
(i)
KNO3 → KNO2 + ½O2
(ii)
Mg(NO3)2 → MgO + 2NO2 + ½O2
NO2, which is produced from decomposition of magnesium nitrate, is toxic/a pollutant.
OR
Decomposition of magnesium nitrate occurs more readily than potassium nitrate,
resulting in a loss in control of burning rate at the high temperatures reached in
fireworks.
OR
Potassium nitrate releases more mols of O2 per mass than magnesium nitrate because
of its lower Mr.
(b)
Let the unknown Group II metal be M.
2 Al +
3
O2 → Al2O3
2
No. of mols of O2 =
0.0141 3
× = 0.01058
2
2
Mass of KNO3 = 2 × 0.01058 [39.1 + 14.0 + 3(16.0)] = 2.138 g
Mass of MCO3 = 2.75 – 2.138 = 0.612 g
MCO3 → MO + CO2
No. of mols of CO2 =
Mr(MCO3) =
100
= 4.167 × 10-3
24000
0.612
4.167 × 10 −3
= 146.9
Ar(M) = 146.9 – 12.0 – 3(16.0) = 86.9 ≈ 87.6
Therefore, M is strontium.
150
Group II Chemistry - Suggested Solutions
DHS/2009/P2/Q2a-f
5
This question is about a Group II element, barium and the reactions of its compounds.
(a)
Ba(NO3)2 BaO(s) + 2NO2(g) + ½ O2(g)
(b)
Decomposition temperature of barium nitrate will be higher than calcium
nitrate. Ba2+ has lower polarizing power, the nitrate electron cloud
polarized to a lower extent, therefore easier to decompose, hence lower
decomposition temp. Ca2+ higher polarizing power due to smaller size
and hence nitrate anion is polarized to a larger extent.
(c)
(d)
BaCl2(s) + conc H2SO4(l) → 2HCl(g) + BaSO4(s)
(e)
HBr, SO2 or Br2
(f)
Cl2+2e
Br2 + 2e
2Cl- Eo =+1.36V
2Br- Eo =+1.07V
Br– is a better reducing agents than Cl–. Br– can reduce conc. H2SO4 to
SO2 but not Cl–.
⇒ Ease of oxidation from X – → X2 increases from Cl– to Br- due to the less
positive Eo value.
HCI/2009/P2/Q1a
6 (a) The alkaline earth metals exhibit +2 oxidation state in their compounds because
they have 2 valence electrons which are lost to achieve stable octet.
They do not exhibit +1 oxidation state due to the low lattice energy of the
compounds formed.
+3 oxidation state is also not favourable due to the very high 3rd ionization energy
required to remove the third electron from the inner quantum shell.
151
Group II Chemistry - Suggested Solutions
HCI/2009/P3/Q3b
7
(b) Mg(OH)2 is basic, and reacts only with acids:
Mg(OH)2(s) + 2HCl (aq) → MgCl2(aq) + 2H2O(l)
Be(OH)2 is amphoteric, and reacts with both acids and bases:
Be(OH)2(s) + 2HCl (aq) → BeCl2(aq) + 2H2O(l)
Be(OH)2(s) + 2NaOH(aq) → Na2Be(OH)4(aq)
IJC/2009/P3/Q1c
8
(c)
MI/2009/P2/Q3
(a)
Size of Sr2+ is smaller than that of Ba2+,
Charge density of Sr2+ is greater than that of Ba2+,
[½]
[½]
∴ Sr2+ is better able to distort the electron cloud of CO32-,
[½]
weakening the C-O bond, making it easier to decompose SrCO3.
[½]
⇒ Decomposition temperature of SrCO3 is lower than that of BaCO3.
;
MJC/2009/P3/Q2c
10
(iii) 5Ba(IO3)2
Ba5(IO6)2 + 4I2 + 9O2
Ionic size hence charge density hence polarising power of of Ca2+ > Ba2+
Hence, Ca2+ has greater ability to distort the anion IO3- charge/electron cloud
Hence, thermal stability of Ca(IO3)2 > Ba(IO3)2.
[3]
152
Group II Chemistry - Suggested Solutions
PJC/2009/P2/Q1d
11
RI/2009/P2/Q3c
12
(c)(i) CaCO3 → CaO + CO2
Amt of CaO = 1.00/100.1 x 56.1 = 0.560 g
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Group II Chemistry - Suggested Solutions
(c)(ii)
Mass/ g
1.00
Decomposition curve of
CaCO3
x
Decomposition curve of
MgCO3
0
time
(c)(iii) From Data Booklet
Mg2+
0.065
ion
Ionic radius
Ca2+
0.099
The thermal stability depends on the charge density of the cation.
The greater the charge density,
the thermally less stable the carbonate.
Since charge density and hence polarsing power of Mg2+ is higher than that of
Ca2+,
the distortion of the electron cloud of the carbonate anion, thus weakening effect
of the carbon−
−oxygen bonds in the magnesium carbonate occurs to a greater
extent.
Hence MgCO3 is more unstable than CaCO3 and should decompose to MgO and CO2
at a faster rate
(since its decomposition temperature is lower than that of CaCO3.)
The mass of MgO obtained is 0.478 g, lower than x (i.e. 0.560 g)
SAJC/2009/P3/Q4b
13
(i)
2Ba(NO3)2
(ii)
2BaO + 4NO2 + O2
The temperature of the thermal decomposition increases down the
group. This is because down the group,
-
the size of the cations increase
-
hence polarizing power of cations decrease
-
ability of cations to distort the large anion decrease
-
nitrates are more stable to heat
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Group II Chemistry - Suggested Solutions
(iii)
L.E is proportionate to ionic charge but inversely proportionate to
ionic radius
or L.E. α
[5]
q+q−
r+ + r−
- Ionic size of nitrate ion bigger than that of chloride
- Hence, LE for barium nitrate is SMALLER than barium chloride.
SRJC/2009/P3/Q2a,b
14 (ai) MgO
has a giant ionic lattice structure
large amount of energy is needed to
overcome the strong ionic bonds
OR electrostatic forces of attraction between oppositely charged ions.
has a high boiling point
P 4O 6
has a simple molecular structure
small amount of energy is needed to
overcome the weak intermolecular Van der Waals’ forces of attraction
has a low boiling point
(b)
Al2O3 (s) + 6HCl (aq) 2AlCl3(aq) + 3H2O (l)
{no need state symbol}
{½ m species, ½ m for coefficient}
SO2 (g) + 2NaOH (aq) Na2SO3 (aq) + H2O (l)
{no need state symbol}
{½ m species, ½ m for coefficient}
Al2O3(s) + 2NaOH(aq) + 3H2O(l) 2Na[Al(OH)4] (aq) {no need state symbol}
{½ m species, ½ m for coefficient}
Al2O3 behaves as an amphoteric oxide or
Al2O3 is able to react with both acid and base
Mg(NO3)2(s) MgO(s) + 2NO2 (g)+ ½ O2 (g) {no need state symbol}
{½ m species, ½ m for coefficient}
Down the group,
Ionic radius of cation: Mg2+ < Sr2+.
Charge density of cation: Mg2+ >Sr2+ / decreases
Polarising power of cation: Mg2+ > Sr2+ / decreases
or
Polarising effect on NO3- anion: Mg2+ > Sr2+ / decreases
Mg(NO3)2 is less stable than Sr(NO3)2, hence it decomposes at a lower
temperature
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Group II Chemistry - Suggested Solutions
Thermal stability of Group II nitrates increases down the group; hence barium nitrate
would decompose at a higher temperature compared to strontium nitrate.
TJC/2009/P3/Q4a
15 (a) (i)
MgO (s) + 2HNO3 (aq) → Mg(NO3)2 (aq) + H2O (l)
0.500
= 0.0124 mol
Amount of MgO =
24.3 + 16.0
•
•
(ii)
50
× 0.100 = 0.00500 mol (limiting reagent)
1000
0.005
Amount of Mg(NO3)2 produced =
= 0.00250 mol
2
Amount of HNO3 =
Mg(OH)2 + 2NH4NO3 → Mg(NO3)2 + 2NH3 + 2H2O
1. 5
= 0.0101 mol
24.3 + 28.0 + 96.0
•
Amount of Mg(NO3)2 =
•
Volume of NH3 produced = 0.0101 × 2 × 24 = 0.485 dm3
(iii)
2Mg(NO3)2 → 2MgO + 4NO2 + O2
•
Brown gas of NO2 will be observed.
(iv)
•
•
Ba(NO3)2 is more thermally stable than Mg(NO3)2, hence it will not
decompose at 330°C.
Ba2+ is less polarizing / lower charge density than Mg2+ as it has a larger
size, hence the NO3- is polarised to a smaller extent in Ba(NO3)2.
156
Group II Chemistry - Suggested Solutions
TPJC/2009/P3/Q2
16
157
Group II Chemistry - Suggested Solutions
YJC/2009/P2/Q2
17
158