Chemistry - p-Block elements
[Time: 90Mins]
SOLUTION OF ASSIGNMENT
[Max. Marks : 35]
General Instructions
1.
All questions are compulsory.
2.
Question nos. 1 to 5 are very short answer questions and carry 1 mark each.
3.
Question nos. 6 to 10 are short answer questions and carry 2 marks each.
4.
Question nos. 11 to 15 are also short answer questions and carry 3 marks each
5.
Question nos. 16 is long answer questions and carry 5 marks each
6.
Use log tables if necessary, use of calculators is not allowed.
7.
p-Block elements elements chapter Weight-age 8 marks.
1.
Sol:
Explain why phosphorus form PF5 while Nitrogen does not form NF5.
[1 MARK]
Phosphorus has vacant d-orbital whereas Nitrogen does not has d-orbital. Hence, due to non-availabiltiy of dorbital phosphorus forms PF5 but nitrogen does not form NF5.
[1 MARK]
2.
Sol:
The solubility of noble gases in water increases on going down the group. Explain.
[1 MARK]
The solubility of noble gases in water, increases form He to Xe because the size of the atom increases and,
hence, the ease of polarization of the atom also increases down the group.
[1 MARK]
3.
Sol:
Why does PCl3 fume is moisture?
In presence of moisture (H2O), PCl3 undergoes hydrolysis giving fumes of HCl.
[1 MARK]
4.
Sol:
PCl3 + 3H2O → H3PO3 + 3HCl
Why fluorine forms only one oxoacid, HOF
Due to high electro negativity and small size fluorine forms only one oxoacids.
[1 MARK]
[1 MARK]
[1 MARK]
5.
Sol:
Noble gases have largest radii. Explain.
In noble gases, we can measure only vander Waals' radii which are larger than covalent radii.
[1 MARK]
[1 MARK]
6.
Sol:
Give two examples to show the anomalous behaviour of fluorine.
(1/2 Mark for each point)
The anomalous behavior of fluorine is due to its
1. small size
2. highest electornegativity
3. low F-F bond dissociation enthalpy
4. non-availability of d-orbitals in its valence shell.
[2 MARKS]
7.
Sol:
Write two uses of ClO2.
[2 MARKS]
(1 Mark for each point)
1. ClO2 is a powerful oxidizing agent and chlorinating agent.large quantities of clo2 are used for bleaching
wood pulp and cellulose and for purifying drinking water.
2. It is an excellent bleaching agent. its bleaching power is about 30 times higher than that of cl2and is used
for bleaching flour to make white bread.
8.
What is catenation ? In which element this property is maximum ?
Ideal 21st Century Competitions
[2 MARKS]
www.21stideal.com 1
Sol:
9.
Sol:
Catenation is the self linking property of an element. This property is maximum in carbon because:
[1 MARK]
(i) Valency of carbon is greater than two.
[1/2 MARK]
(ii) Carbon can form strong covalent bond with itself as compared to other element like oxygen.
[1/2 MARK]
Decolourization of flowers
ers by SO2 is temporary while chlorine decolourize permanently. Why ?
[2 MARKS]
Chlorine decolourize flowers by the process of oxidation, so it is stable.
Cl 2 + H 2O→
→HC1 + HOC1
HOC1→
→HC1 + [O]
Coloured flower+ [O] → Colourless
[1 MARK]
On the other hand decolourization by SO2, is unstable because the process is reduction and decolourized
flowers again oxidize with oxygen of atmosphere.
SO2 + 2H2O→
→ H2SO4 + 2[H]
Coloured flower+[H]→
→ Colourless flower
[1 MARK]
10.
Sol:
Give the resonating structures of NO2 and N2O5.
Resonating structures of NO2 are :
[2 MARKS]
[1 MARK]
Resonating structures of N2O5 are :
etc.
11.
Sol:
[1 MARK]
Give reasons for the following :
[3 MARKS]
(i) Nitric oxide becomes brown when released in air.
(ii) Solid phosphorus pentachloride exhibits some ionic character. Or PCI5 is ionic in nature in the solid state,
(iii) Ammonia is a good complexing agent.
(1 Mark for each point)
(i) Nitric oxide readily combines with O2 of air to form nitrogen dioxide which is brown in colour.
(ii) Solid PCl5 exist as [PC14]+, [PC16]- and hence exhibits some ionic character,
(iii) Due to the presence of lone pair of electrons on N, NH3 acts as a ligand.
12.
Why does the reactivity of nitrogen differ from phosphorus?
phosphorus
Sol:
Nitrogen exists as a diatomic molecule (N ≡ N). Due to the presence of a triple bond between the two NN
atoms, the bond dissociation energy is large (941.4 kJ mol-1). Thus, nitrogen is inert and uncreative in its
elemental state.
Ideal 21st Century Competitions
[3 MARKS]
2
w
www.21stideal.com
In contrast, white or yellow phosphorus exists as a tetratomic molecule (P4). Since the P-P single bond is
much weaker (213 kJ mol-1) than N ≡ N triple bond (941.4 kJ mol-1), therefore, phosphorus is much more
reactive than nitrogen.
[3 MARKS]
13
Sol:
14
Sol:
Deep sea divers now-a-days use a mixture of He (80%) and O2 (20%) in their breathing apparatus
instead of air (80% N2, 20% O2). Give reasons.
[3 MARKS]
Under the high pressure in the sea, He is less soluble in blood than N2. So, when a mixture of He and O2 is
used, the diver does not suffer from severe pain due to bubbling out of the gas from the blood when he comes
from the high pressure under the sea to the surface under atmospheric pressure. When a mixture of O2 and N2
is used, N2 dissolves in the blood of the diver under pressure in deep sea waters and he suffers a severe pain
due to bubbling out of large quantity of N2 from the blood when he reaches the surface. Thus, to avoid the
pains (called bends) a mixture of He and O2 is used.
[3 MARKS]
Present a comparative account of the following:
(i) Structures of the pentoxides of nitrogen and phosphorus.
(ii) Oxidising powers of fluorine and chloride.
(iii) Bond angles of H2O and H2S.
(1 Mark for each point)
[3 MARKS]
(i) Structures of N2O5 and P2O5 : In nitrogen pentoxide, pπ-pπ bonds are present but these bonds are absent
in phosphorus pentoxide. Therefore, N2O5 differs from P2O5.
(ii) Oxidising powers of fluorine and chlorine: the dissociation energy E1 of fluorine is very less as compare
to dissociation energy E2 of chlorine because fluorine is far smaller than chlorine molecule. Hence, fluorine
molecule is very powerful oxidizing agent than chlorine.
(iii) Bond angles of H2O & H2S : Bond angle of H2O is 105o while bond angle of H2S is 92.5o. It is because
oxygen is more electronegative than sulphur. Thus, higher polarity is developed on O & H atoms.
15.
Sol:
16.
Sol:
Discuss the behaviour of hydrides of halogen under following points:
[3 MARKS]
(i) Thermal stability,
(ii) Reducing property and
(iii) Acidic strength.
(1 Mark for each point)
(i) Thermal stability : Thermal stability of hydrides decreases down the group. This is due to decrease in
bond strength (bond length increases) down the group.
(ii) Reducing properties: Hydrides are strongly reducing in nature due to decrease in thermal stability
reducing character increases down the group.
(iii) Acidic strength: As the thermal stability decreases acidic strength increases down the group in the order
given below :
HF < HCl < HBr < HI
(least acidic) (most acidic)
Write three use each of (1) Helium (2) Neon (3) Argon (4) Krypton (5) Radon
[5 MARKS]
(1 Mark for each point)
1. Use of Helium
(i) The chief use of helium is in filling of balloons which are employed for meteorological observations.
Although lifting power of helium is 8% less than that of hydrogen, yet it is preferred because it is noninflammable.
(ii) Liquid helium is used as a cryogenic agent for carrying out various experiments at low temperatures. It is
used to produce and sustain powerful superconducting magnets which form essential part of modern NMR
spectrometers and Magnetic Resonance Imaging (MRI) systems used for clinical diagnosis.
(iii) Because of very low solubility of helium in blood, an oxygen – helium mixture is used for artificial
Ideal 21st Century Competitions
www.21stideal.com 3
respiration in deep sea diving instead of air because nitrogen present in air dissolves in air dissolves in blood
under high pressure when sea diver goes into deep sea. When he comes to the surface, nitrogen bubbles out
of the blood due to sudden release of pressure causing pain. This disease in called "bends".
(iv) Oxygen-helium mixture is also used in the treatment of asthma.
(v) Helium is also used for creating inert atmosphere during welding of Mg and Al which are easily
oxidizable.
(vi) Helium is chemically inactive and does not become radioactive. Hence it is used in gas cooled atomic
reactors as a heat transfer agent.
2. Uses of Neon
(i) Neon is mainly used in discharge tubes and fluorescent lamps for advertising purposes. Neon when used
in a discharge tube produces an orange red glow which can be seen at long distances even in mists and fogs.
However, when neon is mixed with other gases and used in discharge tubes, glows of different colours
known as neon signs are produced. These are widely used for advertising purposes. Neon bulbs are also used
in botanical gardens and green houses.
(ii) Neon has a remarkable property of carrying extremely high currents even under high voltage. Therefore,
neon is used in safety devices for protecting electrical instruments such as volt- meters, relays and rectifiers
from high voltage.
(iii) It is used in becon light as saftey signal for air navigators since its light has fog penetration power.
(iv) It is used for filling sodium vapour lamps.
3. Uses of Argon
(i) Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes such as
arc welding of metals and alloys. In the laboratory, it is used for handling substances which are air sensitive.
(ii) It is used infilling incandescent and fluorescent lamps where its presence retards the sublimation of the
filament and thus increases the life of the lamp.
(iii) It is also used in "neon signs" for obtaining lights of different colours.
4. Uses of Krypton
(i) Krypton and xenon are more efficient than argon in gas filled lamps because of their lower thermal
conductivities but due to their scarcity and high cost they are used to a much smaller extent.
(ii) A mixture of krypton and xenon has also been used in some tubes for high speed photography.
5. Uses of Radon
(i) Being radioactive, radon is used in radioactive research.
(ii) It is used for normal treatment of cancer and other malignant growths.
(iii) It is used in X-ray photography for the detection of flaws in metals and other solids
Ideal 21st Century Competitions
www.21stideal.com 4