1. Why are pentahalides more covalent than trihalides? In
pentahalides, the oxidation state is more (+5) than in trihalides (+3).
As a result of higher positive oxidation state of central atom, they
have larger polarizing power and can polarise the halide ion (X-)
to a greater extent than in the corresponding trihalide. Since larger
the polarisation, larger is the covalent character, therefore,
pentahalides are more covalent than trihalides.
2. Why is BiH3 the strongest reducing agent amongst all the hydrides of the group 15?
Among the hydrides of group 15, BiH3
is least stable because Bi has largest size in the group and has least
tendency to form covalent bond with small hydrogen atom. Therefore, it
can readily lost H atom and has strongest tendency to act as reducing
agent.
3. Mention the conditions required to maximise the yield of ammonia.
Ammonia is formed according to the reaction:
The conditions for maximum yield of ammonia are: - Low temperature of the order of about 700 K.
- High pressure of 200 × 105 Pa (about 200 atm).
- Presence of catalyst such as iron oxide with small amount of K2O and Al2O3
4. How does ammonia react with a solution of Cu2+? Ammonia reacts with a solution of Cu2+ to form deep blue coloured complex, tetraamminecopper (II) ion: 
5. Bond angle in PH4+ is higher than that in PH3. Why?
Both PH4+and PH3 involves sp3 hybridisation of P atom. In PH4+all the four orbitals are bonded, whereas in PH3 there is a long pair of electrons on P. In PH4+,the HPH bond angle is tetrahedral angle of 109.5o. But in PH3,
lone pair-bond pair repulsion is more than bond pair-bond pair
repulsion so that bond angles become less than normal tetrahedral angle
of 109.5o. The bond angle in PH3 has been found to be about 93.6o.
6. Why is H2O a liquid and H2S a gas?
Due
to high electronegativity of oxygen and its small size, there are
strong hydrogen bonding in water. As a result, the molecules exists as
associated and is liquid at room temperature. But there is negligible
hydrogen bonding in H2S because of low electronegativity of S.
7. Why does O3 act as a powerful oxidising agent?
Ozone acts as a powerful oxidising agent because it has higher energy content and decomposes readily to give atomic oxygen as:
O3 ? O2 + O
Therefore, ozone can oxidise a number of non-metals and other compounds. For example:
PbS (s) + 4O3(g) ? PbSO4 (s) + 4O2(g)
2I- (aq) + H2O(l) + O3(g) ? 2OH (aq) + I2(s) + O2(g)
8. How is ozone estimated quantitatively?
Ozone oxidises potassium iodide to iodine as:
2KI + O3 + H2O ? 2KOH + O2 + I2
The liberated iodine may be titrated against a standard solution of sodium thiosulphate.
I2 + 2Na2S2O3 ? Na2S4O6 + 2Nal
Thus, to estimate O3
quantitatively, ozone is allowed to react with known amount of excess
potassium iodide solution buffered with a borate buffer (pH = 9.2). The
liberated I2 is titrated against Na2S2O3 solution using starch as an indicator. From this amount of ozone can be calculated.
9. How is the presence of SO2 detected? - It has a pungent characteristic smell.
- It decolourises acidified potassium permanganate solution.
- It turns acidified potassium dichromate solution green.
- It turns blue litmus red.
10.
Considering the parameters such a bond dissociation enthalpy, electron
gain enthalpy and hydration enthalpy, compare the oxidising power of F2 and Cl2. F2, is stronger oxidising agent than Cl2.
This can be explained on the basis of bond dissociation enthalpy,
electron gain enthalpy and hydration enthalpy. The process of oxidising
behaviour may be expressed as: 
The
overall tendency for the change (i.e., oxidising behaviour) depends
upon the next effect of three steps. As energy is required to dissociate
or convert molecular halogen into atomic halogen, the enthalpy change
for this step is positive. On the other hand, energy is released in
steps (II) and (III), therefore, enthalpy change for these steps is
negative. Now although fluorine has less negative electron gain
enthalpy, yet it is stronger oxidising agent because of low enthalpy of
dissociation and very high enthalpy of hydration. In other words, large
amount of energy released in step (III) and lesser amount of energy
required in step (I) overweigh the smaller energy released in step (II)
for fluorine. As a result, the ?H overall is more negative for F2 than for Cl2. Hence F2 is stronger oxidising agent than Cl2.
11. Give two examples to show the anomalous behaviour of fluorine. - Since
fluorine is most electronegative element, it shows only a negative
oxidation state of -1. It does not show any positive oxidation state. On
the other hand, the other halogens show positive oxidation states also
such as +1, +3, +5, +6 and +7.
- Maximum covalency of fluorine is
one because it cannot expand its valency shall beyond octel because
there are no d-orbitals in the valence shell. On the other hand, other
elements can exercise covalencies upto because of availability of vacant
d-orbitals.
12. Give the reason for bleaching action of chlorine. Bleaching action of chlorine is due to its oxidation. In the presence of moisture, chlorine gives nascent oxygen Cl2 + H2O ? 2HCl + O Because of nascent oxygen, it bleaches colouring substance as : Clouring
substance + O ? Colourless substance. It bleaches vegetables or organic
matter. The bleaching action of chlorine is permanent. 13. Why does the reactivity of nitrogen differ from phosphorus? Nitrogen
has a unique ability to form p?-p? multiple bonds with itself and with
other elements having small size and high electronegativity (e.g., C,
O). Therefore, it has triple bond between two nitrogen atom (N ? N) and
is non-polar. Due to triple bond it has very high bond enthalpy (941.4
kJ) and therefore, it does not react with other elements under normal
conditions and is very unreactive. On the other hand, phosphorus forms
single bond (P-P) and is reactive in comparison to nitrogen. 14. Why does NH3 forms hydrogen bonds but PH3 does not ? Because
of high electronegativity and small size of nitrogen, ammonia forms
hydrogen bonds. On the other hand, P has low electronegativity and large
size and hence cannot form hydrogen bonds. 15. Illustrate how copper metal can give different products on reaction with HNO3. Concentrated
nitricacid is a strong oxidising agent and reacts with metals. The
products of oxidation depends upon the concentration of the acid,
temperature and the nature of the material undergoing oxidation. For
example, copper reacts with HNO3 giving different products as: Conc. HNO3 gives copper nitrate and nitrogen dioxide. Cu + 4HNO3 ? Cu(NO3)2 + 2NO2 + 2H2O
conc. Dilute HNO3 gives copper nitrate and nitric oxide. 3Cu + 8HNO3 ? 3CU (NO3)2 + 2NO + 2H2O 16. Explain why NH3 is basic while BiH3 is only feebly basic. Both N and Bi have a lone pair of electrons in NH3 and BiH3 respectively. They can donate the electron pair and therefore behave as Lewis base. In NH3,
N has small size and the lone pair is concentrated on a small region
and electron density on it is maximum. Consequently, it has greater
electron releasing tendency. But the size of Bi is large and the
electron density of the lone pair is less. As a result, it has lesser
tendency to donate electron pair. Hence, NH3 is basic while BiH3 is only feebly basic. 17. Can PCI5 act as an oxidising as well as a reducing agent? Justify. Phosphours can show maximum odixation state of +5 in its compounds. In PCI5,
oxidation state is +5. Since it, cannot increase its oxidation state
beyond +5, it cannot act as a reducing agent. However, it can act as an
oxidising agent by undergoing decrease in its oxidation state from +5 to
+3. For example, it oxidises silver to AgCl, Sn to SnCl4 etc.
+5 +1 +3
2Ag + PCl5 ? 2AgCl + P Cl3
18. Why is dioxygen a gas but sulphur a solid?
Due
to small size and high electronegativity, oxygen atom forms p?-p?
double bond, O = O. The intermolecular forces in oxygen are weak van der
Waals forces and therefore, oxygen exists as a gas. On the other hand,
sulphur does not form stable p?-p? bonds and do not exists as S2. It is linked by single bonds and form polyatomic complex molecules having either atoms per molecule (S8) and have puckered ring structure. Therefore, S atoms are strongly held together and it exists as a solid.
19. Why are halogens coloured?
All
the halogens are coloured. This is due to absorption of radiations in
the visible region which results in the excitation of outer electrons to
higher energy levels. By absorbing different quanta of radiations, they
display different colours. Fluorine atom is the smallest and the force
of attraction between the nucleus and the outer electrons is very large.
As a result, it requires large excitation energy and absorbs violet
light (high energy) and therefore, appears pale yellow. On the other
hand, iodine needs very less excitation energy and absorbs yellow light
of low energy. Thus, it appears dark violet. Similarly, we can explain
the greenish yellow colour of chlorine and reddish brown colour of
iodine.
20.. Why do noble gases have comparatively large atomic sizes?
In case of noble gases, the atomic radii correspond to van der Waals radii, which are always large.
21.. Give reasons: - PCl5 is known but NCl5 is not known.
- Amongst all noble gases only xenon is known to form compounds with oxygen and fluorine.
- Nitrogen
does not have vacant d-orbitals in its valence shell. Therefore, it
cannot extend its valency beyond 3. On the other hand, phosphorus has
vacant 3d-orbitals in the valence shell and therefore, one electron can
be easily promoted from valence 3s-orbital to vacant 3d-orbital. Thus,
it can exhibit pentavalency in its compounds.
- Xenon has bigger
size and lower ionisation enthalpy than other noble gases. As a result,
it is the only noble gas which is able to form compounds with
electronegative oxygen and fluorine.
22. Why is SO2 a better reducing agent in alkaline sodium than in acidic medium?
SO2 acts as a reducing agent because of the liberation of nascent hydrogen in the presence moisture:
Alkali neutralizes the acid (H2SO4)
and shifts the equilibrium to the forward direction producing more
nascent hydrogen. However, in acidic medium, the equilibrium is
suppressed resulting in a lesser amount of nascent hydrogen.
Consequently, SO2 is better reducing agent in alkaline medium than in the acidic medium.
23. Elemental phosphorus does not exist as P2 like N2, why?
Nitrogen
has a strong tendency to form multiple bonds because of its small size
and high electronegativity. Therefore, it exists as a diatomic molecule,
N ? N. On the other hand, phosphorus because of its large size and
small electronegativity des not show any tendency to form multiple bonds
and therefore, diatomic molecule like P ? P is not formed. Instead, it
prefers to form stable tetra atomic, P4 molecules in which each P is linked to three other P atoms by three single covalent bonds. The four atoms in P4 molecule lie at the corners of a regular tetrahedron.
24. NCl3 is readily hydrolysed while NF3 does not. Explain.
In NCl3, Cl has vacant d-orbitals to accept the lone pair of electrons donated by oxygen atom of H2O molecules. But in NF3, F does not have vacant d-orbitals. Therefore, NCl3 undergoes hydrolysis but NF3 does not.
2NCl3 + 3H2O ? NH3 + 3HOCl
NF3 + H2O ? No reaction
25.. H3PO2 and H3PO3 act as good reducing agents but H3PO4 does not. Why?
The structures of H3PO2 and H3PO4 are:
Due to the presence of P - H bonds, both H3PO2 and H3PO3 act as reducing agents. On the other hand, H3PO4 does not have any P - H bond and hence it does not act as a reducing agent.
26. Why does chlorine water lose its yellow colour on standing ?
Chlorine
water is yellow due to the presence of hypochlorous acid (HClO) in it.
HClO is unstable and decomposes to form HCI on standing. Therefore,
yellow colour disappears.
Cl2 + H2O ? HCl + HClO
Yellow
HClO ? HCl + O
Colourless
27.. Iodine is liberated when KI is added to CuSO4 solution but Cl2 is not liberated when KCl is added to CuSO4 solution. Why?
The I-is a strong reducing agent and therefore, it reduces Cu2+ to Cu+ and itself gets oxidised to I2.
2Cu2+ + 4KI ? Cu2I2 + I2 + 4K+
But Cl- does not act as a reducing agent and therefore, it cannot reduce Cu2+ to Cu+ and hence Cl2 is not liberated.
28.. SOCl2 can act as a weak Lewis acid as well as a weak Lewis base. Explain.
The basic character of SOCl2 is due to the presence of a lone pair of electrons on S atom, like ammonia. It has a pyramidal structure involving sp3 hybridisation with a lone pair of electrons as:
So, Lewis basic character is due to the presence of a lone pair. In addition, SOCl2 has also empty d-orbitals which can be used to accept electron pairs and hence it behaves as a Lewis acid.
29. Why is nitrous oxide called laughing gas?
When nitrous oxide (N2O) is inhaled in minor quantities, it causes hysterical laughter and therefore, it is called laughing gas.
30. What is the action of heat on - Pyrophosphoric acid
- Metaphosphoric acid
- Phosphorous acid
- Pyrophosphoric acid (H4P2O7) on heating gives metaphosphoric acid.
2. Metaphosphoric acid (HPO3) on heating gives phosphorous pentoxide.
3. Phosphorous acid (H3PO3) on heating gives phosphine and phosphoric acid.
31.. What structures does PCI5 adopt in the solid state and vapour state?
In the solid state PCI5 is ionic compound consisting of [PCI4]+ (tetrahedral) and [PCl6]- (octahedral) ions. In vapour state, PCI5 is monomeric having trigonal bipyramidal geometry.
. SF6 has zero dipole moment while SF4 has non-zero dipole moment. Why?
SF6
has symmetrical octahedral structure. In this structure, bond moments
cancel each other and resulting dipole moment is zero. On the otherhand,
SF4 has a trigonal bipyramidal structure in which one
position is occupied by a lone pair. In this case the resultant dipole
moment of individual bond moments is not zero.
 ? ? 0 ? = 0
32.. Molten ICl has high electrical conductivity, why?
The high electrical conductivity of molten ICl is because of its ionization into ICl2+ and ICl4- ions. 
33.
A translucent white waxy solid (A) on heating in an inert atmosphere is
converted to its allotropic form (B). Allotrope (A) on reaction with
very dilute aqueous KOH liberates a highly poisonous gas (C) having
rotten fish smell. With excess of chlorine forms (D) which hydrolyses to
compound (E). Identify compounds (A) to (E). (A) The white waxy solid (A) is white phosphorus. When white phosphours is heated in an inert atmosphere at 573K, it changes to red phosphorus. (B) is red phosphorus. (A) on heating with KOH liberates phosphine (C) which is poisonous gas with rotten fish smell. P4 + 3KOH + 3H2O ? PH3 + 3KH2PO2 Phosphine (C) white phosphorus (P4) burns with excess of Cl2 to form phosphorus pentachloride (D).  (D) Hydrolysis of (D) gives phosphoric acid (E). PCl5 + 4H2O ? H3PO4 + 5HCl (E)
34..
When a mixture of ammonium chloride and potassium dichromate are
heated, a stable colourless gas (A) was evolved which did not support
combustion but magnesium continued
to burn in it. The gas (A) reacted with calcium carbide in an electric
furnace forming a solid (B). The compound (B) was slowly hydrolysed by
water forming an insoluble substance (C) and the solution of substance
(D) which turned Nessler's reagent brown. Identify (A) to (E) and give
the reactions involved. 
35.. When conc. H2SO4
was added to an unknown salt present in a test tube, a brown gas (A)
was evolved. The gas intensified when copper turnings were also added
into this test tube. On cooling the gas A changed into a colourless gas
(B). (i) Identify the gases (A) and (B). (ii) Write the equations for the reactions involved. The given salt is a nitrate salt which on reaction with conc. H2SO4 gives first vapours HNO3 which decompose to give brown gas (NO2) 
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