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Title: chapter 10 s-block elements
Description: easy questions of ncerts solutions
Description: easy questions of ncerts solutions
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
Question 10
...
(1) They are quite soft and can be cut easily
...
(2) They are light coloured and are mostly silvery white in appearance
...
The density increases down
the group from Li to Cs
...
(4) The metallic bonding present in alkali metals is quite weak
...
(5) Alkali metals and their salts impart a characteristic colour to flames
...
When this excited electron reverts back to the ground state, it emits excess
energy as radiation that falls in the visible region
...
When metals such as Cs and K are irradiated
with light, they lose electrons
...
As we move down
the group, the reactivity increases
...
As we move down
the group, the reaction becomes more and more spontaneous
...
The
general reaction for the same is given as
(3) They react with dihydrogen to form metal hydrides
...
(4) Almost all alkali metals, except Li, react directly with halogens to form ionic halides
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
Since Li+ ion is very small in size, it can easily distort the electron cloud around the
negative halide ion
...
(5) They are strong reducing agents
...
However, lithium is an exception
...
It is because of its high hydration energy
...
These
solutions are conducting in nature
...
These solutions are
paramagnetic and if allowed to stand for some time, then they liberate hydrogen
...
In a highly concentrated solution, the blue colour changes to bronze and the solution
becomes diamagnetic
...
2:
Discuss the general characteristics and gradation in properties of alkaline earth metals
...
(i) The general electronic configuration of alkaline earth metals is [noble gas] ns2
...
Therefore, their oxidation state is +2
...
Also,
when moved down the group, the effective nuclear charge decreases and this causes an
increase in their atomic radii and ionic radii
...
However, their first ionization enthalpies are higher than the
corresponding group 1 metals
...
They are relatively less
soft as compared to alkali metals
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
(vi)Atoms of alkaline earth metals are smaller than that of alkali metals
...
These two factors cause alkaline
earth metals to have high melting and boiling points as compared to alkali metals
...
This is due to their low ionization
enthalpies
...
(viii) Ca, Sr, and Ba impart characteristic colours to flames
...
Hence, these do not
impart any colour to the flame
...
Chemical properties of alkaline earth metals are as
follows
...
(a) Powdered Be burns in air to form BeO and Be3N2
...
(c) Ca, Sr, and Ba react readily with air to form respective oxides and nitrides
...
(ii) Alkaline earth metals react with halogens at high temperatures to form halides
...
(iv) They react readily with acids to form salts and liberate hydrogen gas
...
However, their reducing power is less than that of
alkali metals
...
(vi) Similar to alkali metals, the alkaline earth metals also dissolve in liquid ammonia to
give deep blue coloured solutions
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
Question 10
...
These
metals have only one electron in their valence shell, which they lose easily, owing to
their low ionization energies
...
Question 10
...
Answer
Let the oxidation state of Na be x
...
Therefore,
Therefore, the oxidation sate of sodium is +1
...
5:
Explain why is sodium less reactive than potassium?
Answer
In alkali metals, on moving down the group, the atomic size increases and the effective
nuclear charge decreases
...
Hence, potassium is more reactive than
sodium
...
6:
Compare the alkali metals and alkaline earth metals with respect to (i) ionization
enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
Answer
Alkali metals
(i)
Alkaline earth metals
Ionization enthalpy:
These
have
(i)
lowest
ionization
Ionization enthalpy:
Alkaline
earth
metals
have
smaller
enthalpies in respective periods
...
atomic sizes
...
However, their second
configuration after losing it
...
This
is
because alkali metals, after losing one
electron,
acquires
noble
gas
configuration, which is very stable
...
This is because alkaline
electropositive nature of alkali
earth metals are less electropositive
metals,
than alkali metals
...
to
the
makes
these
oxides highly ionic
...
(iii)
Solubility of hydroxides:
(iii)
Solubility of hydroxides:
The hydroxides of alkali metals
The hydroxides of alkaline earth metals
are more soluble than those of
are less soluble than those of alkali
alkaline earth metals
...
This is due to the high lattice
energies of alkaline earth metals
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
energies
...
7:
In what ways lithium shows similarities to magnesium in its chemical behaviour?
Answer
Similarities between lithium and magnesium are as follows
...
(ii) The oxides of both Li and Mg are much less soluble in water and their hydroxides
decompose at high temperature
...
(iv) Neither Li nor Mg form peroxides or superoxides
...
Also, these decompose on heating
...
(vii) Both LiCl and MgCl2 are soluble in ethanol owing to their covalent nature
...
They crystallize from aqueous
solutions as hydrates, for example,
and
...
8:
Explain why alkali and alkaline earth metals cannot be obtained by chemical reduction
methods?
Answer
In the process of chemical reduction, oxides of metals are reduced using a stronger
reducing agent
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
reducing agents and the reducing agents that are stronger than them are not available
...
Question 10
...
Still, K and Cs are used
in the photoelectric cell and not Li
...
While on the other hand, K and Cs have low ionization
energy
...
This property of K and Cs is utilized in
photoelectric cells
...
10:
When an alkali metal dissolves in liquid ammonia the solution can acquire different
colours
...
Answer
When an alkali metal is dissolved in liquid ammonia, it results in the formation of a deep
blue coloured solution
...
Therefore, the transmitted light is blue in colour
...
This causes the
solution to attain a copper–bronze colour and a characteristic metallic lustre
...
11:
Beryllium and magnesium do not give colour to flame whereas other alkaline earth
metals do so
...
When this excited electron comes back to its lower energy level, it radiates
energy, which belongs to the visible region
...
In Be and Mg,
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The energy required to excite these electrons is very
high
...
Hence, no colour in the flame is seen
...
12:
Discuss the various reactions that occur in the Solvay process
...
When carbon dioxide gas is bubbled through a brine solution saturated with ammonia,
sodium hydrogen carbonate is formed
...
Step 1: Brine solution is saturated with ammonia
...
Step 2: Carbon dioxide is reacted with this ammoniated brine to result in the formation
of insoluble sodium hydrogen carbonate
...
Step 4: NaHCO3 is heated strongly to convert it into NaHCO3
...
The
overall
reaction
taking
place
in
Solvay
process
is
Question 10
...
Why?
Answer
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This is because unlike
sodium bicarbonate, potassium bicarbonate is fairly soluble in water and does not
precipitate out
...
14:
Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at higher
temperature?
Answer
As we move down the alkali metal group, the electropositive character increases
...
However, lithium carbonate is not
so stable to heat
...
Lithium ion, being very
small in size, polarizes a large carbonate ion, leading to the formation of more stable
lithium oxide
...
Question 10
...
(a) Nitrates (b) Carbonates (c) Sulphates
...
LiNO3, on decomposition, gives oxide
...
As we move down group 1 and group 2, the thermal stability of nitrate increases
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
Solubility
Nitrates of both group 1 and group 2 metals are soluble in water
...
However, carbonate of lithium,
when heated, decomposes to form lithium oxide
...
Solubility
Carbonates of alkali metals are soluble in water with the exception of Li2CO3
...
Carbonates of alkaline earth metals are insoluble in water
...
Solubility
Sulphates of alkali metals are soluble in water
...
BeSO4 Fairly soluble
MgSO4 Soluble
CaSO4 Sparingly soluble
SrSO4 Insoluble
BaSO4 Insoluble
In other words, while moving down the alkaline earth metals, the solubility of their
sulphates decreases
...
16:
Starting with sodium chloride how would you proceed to prepare (i) sodium metal (ii)
sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?
Answer
(a) Sodium can be extracted from sodium chloride by Downs process
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
This process involves the electrolysis of fused NaCl (40%) and CaCl2 (60 %) at a
temperature of 1123 K in Downs cell
...
Metallic Na and Ca are
formed at cathode
...
(ii) Sodium hydroxide can be prepared by the electrolysis of sodium chloride
...
In this process, the brine solution is electrolysed using a
carbon anode and a mercury cathode
...
(iii) Sodium peroxide
First, NaCl is electrolysed to result in the formation of Na metal (Downs process)
...
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Sodium hydrogen carbonate is
precipitated in a reaction of sodium chloride and ammonium hydrogen carbonate
...
Question 10
...
(ii) Quick lime (CaO) combines with silica (SiO2) to form slag
...
(iv) Calcium nitrate, on heating, decomposes to give calcium oxide
...
18:
Describe two important uses of each of the following: (i) caustic soda (ii) sodium
carbonate (iii) quicklime
...
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(ii) Uses of sodium carbonate
(a) It is generally used in glass and soap industry
...
(iii) Uses of quick lime
(a) It is used as a starting material for obtaining slaked lime
...
Question 10
...
Answer
(a) Structure of BeCl2 (solid)
BeCl2 exists as a polymer in condensed (solid) phase
...
Question 10
...
Explain
...
Thus, the lattice energies of carbonates and hydroxides formed by calcium and
magnesium are much more than those of sodium and potassium
...
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21:
Describe the importance of the following: (i) limestone (ii) cement (iii) plaster of paris
...
Importance of limestone
(a) It is used in the preparation of lime and cement
...
(ii) Chemically, cement is a mixture of calcium silicate and calcium aluminate
...
(b) It is used in concrete
...
H2O
...
(b) It is also used for making casts and moulds
...
22:
Why are lithium salts commonly hydrated and those of the other alkali metal ions usually
anhydrous?
Answer
Lithium is the smallest in size among the alkali metals
...
As a result, water molecules get attached
to lithium salts as water of crystallization
...
3H2O) are commonly hydrated
...
Hence, other alkali metal ions usually form anhydrous
salts
...
23:
Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in
acetone?
Answer
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On the contrary, LiCl is soluble not only in water, but also in
acetone
...
The solubility of a compound in water depends on the balance between lattice energy
and hydration energy
...
Also, there is not much difference
between the hydration energies of fluoride ion and chloride ion
...
Hence, low lattice energy and greater covalent character are
the factors making LiCl soluble not only in water, but also in acetone
...
24:
Explain the significance of sodium, potassium, magnesium and calcium in biological
fluids
...
They are also found in the
interstitial fluids surrounding the cells
...
(b) They help in regulating the flow of water across the cell membranes
...
(ii) Potassium (K):
Potassium ions are found in the highest quantity within the cell fluids
...
(b) They also participate in oxidising glucose to produce ATP
...
(iii) Magnesium (Mg) and calcium (Ca):
Magnesium and calcium are referred to as macro-minerals
...
(a) Mg helps in relaxing nerves and muscles
...
(c) Mg maintains normal blood circulation in the human body system
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
(e) Ca also helps in maintaining homeostasis
...
25:
What happens when
(i) sodium metal is dropped in water ?
(ii) sodium metal is heated in free supply of air ?
(iii) sodium peroxide dissolves in water ?
Answer
(i) When Na metal is dropped in water, it reacts violently to form sodium hydroxide and
hydrogen gas
...
The chemical equation involved in the reaction is:
(iii) When sodium peroxide is dissolved in water, it is readily hydrolysed to form sodium
hydroxide and water
...
26:
Comment on each of the following observations:
(a) The mobilities of the alkali metal ions in aqueous solution are Li+ < Na+ < K+ < Rb+ <
Cs+
(b) Lithium is the only alkali metal to form a nitride directly
...
Answer
(a) On moving down the alkali group, the ionic and atomic sizes of the metals increase
...
Since Li+ is the smallest, it
gets heavily hydrated in an aqueous solution
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
so it is the least hydrated
...
Therefore, hydrated
Li+ is the least mobile and hydrated Cs+ is the most mobile
...
This is because Li+ is very small in size and so its size is the most compatible
with the N3– ion
...
This energy also
overcomes the high amount of energy required for the formation of the N3– ion
...
Hence, their electrode potentials are nearly constant
...
27:
State as to why
(a) a solution of Na2CO3 is alkaline ?
(b) alkali metals are prepared by electrolysis of their fused chlorides ?
(c) sodium is found to be more useful than potassium ?
Answer
(a) When sodium carbonate is added to water, it hydrolyses to give sodium bicarbonate
and sodium hydroxide (a strong base)
...
(b) It is not possible to prepare alkali metals by the chemical reduction of their oxides as
they themselves are very strong reducing agents
...
This is
because these elements are highly electropositive
...
This is because the liberated metals react
with water
...
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Class XI
Chapter 10 – The s-Block Elements
Chemistry
Hence, to overcome these difficulties, alkali metals are usually prepared by the
electrolysis of their fused chlorides
...
Potassium ions are located within the cell fluids
...
Hence, sodium is found to be more useful than potassium
...
28:
Write balanced equations for reactions between
(a) Na2O2and water
(b) KO2 and water
(c) Na2O and CO2
Answer
(a) The balanced chemical equation for the reaction between Na2O2 and water is:
(b) The balanced chemical equation for the reaction between KO2 and water is:
(c) The balanced chemical equation for the reaction between Na2O and CO2 is:
Question 10
...
Answer
(i) BeO is almost insoluble in water and BeSO4 is soluble in water
...
The size compatibility of Be2+ and
O2– is high
...
When BeO is dissolved in water, the hydration energy of its ions is not sufficient to
overcome the high lattice energy
...
On the other
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Hence, Be2+ can easily polarise
ions, making
BeSO4 unstable
...
(ii) BaO is soluble in water, but BaSO4 is not
...
The size compatibility of Ba2+ and O2– is not high
...
The lattice energy released during its formation is also not very large
...
Therefore, BaO is soluble in water
...
The lattice energy released is high
...
(iii) LiI is more soluble than KI in ethanol
...
It polarises the electron cloud of
the iodide ion to a much greater extent than the potassium ion
...
Hence, LiI is more soluble in ethanol
...
30:
Which of the alkali metal is having least melting point?
(a) Na (b) K (c) Rb (d) Cs
Answer
Atomic size increases as we move down the alkali group
...
Also, the strength of metallic
bonds decreases on moving down a group in the periodic table
...
Among the given metals, Cs is the largest and has the least melting
point
...
31:
Which one of the following alkali metals gives hydrated salts?
(a) Li (b) Na (c) K (d) Cs
Answer
Smaller the size of an ion, the more highly is it hydrated
...
Also, it has the highest charge density and highest polarising
power
...
As
a
result,
it
forms
hydrated
salts
such
as
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2 H2O
...
Hence, they usually do not form hydrated salts
...
32:
Which one of the alkaline earth metal carbonates is thermally the most stable?
(a) MgCO3 (b) CaCO3 (c) SrCO3 (d) BaCO3
Answer
Thermal stability increases with the increase in the size of the cation present in the
carbonate
...
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Title: chapter 10 s-block elements
Description: easy questions of ncerts solutions
Description: easy questions of ncerts solutions