S Block Elements(12th Grade > Chemistry ) Questions and answers for exam Preparation

Question 1. Potassium when heated strongly in oxygen, it forms

K2O
KO2
K2O2
KO3

Explanation:-

Answer: Option B. -> KO2
:
B
Whenever potassium is heated strongly in a good supply of air or oxygen, it forms

K O_{2}

.

K O_{2}

is superoxide.

K_{2} O

is obtained by heating potassium nitrate with potassium.
Controlled oxidation of potassium in excess air or oxygen at 300

Question 2. Halides of alkaline earth metals form hydrates such as MgCl₂.6H₂O,CaCl₂.6H₂O, BaCl₂.2H₂O and SrCl₂.2H₂O. This shows that halides of group 2 elements

Are hygroscopic in nature
Can act as dehydrating agents
Can absorbs moisture from air
All of the above

Explanation:-

Answer: Option D. -> All of the above
:
D
The tendency to form hydrates decreases in the group. All the options reflect the fact that the chlorides do form hydrates.

Question 3. When chloro - bridged dimeric form of beryllium chloride is heated at 1200

^{\circ}

C, then in the product is formed, hybridization of central atom & shape of molecule is

sp2 , angular
sp3, pyramidal
sp, linear
sp2, triangular shape

Explanation:-

Answer: Option C. -> sp, linear
:
C
It becomes monomer above 1200

^{\circ}

C. The monomer is a linear molecule with

s p

hybridization.

Question 4. Look at the following reaction:

L i H + A l H_{3} ⟶ L i A l H_{4}

Here

A l H_{3}

and

L i H

act as:

Lewis acid and Lewis base
Lewis base and Lewis acid
Bronsted base and Bronsted acid
None of these

Explanation:-

Answer: Option A. -> Lewis acid and Lewis base
:
A

Look At The Following Reaction:LiH+AlH3⟶LiAlH4Here AlH3 a...

As shown in the figure, the central metal atom in

A l H_{3}

has only six valence electrons. Hence, it could accept a pair of electrons (Lewis acid) to complete its octet. On the other hand,

H^{-}

is a Lewis base because it has two valence electrons thus having a completely filled shell. Further, since the ion readily shares or donates electrons - it acts as a Lewis base.

Question 5. Statement-1 : Alkali metals dissolve in liquid ammonia to give blue solutions.
Statement-2 : Alkali metals in liquid ammonia give solvated species of the type

[M (N H_{3})_{n}]^{+}

(M =alkali metals).

Statement-1 is True, Statement-2 is True ; Statement-2 is a correct explanation for Statement-1.
Statement-1 is True, Statement-2 is True; Statement-2 is Not a correct explanation for Statement-1
Statement-1 is True, Statement-2 is False
Statement-1 is False, Statement-2 is True

Explanation:-

Answer: Option B. -> Statement-1 is True, Statement-2 is True; Statement-2 is Not a correct explanation for Statement-1
:
B
Alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature. The blue colour of the solution is due to ammoniated electrons which absorb energy in the visible region of light. The second statement describes the solvated cations (positively charged species) whereas the colour is due to the solvated electrons. Do note howeverthat both the statements are accurate.

Question 6. The correct order of thermal stability of alkali metal hydrides is:

CsH>RbH>KH>NaH>LiH
LiH>NaH>KH>RbH>CsH
CsH>NaH>KH>RbH>LiH
LiH>KH>NaH>RbH>CsH

Explanation:-

Answer: Option B. -> LiH>NaH>KH>RbH>CsH
:
B
The stability of alkali metal hydrides decreases with increase in the size of the metal cation. In other words, as we go down the group - the stabilityof the hydrides decreases.

Question 7.

N a H

reacts with water to give:

Alkaline solution
NH3 gas
Acidic solution
Neutral solution

Explanation:-

Answer: Option A. -> Alkaline solution
:
A

N a H + H_{2} O \to N a O H + H_{2} (g)

As you can see - we get

N a O H

which is very strongly basic.

Question 8. The correct order of density of alkali metals is:

Explanation:-

Answer: Option A. -> Alkaline solution
:
A
As we go down a group - density increases generally. However, there is a notable exception in potassium, which is less dense than sodium. Simply put, in the case of potassium the increase in shell size outweighs the pull of the core on the outer shell electron and so potassium is less dense than sodium.

Question 9.

Paramagnetic, paramagnetic
Paramagnetic, diamagnetic
Diamagnetic, paramagnetic
Diamagnetic ,diamagnetic

Explanation:-

Answer: Option A. -> Paramagnetic, paramagnetic
:
A
X is NO₂& Y is O₂ .Both the species have unpaired electrons in their electronic configurations (For

O_{2}

you will have to use MOT).
Due to the presence of unpaired electrons, both

N O_{2}

and

O_{2}

are paramagnetic.

Question 10. The reaction of sodium is highly exothermic with H₂O. The rate of reaction is lowered by

Lowering the temperature
Mixing with alcohol
Mixing with acetic acid
Making an amalgam

Explanation:-

Answer: Option D. -> Making an amalgam
:
D
Sodium amalgam, commonly denoted Na(Hg), is analloyofmercuryandsodium.
Na(Hg) is less reactive than Na. It will decrease the rate of reaction.
Sodium is highly reactive with water and decreasing the temperature does not decrease the rate of the reaction if we consider Le Chatelier's principle.
Sodium reacts with alcohol and acetic acid also. They will not affect the rate of reaction of sodium with water.