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Chemistry 1 Marks Question

p- Block Elements · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 23 questions.

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Question 11 Mark
Why does $ NH_{3} $ form hydrogen bond but $ PH_{3} $ does not?
Answer
The electronegativity of nitrogen is more than phosphorus. Due to the small size of nitrogen the intensity of charge is also high in comparison phosphorus. So $ NH_{3} $ form hydrogen bond but $ PH_{3} $ does not.
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Question 21 Mark
Write balanced equations for the following : (i) NaCl is heated with sulphuric acid in the presence of $MnO _2$, (ii) Chlorine gas is passed into a solution of NaI in water.
Answer
(i) $4 NaCl + MnO _2+4 H _2 SO _4 \longrightarrow MnCl _2+ Cl _2 +2 H _2 O +4 NaHSO _4$
(ii) $2 NaI + Cl _2 \longrightarrow 2 NaCl + I _2$
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Question 31 Mark
On heating $Pb \left( NO _3\right)_2$ a brown gas is evolved which undergoes dimerization on cooling. Identify the gas.
Answer
Nitrogen dioxide gas ( $NO _2$ )
$2 Pb\left( NO _3\right)_2 \xrightarrow{\Delta} 2 PbO +4 NO _2+ O _2$
$2 NO _2 \longrightarrow N_2 O _4$
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Question 41 Mark
Write the electronic configurations of Kr $(Z=36)$ and $X e(Z=54)$.
Answer
Krypton (Kr) : $1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6 3^{10}, 4 s ^2 s p ^6$
Xenon (Xe) : $1 s^2, 2 s^2 2 p^6 3 s^2 3 p^6 3 d^{10}, 4 s^2 4 p^6 4 d^{10}$,$5 s^2 5 p^6$
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Question 51 Mark
Bond angle in $ NH_{3} $ is greater than in $ PH_{3} $. Why?
Answer
The size of nitrogen is small than phosphorus. As a result, the force of repulsion between the bonded pair of electrons in $PH _3$ is more than in $NH _3$. Thus the bond angle in $PH _3$ is lesser than that in $NH _3$.
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Question 71 Mark
Write the name of catalyst used in Contact process and Ostwald Method?
Answer
(i) Contact Process: $ V_{2}O_{5} $
(ii) Ostwald Method: Pt
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Question 81 Mark
What happens when $ PCl_{5} $ is heated? Give reaction.
Answer
$ PCl_{5} \xrightarrow{\Delta} PCl_{3} + Cl_{2} $
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Question 91 Mark
Why $ O_{3} $ acts as a strong oxidising agent.
Answer
$ O_{3} $ being endothermic compound decomposes on heating to give nascent oxygen. Since nascent oxygen is very reactive, therefore $ O_{3} $ acts as a strong oxiding agent.
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Question 101 Mark
Sulphur forms $ SF_{6} $ but not $ SCl_{6} $, Why?
Answer
The size of chlorine atom is large while size of fluorine atom is smaller. Also chlorine is less electronegative as compared to fluorine. Chlorine fails to pair in empty $d$-orbitals of sulphur. Fluorine easily combine 6 atoms with on sulphur atom due to its small size.
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Question 121 Mark
$ PH_{3} $ has lower boiling point than $ NH_{3} $. Why?
Answer
This is due to the fact that $NH _3$ has interatonic hydrogen bonding which increases the bond strength and thus increases the boiling point. While in case of $PH _3$ there are weak Van der Walls force.
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Question 131 Mark
Arrange the following in order of property indicated for each set :
(i) $H _2 O , H _2 S, H _2 Se , H _2 Te$ : increasing acidic character
(ii) $HF , HCl , HBr , HI$ : decreasing bond enthalpy
Answer
(i) $H _2 O < H _2 S< H _2 Se < H _2 Te$
(ii) $HF > HCl > HBr > HI$
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Question 141 Mark
Why is HF weaker acid than HCl?
Answer
HCl is stronger acid than HF in aqueous medium because with increase in H-X bond length, bond dissociation energy decreases and $ H^{+} $ ion is easily produced.
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Question 151 Mark
Explain what happens when :
(a) Sodium acetate is heated with sodalime?
(b) Calcium carbide is allowed to react with water?
Answer
(a) $CH _3 COONa + NaOH \longrightarrow CH _4+ Na _2 CO _3$
(b) $CaC _{2(s)}+2 H _2 O _{(l)} \longrightarrow Ca ( OH )_{2(s)}+ C _2 H _{2(g)}$
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Question 161 Mark
Why does fluorine exhibit the oxidation state of -1 only?
Answer
Fluorine is most electronegative element and is the strongest oxidising agent so it gains one electron and exhibit -1 oxidation state only.
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Question 171 Mark
$ H_{2}O $ is a liquid whereas $ H_{2}S $ is a gas at room temperature. Give reason.
Answer
There is hydrogen bonding in $ H_{2}O $ due to high electronegativity and small size of O atom but there is no hydrogen bonding possible in $ H_{2}S $.
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Question 191 Mark
Why are group 16 elements called as chalcogens?
Answer
Because many metals are found as oxides and sulphides and some are solenides and tellurides thus the group 16 elements are chalcogens (Chalcogens = Ore forming).
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Question 201 Mark
What is the oxidation number of phosphorus in the following? $ H_{3}PO_{3} $
Answer
Let P oxidation number x.
$3(+1)+x+3(-2)=0$
$\Rightarrow \quad 3+x-6=0$
$\Rightarrow \quad x-3=0$
$\therefore \quad x=+3$
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Question 211 Mark
Explain why phosphorus forms $ PF_{5} $ whereas nitrogen does not form $ NF_{5} $.
Answer
Phosphorus have vacant d-orbital so it form $ PF_{5} $ but Nitrogen does not form $ NF_{5} $ because of non-availability of vacant d-orbital.
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Question 221 Mark
Why nitrogen exists as diatomic molecule $N _2$, whereas phosphorus exists as tetratomic molecule, $P _4$.
Answer
Nitrogen from $p \pi-p \pi$ multiple bond easily so it is found in $N _2$ form. The tendency to form $p \pi-p \pi$ multiple bonds decrease down the group due to large size and lower electro-negativity of other members. So phosphorous exist in $P _4$ form.
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Question 231 Mark
Why is the bond angle of$PH _4^{+}$ more than $PH _3$ ?
Answer
It is because according to the VSPER theory the lone pair-bond pair repulsion in $PH _3$ is higher than that of bond pair bond pair repulsion. But in $PH _4^{+}$there is only bond pair-bond pair repulsion. Hence bond angle of $PH _4^{+}$is more than $PH _3$.
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