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Chemistry 2 Marks Questions

d-and f-Block Elements · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 6 questions.

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Question 12 Marks
$Cu ^{+}$ion is not stable in aqueous solution. Why?
Answer
In aqueous solution $Cu ^{+}$undergoes disproportionation to form a more stable $Cu ^{2+}$ ion.
$2 Cu ^{+}{ }_{(a q)} \longrightarrow Cu ^{2+}{ }_{(a q)}+ Cu _{(s)}$
The higher stability of $Cu ^{2+}$ in aqueous solution may be attributed to its greater negative $\Delta_{h y d} . H ^0$ than that of $Cu ^{+}$. It compensates the second ionisation enthalpy of Cu involved in the formation of $Cu ^{2+}$ ions.
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Question 22 Marks
Why do transition elements form complex compounds?
Answer
The tendency of transition metal ion to form complexes is due to following factors :
(i) The transition metal cations are relatively very small in size and have high effective nuclear charge. Thus they have a high positive charge density (charge/size ratio) which facilitates the acceptance of lone pairs of electrons from other molecules or ions.
(ii) The transition metal cations have vacant inner $d$-orbitals which are of appropriate energy to accept lone pair of electrons from the ligands.
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Question 32 Marks
How is it that several transition metal act as catalysts ? Give two examples of reactions catalysed by them ?
Answer
Transition metals have unpaired electrons and they exhibit multiple oxidation state. So they can form intermediate compounds with the reactants. Examples are
(i) $2 SO _2+ O _2 \xrightarrow{V_2 O _5} 2 SO _3$
(ii) $N _2+3 H _2 \underset{\substack{200-1000 atm \\ 625 K}}{\stackrel{ Fe / Mo }{\rightleftharpoons}} 2 NH _3$
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Question 42 Marks
Why do transition elements form coloured compounds explain?
Answer
Transition elements and their compounds form coloured ions and complexes because they have incomplete $d$-sub-sell and due to the approach of anions $d$-orbitals of transition metals split into 2 sets of lower and higher energy so electrons absorb certain amount of energy and jump into orbitals of higher energy. To gain energy they absorb certain radiations from visible light and emit out the remainder as coloured light. So they look coloured.
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Question 52 Marks
Copper(I) compounds are white and are dimagnetic. While copper(II) compounds are coloured and paramagnetic, explain.
Answer
The electronic configuration of $Cu ^{+}( Z =28)$ is $1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6 3 d^{10}$, i.e., all orbitals are completely filled so it is dimagnetic in nature and form white compounds. The electronic configuration of $Cu ^{2+}( Z =27)$ is $1 s^2, 2 s^2 2 p^6, 3 s^2 3 p^6 3 p^9$ so it has 1 unpaired electron. Hence it is paramagnetic in nature and form coloured compounds.
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Question 62 Marks
Explain the following :
(a) Transition elements show variable oxidation states.
(b) The paramagnetic character in $3 d$ transition series elements increases upto Mn and then decreases.
Answer
(a) Transition metals have two orbitals to accommodate their valency electron. The difference in energy between 2-orbital is very less and both can be used for bond formation. $s$-orbital can be used up to +2 oxidation state and $d$-orbital is used for higher oxidation.
(b) In $3 d$ transition series the number of unpaired electrons increases up to Mn and then decreases.
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