3 Marks Question

Question 13 Marks

Write down the electronic configuration of :
(a) $Cr ^{3+}$$\quad$(b) $Cu ^{+}$$\quad$(c) $Co ^{2+}$$\quad$(d) $M n ^{ 2 + }$
(e) $P m ^{ 3 + }$$\quad$(f) $Ce ^{ 4 +}$$\quad$(g) $Lu ^{2+}$$\quad$(h) $Th ^{4+}$.

Answer

(a) $Cr ^{3+}:[ Ar ] 3 d^3$,
(b) $Cu ^{+}:[ Ar ] 3 d^{10}$,
(c) $Co ^{2+}:[ Ar ] 3 d^7$,
(d) $Mn ^{2+}:[ Ar ] 3 d^5$,
(e) $Pm ^{3+}:[ Kr ] 4 d^{10} 4 f^5 5 s^2 5 p^5$,
(f) $Ce ^{4+}:[ Kr ] 4 d^{10} 4 f^2 5 s^2 5 p^4$,
(g) $Lu ^{2+}:[ Xe ] 4 f^{14} 5 d^1$,
(h) $Th ^{4+}:[ Rn ]$

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Question 23 Marks

(a) Account for the following :
(i) Copper (I) compounds are white whereas Copper (II) compounds are coloured.
(ii) Chromates change their colour when kept in an acidic solution.
(iii) $Zn , Cd , Hg$ are considered as $d$-block elements but not as transition elements.
(b) Calculate the spin only moment of $Co ^{2+}( Z =27)$ by writing the electronic configuration of Co and $Co ^{2+}$.

Answer

Ans. (a) (i) Cu (I) compounds are white because of absence of unpaired electrons while Cu (II) compounds are coloured due to unpaired electrons.
(ii) Chromate $\left( CrO _4^{2-}\right)$ changes to dichromate $\left( Cr _2 O _7^{2-}\right)$ ion in acidic medium.
(iii) Due to completely filled $d$-orbitals in their ground state or in oxidized state.
(b)$Co =[ Ar ] 4 s^2 3 d^7$
$\left.Co ^{2+}=[ Ar ] 3 d^7\right\}$
$u=\sqrt{n(n+2)}$
$=\sqrt{3(3+2)}$
$=\sqrt{15}$
= 3.92 BM

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Question 33 Marks

Give the reasons for following:
(i) $E ^{\circ}$ value for $\left( mn ^{+2} / mn \right)$ is negative whoreas for $\left( cn ^{2+} / cu \right)$ is positive.
(ii) Actinoids show irregularities in their electronic configuration.

Answer

(i) $E ^{\circ}$ values are indicative of the stability of the oxidized from of element. The lower the $E ^{\circ}$ value, more stable the oxidized form of the element. $Mn ^{2+}$ with a half filled d-subshell ( $d^5$ ) is stable, so Mn is easily oxidized to $Mn ^{2+}$, making the $E ^{\circ}$ value negative. $Cu ^{2+}$ with a partially filled d subshell ( $d^9$ ) is not stable, and is relatively easily reduced to element form. This makes its $E ^{\circ}$ value positive.
(ii) The electronic configurations of actinoids show irregularities because the energies of their $5 f, 6 d$ and $7 s$ orbitals are close to each other. Electrons can easily move between these subshells.

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Question 43 Marks

Complete the following equations :
(a) (i) $2 MnO _2+4 KOH + O _2 \xrightarrow{\Delta}$
(ii) $Cr _2 O _7^{2-}+14 K^{+}+6 I ^{-} \longrightarrow$
(b) The elements of 3rd transition series are given as :
Sc Ti V Cr Mn Fe Co Ni Cu Zn
(i) Write the element which is not regarded as a transition element. Give reason.
(ii) Which element has the highest melting point.
(iii) Write the element which can show an oxidation state of +1 .
(iv) Which element is a strong oxidizing agent in +3 oxidation state and why?

Answer

(a) (i) $2 MnO _2+4 KOH + O _2 \xrightarrow{\text { Fuse }} 2 K_2 MnO _4+2 H _2 O$
(ii) $Cr _2 O _7^{-2}+14 H ^{+}+6 I ^{-} \longrightarrow 2 Cr ^{3+}+7 H _2 O +3 I _2$
(b) (i) $Zn \longrightarrow$ because Zn does not shows variable oxidation state.
(ii) $Cr \longrightarrow$ it is due to high enthalpy of atomization.
(iii) Copper $( Cu )$.
$ Mn \underset {\downarrow} \longrightarrow \text { (Ar) } 4 s^2 3 d^5 $
$ Mn ^{3+} \longrightarrow \text { (Ar) } 4 s^{\circ} 3 d^4$
$Mn ^{3+}$ having 4 electrons in 3 d subshell it required one electron to half filled configuration in $3 d$ subshell it thus acts as strong oxidizing agent.

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Question 53 Marks

Write balanced chemical equations for the following reactions:
(i) A mixture of potassium dichromate and sodium chloride is heated with concentrated $ H_{2}SO_{4} $.
(ii) Potassium permanganate is added to a hot solution of manganous sulphate.

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