3 Marks Question

Question 13 Marks

Permanganate ion $\left( MnO _4^{-}\right)$gives different reduction products at different concentration of hydrogen ions, write their equations.

Answer

The products formed in the reduction reaction of permanganate ion depend on the concentration of hydrogen ion. The reactions occurring at different concentration of $H ^{+}$ are as follows:
$
\begin{aligned}
MnO_{+7}^{-}+e^{-} & \rightarrow \underset{+6}{MnO_4^{2-}} \\
MnO_4^{-}+4 H^{+}+3 e^{-} & \rightarrow \underset{+4}{MnO_2}+2 H_2 O \\
MnO_4^{-}+8 H^{+}+5 e^{-} & \rightarrow Mn^{2+}+4 H_2 O
\end{aligned}
$
Water should be oxidized by permanganate ion at $\left[ H ^{+}\right]$ $=1$ but this reaction happens very slowly but in this. The rate of reaction increases by using $Mn ^{2+}$ ion as a catalyst or by increasing the temperature.

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Question 23 Marks

(a) Which is the salt of Mn which is isostructural to $KClO _4$ ?
(b) Explain the structure and magnetic properties of manganese and permanganate ions.

Answer

(a) $KMnO _4$ (potassium permanganate) is isostructural to $KClO _4$ (potassium chlorate), that is, both have the same structure.
(b) Manganate and permanganate ions are tetrahedral in which $\pi$-bond is formed due to overlap between the $p$ orbital of oxygen and $d$-orbitals of Mn and $sp ^3$ hybridization occurs on Mn . Their structure is as follows :

Permanganate ion is diamagnetic, when heated at 513 K it gets converted into manganese ion which is paramagnetic because it has one unpaired electron.

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Question 33 Marks

(a) $Cu ^{+} X ^{-}$is unstable, why?
(b) Explain the structure of $Mn _2 O _7$ and $MnO _4^{-}$.

Answer

(a) $Cu ^{+} X ^{-}$is unstable because the disproportionation of Cu in aqueous solution occurs.
$
2 Cu_{(aq)}^{+} \longrightarrow Cu_{(aq)}^{2+}+Cu_{(s)}
$
Therefore, $Cu ^{2+}$ is more stable than $Cu ^{+}$in aqueous solution because the value of hydration enthalpy of $Cu ^{2+}$ is much more negative than that of $Cu ^{+}$, which can easily balance the energy (second ionization enthalpy) required to form $Cu ^{2+}$ from $Cu ^{+}$.
(b) In $Mn _2 O _7$, each Mn atom is tetrahedrally surrounded by oxygen atoms and an $Mn - O - Mn$ bridge is also found in it and the structure of $MnO _4^{-}$is also tetrahedral.

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Chemistry 3 Marks Question

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