1
MCQ (Single Correct Answer)

JEE Main 2017 (Online) 9th April Morning Slot

Which of the following ions does not liberate hydrogen gas on reaction with dilute acids ?
A
Ti2+
B
V2+
C
Cr2+
D
Mn2+

Explanation

The third ionisation energy of Mn is too high due to stable half filled 3d-orbital. Hence, it cannot further get oxidised to liberate hydrogen.
2
MCQ (Single Correct Answer)

JEE Main 2018 (Online) 16th April Morning Slot

When XO2 is fused with an alkali metal hydroxide in presence of an oxidizing agent such as KNO3 ; a dark green product is formed which disproportioates in acidic solution to afforda dark purple solution. X is :
A
Ti
B
V
C
Cr
D
Mn

Explanation

X is Mn.

MnO2  +  2KOH  +  KNO3  $$ \to $$  K2MnO4  +  KNO2  +  H2O

Here   K2MnO4  is dark green.

3MnO4$$-$$2  +  4H+  $$ \to $$  2MnO4$$-$$  +  MnO2  +  2H2O

In acidic solution, K2MnO4 changes to dark purple solution of KMnO4.
3
MCQ (Single Correct Answer)

JEE Main 2019 (Online) 9th January Evening Slot

The transition element that has lowest enthalpy of atomisation, is :
A
Fe
B
Cu
C
V
D
Zn

Explanation

Since Zn is not a transition element so transition element having lowest atomisation energy out of Cu, V, Fe is Cu.
4
MCQ (Single Correct Answer)

JEE Main 2019 (Online) 10th January Morning Slot

Hall- Heroult's process is given by
A
Cu2+ (aq) + H2(g) $$ \to $$ Cu(s) + 2H+ (aq)
B
Cr2O3 + 2AI  $$ \to $$ Al2O3 + 2Cr
C
2Al2O3 + 3C  $$ \to $$ 4Al + 3CO2
D
ZnO + C   $$\buildrel {Coke,\,1673\,K} \over \longrightarrow $$ Zn + CO

Explanation

In Hall-Heroult's process is given by

At cathode :- $$4Al_{\left( \ell \right)}^{3 + }{\mkern 1mu} {\mkern 1mu} + {\mkern 1mu} {\mkern 1mu} 12{e^ - }{\mkern 1mu} {\mkern 1mu} \,\mathrel{\mathop{\kern0pt\longrightarrow} \limits_\,} \,\,4Al\left( \ell \right)$$

At Anode :- $$6O_{\left( \ell \right)}^{2 - }\,\,\mathrel{\mathop{\kern0pt\longrightarrow} \limits_\,} \,\,3{O_2}(g) + 12{e^ - }$$

$$3C + 3{O_2}\,\,\mathrel{\mathop{\kern0pt\longrightarrow} \limits_\,} \,\,3C{O_2}\left( \uparrow \right)$$

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