Electrochemistry Test

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Electrochemistry Test - 18

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Weekly Quiz Competition

Question 1
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Only One Option Correct Type
This section contains multiple choice questions. Each question has four choices (a), (b), (c) and (d), out of which ONLY ONE is correct.
Q.
Cl₂ gas is passed into a solution containing KF, Kl and KBr, and CHCI₃ is added. There is a colour in CHCI₃ (lower) layer. It is due to

A
formation of l₂ (violet)

B
formation of Br₂ (orange)

C
formation of l₂ and Br₂ both

D
formation of F₂ (colourless)

Solution

Based on electro chemical series, oxidising power of
F₂ > Cl₂ > Br₂ > I₂

On passing Cl₂ in to a solution containing KF, Kl and KBr,
2KBr + Cl₂ → 2KCI + Br₂ (orange)
2KI + Cl₂→ 2KCI + l₂ (violet)
2KI + Br₂ → 2KBr + l₂ (violet)
Br₂ formed also oxidises Kl to l₂ (violet)
Question 2
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Select the correct statement(s) about galvanic cell (Daniell cell) with E^° _cell = 1.10 V

A

B

C

D
All of the above

Solution

E° _cell = 1.10 V
(a) Hence , when E_ext < 1.10 V, reaction continues till the E_extreaches 1.10 V and
Zn(s) + Cu²⁺(aq) → Cu(s) + Zn²⁺(aq)
thus , zinc dissolves at anode and Cu deposits at cathode.
(b) When E_ext = 1.10 V , no reaction.
(c) When E_ext > 1.10V, reverse reaction takes place,

It now acts as electrolytic cell.
Zinc deposits at zinc plate which is now the cathode (+ ve plate) and Cu dissolves as Cu²⁺ at copper plate which is now the anode (- ve plate)
Question 3
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Given,
E° _Ag⁺/Ag = 0.80 V
E° _Mg²⁺/Mg = -2.37 V
E°_Cu²⁺/Cu = + 0.34V
E° _Hg²⁺/Hg = 0.79V
Q.
Which of the following statement(s) is/are correct?

A
AgNO₃can be stored in a copper vessel

B
Cu(NO₃)₂can be stored in a magnesium vessel

C
CuCl₂can be stored in a silver vessel

D
HgCl₂ can be stored in a copper vessel

Solution

Based on electro chemical series, reactivity series is

(a) 2Ag⁺ + Cu → Cu²⁺ 2Ag
Cu is a better reducing agent than Ag hence Ag⁺ is reduced by Cu. Thus AgNO₃ can not be stored in copper vessel.
(b) Cu²⁺ + Mg → Mg²⁺ + Cu
Mg will reduce Cu²⁺ to Cu thus can not be stored.
(c) Cu²⁺ + 2Ag → Cu + 2 Ag⁺
Cu is better reducing agent hence CuCI₂ can be stored in silver vessel.
(d) Hg²⁺ + Cu → Cu²⁺ + Hg
Cu is better reducing agent. Hence, HgCI₂ can not be stored in copper vessel.
Question 4
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One mole of Ag₂CO₃ is strongly heated in an open vessel. Residue wll be

A
1 mole of Ag₂O

B
2 moles of Ag

C
1 mole of Ag₂O and 1 mole of CO₂

D
2 moles of Ag , 0.5 mole of O₂and and 1 mole of CO₂

Solution

Oxides of the metals are decomposed to metals if in electro chemical series , E^o_red > 0 .7 9 V
For A g, E°_Ag⁺/Ag = 0.80 V hence Ag₂O is decomposed to metals. Products are Ag , CO₂ and O₂. Since reaction takes place in an open vessel, CO₂ and O₂ escape from mixture, residue being 2 moles of silver.
Question 5
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Zinc ,silver and iron plates are dipped in CuSO₄solutions placed in different vessels as shown:

Blue colour of CuSO₄ fades in

A
I , II ,III

B
I, II

C
II , III

D
I , III

Solution

Blue colour fades or changes to colourless if Cu²⁺ is reduced to Cu.
In electrochemical series (ECS), reactivity order is Zn, Fe, Cu, Ag
Thus, Zn and Fe reduce Cu²⁺ to Cu but Ag does not reduce Cu²⁺. Thus, in I and III, CuSO₄ changes to colourless or fades by zinc and iron plates.
Question 6
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For the following half -cell reactions ,E° values are:
Mn²⁺(aq) + 2H₂O(l) MnO₂(s) + 4H⁺(aq) + 2e^-, E⁰= -1.23V
MnO^-₄(aq) + 4H+(aq) +3e^- MnO₂(s) + 2H₂O(l), E⁰ = +1.70 V
Thus

A
Mn²⁺ reacts with MnO₄^-in acid solution to form MnO₂

B
Mn(MnO₄)₂ is stable in acid solution

C
MnO₂ disproportionates to Mn²⁺and MnO₄^- in acid solution

D
None of the above

Solution

3Mn²⁺(aq) + 2MnO₄^-(aq)+ 2H₂O (/) → 5MnO₂(s) + 4H⁺ E°_cell= 0.47 V
Since E°_cell > 0, hence spontaneous.
Thus, Mn²⁺ is oxidised to MnO₂ by MnO₄^- in acidic medium.
Question 7
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Which of the following statement is correct?

A
E_cell and Δ_rG of a cell reaction both are extensive properties

B
E_cell and Δ_rG of a cell reaction both are intensive properties

C
E_cell is an intensive property while Δ_rG of a cell reaction is an extensive property

D
E_cell is an extensive property while Δ_rG of a cell reaction is an intensive property

Solution

E°_cell remains unchanged on division , thus intensive property.
Δ_rG is dependent on the division , thus extensive property.
Question 8
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Then

A
Sn²⁺ is unstable and disproportionates to Sn⁴⁺and Sn

B
Sn²⁺ is stable and disproportionation reaction is not spontaneous

C
Sn⁴⁺is easily reduced to Sn

D
None of the above

Solution

Since E° _cell < 0, disproportionation of Sn²⁺ to Sn⁴⁺ (by oxidation) and Sn (by reduction) is not spontaneous.
Question 9
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Consider some facts about Standard Hydrogen Electrode(SHE).
I. It is assigned a zero potential at all temperature corresponding to the reaction.
II. Platinum electrode coated with platinum black is dipped in acidic solution and H₂ gas is bubbled through it.
III. Concentration of both are oxidised and reduced forms of hydrogen is maintained at unity.
Select the correct facts

A
I , II ,III

B
II ,III

C
I ,II

D
I ,III

Solution

It is oxidation half-cell

H₂ (reduced part) is at 1 bar. H⁺ (oxidised part) is at 1 M.

It is reduction half-cell

In this case also, oxidised and reduced parts are at unity.

Then (I), (II) and (III) are correct facts.
Question 10
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The gas X at 1 atm is bubbled through a solution containing a mixture of 1 M Y^- and 1 M Z^- at 298 K.If the standard reduction potential
then,

A
Y will oxidise X and not Z

B
Y will oxidise Z and not X

C
Y will oxidise both X and Z

D
Z^- will reduce both X and Y

Solution

In ECS, pair with more negative values of E°red reducing agent is above oxidising agent.
Thus, Z/Z^- is the best reducing agent

Thus, Z^- will reduce both X and Y and itself will be oxidised to Z .
Question 11
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The standard reduction potential values of three metallic cations X, Y, Z are 0.52, -3.03 and -1.18 V respectively. The order of reducing power of the corresponding metals is

A
Y > Z > X

B
X > Y > Z

C
Z > Y > X

D
Z > X > Y

Solution

E°_x = - 0.52 V
E°_y= -3.03 V
E°_z = - 1.18 V
Their placements in ECS is in order Y > Z > X.
Thus, reducing nature is also in same order y > Z > X
Question 12
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A solution containing one mole per litre each of Cu(NO₃)₂ ,AgNO₃ ,Hg₂(NO₃)₂and Mg(NO₃)₂is being electrolysed using inert electrodes. The value of standard redution potentials are

with increasing voltage , the sequence of deposition of metals on the cathode will be

A
Ag ,Hg,Cu,Mg

B
Mg,Cu,Hg,Ag

C
Ag,Hg,Cu

D
Cu,Hg,Ag

Solution

These metals based on E°_red values are placed in the following order:

Mg will reduce and Ag⁺ to metals Cu, Hg, Ag.
Cu will reduce and Ag⁺ to Hg, Ag, Hg will reduce Ag⁺ to Ag.
Ag⁺ is reduced by all metals thus Ag first. Hg²⁺ is reduced by Cu, Mg thus Hg. Cu²⁺ is reduced by Mg only, thus Cu.
Thus, deposition :
Question 13
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The standard reduction potentials at 298K for the following half-cell are given
Which is the strongest reducing agent?

A
Zn(S)

B
Cr(s)

C
H₂g

D
Fe²⁺(aq)

Solution

In electrochemical series (ECS), elements have been arranged in the increasing standard reduction potential starting from most negative to most positive value w.r.t. SHE (E°_she= 0.00 V).
Most negative standard reduction potential means most easily oxidised and thus is the best reducing agent.
OR Reducing agent in (ECS) is always above oxidising agent. Thus, Zn(s) is the best reducing agent in given set.
Question 14
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The E^o_M³⁺/M^₂₊values for Cr, Mn, Fe and Co are - 0.41 V, +1,57 V, + 0.77 V and m /m 1.97 V respectively. For which one of these metals the change in oxidation state from +2 to +3 is easiest?

A
Cr

B
Mn

C
Fe

D
Co

Solution

Mn³⁺ + e^- → Mn²⁺ + 1.57 V
Fe³⁺ + e^- → Fe²⁺ + 0.77 V
Co³⁺ + e^- → Co²⁺ + 1.97
Cr³⁺/Cr²⁺ with most negative E°_red is the best reducing agent.
Cr²⁺→ Cr³⁺ + e^- , E° = 0.41 V
Thus, is oxidised from Cr²⁺ to Cr³⁺ most easily.
Question 15
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Given the standard electrode potentials
I. K⁺/K = -2.93V,
II. Ag⁺/Ag = 0.80V,
III. Hg^2+/Hg = 0.79 V
IV. Mg²⁺/Mg = -2.37V,
V. Cr³⁺/Cr = - 0.74 V
These metals are arranged in increasing reducing power as

A
I < II < IV < III < V

B
II < III < V < IV < I

C
I < IV < V < III < II

D
None of these

Solution

Most negative E°_red means the Mⁿ⁺/M is at the top of ECS and is the best reducing agent.
Question 16
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Statement Type
Direction : This section is based on Statement I and Statement II. Select the correct answer from the codes given below.
Statement I : CuSO₄ can be stored in a vessel made of zinc.
Statement II : w.r.t SHE

A
Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B
Both Statement I and Statement II are correct but Statement II is not the correct explanation of Statement I

C
Statement I is correct but Statement II is incorrect

D
Statement II is correct but Statement I is incorrect

Solution

E°_Zn²⁺/Zn = -0.76 V
E° _Cu²⁺/Cu = + 0.34 V
In electrochemical series, zinc is above copper and thus zinc is a better reducing agent than copper. When CuS0₄ is placed in zinc vessel, copper is displaced
Thus, CuSO₄ cannot be stored in a vessel made of zinc.
Thus, statement I is incorrect and statement II is correct.
Question 17
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Statement I : When AgNO₃ solution is stirred with a spoon made of copper,solution turns blue.
Statement II : In electrochemical series ,copper is above silver

A
Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B
Both Statement I and Statement II are correct but Statement II is not the correct explanation of Statement I

C
Statement I is correct but Statement II is incorrect

D
Statement II is correct but Statement I is incorrect

Solution

In electrochemical series, copper is above silver thus is a better reducing agent. When AgNO₃ solution is stirred with copper spoon, Ag is displaced and copper is oxidised to Cu²⁺ (blue).
Thus, statement I and II are correct and statement II is the correct explanation of statement I.