Electrochemistry Test

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Electrochemistry Test - 13

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Weekly Quiz Competition

Question 1
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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 2
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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 3
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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.
Question 4
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The positive value of the standard electrode potential of Cu²⁺ /Cu,

(E ° _{Cu²⁺ /Cu} = 0.34 V) indicates that

A
this redox couple is a stronger reducing agent than H⁺ /H₂ couple

B
this redox couple is a stronger oxidising agent than H⁺ /H₂ couple

C
Cu can not displace H₂ from acid

D
Option B,C are Correct

Solution

E °_cell >0 thus spontaneous
Cu²⁺ /Cu couple is thus a stronger oxidising agent. Reverse reaction is non-spontaneous.
Cu + 2H⁺ →Cu²⁺ + H₂ , E °_cell = - 0.34 V
Thus, copper cannot displace H₂ from acid.
Question 5
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Consider the following half-reactions:

Select the correct statements on the basis of the above data

A
In dilute sulphuric acid,hydrogen ion will be reduced at cathode

B
In concentrated sulphuric acid,water will be be oxidised at anode.

C
In dilute sulphuric acid,water will be be oxidised at anode.

D
Option A,C are Correct

Solution

Thus, (c) is correct.
Concentrated H₂ SO₄ ionises as H⁺ and HSO₄^- and then water is not oxidised in concentrated H₂ SO₄ Thus, (b) is incorrect.
is not oxidised to in dilute H₂ SO₄ . Thus, (d) is incorrect.
(d) In dilute sulphuric acid solution, ions will be oxidised to tetrathionate ion () at anode.
Question 6
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Select the correct statement(s) based on the following half-reaction:

A
Copper reacts with conc. H₂ SO₄ forming SO₂

B
Zinc reacts with conc. H₂ SO₄ forming SO₂

C
Zinc reacts with dil. H₂ SO₄ forming H₂

D
All are correct

Solution

If dil. H₂ SO₄ is used, then H⁺ is the reacting species.
If cone. H₂ SO₄ is used, then is the reacting species.

Thus, copper does not react with H⁺ (dil. H₂ SO₄ ) forming H₂ .

Since E °_cell >0, Thus copper reacts with cone. H₂ SO₄ forming SO₂ .
(c) Zn + +4H⁺ →Zn²⁺ + SO₂ + 2H₂ O
E °_cell = 1.93 V, E °_cell >0, thus zinc also reacts with conc.
H₂ SO₄ forming SO₂ .
(d) Zn + 2H⁺ →Zn²⁺ + H₂ , E °_cell = 0.76 V
E °_cell >0, thus zinc reacts with dil. H₂ SO₄ forming H₂ .
Question 7
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Matching List Type

Choices for the correct combination of elements from Column I and Column II are given as options (a), (b), (c) and (d), out of which one is correct

Match the half-cell reaction in Column I with their values in Column II and select the answer from the answer codes at the end.

A
a

B
b

C
c

D
d

Solution

Thus, (i) →(p)
(ii) (B) O₂ (g) + 2H₂ O + 4e^- →40H^- n₁ = 4, E₁ = 0.401 V
(C) O₂ (g)+4H⁺ 4e^- →2H₂ O , n₂ = 4, E₂ = 1.229 V
(G) 4H⁺ + 4e^- →2H₂ , n₃ = 4, E₃ = 0.00 V
(B) + (G) - (C) gives
4 H₂ O + 4e^- →2H₂ + 40H^-
2 H₂ O + 2e^- →H₂ + 2OH^-
∴E₄ = 0.401 + 0.00- 1.229
= -0.83 V
Thus, (ii) →(s)

Thus, (iv) →(s)
Question 8
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Passage I

The most stable oxidised species is

A
Cr³⁺

B
MnO₄^-

C

D
Mn²⁺

Solution

For (I) and (II), E °>0, but for reduced species.
For (III), E °>0, thus Cr³⁺ is the most stable.
More negative value of standard electrode reduction potential, better the reducing agent. Thus,
Mn²⁺ < CI^- < Cr³⁺ < Cr
Question 9
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Passage I

Reducing power of different species is in the order

A
Cr³⁺<Cl^-<Mn²⁺<Cr

B
Mn²⁺<Cl^- <Cr³⁺<Cr

C

D
Mn²⁺ <Cr³⁺ <Cl^- <Cr

Solution

For (I) and (II), E °>0, but for reduced species.
For (III), E °>0, thus Cr³⁺ is the most stable.
More negative value of standard electrode reduction potential, better the reducing agent. Thus,
Mn²⁺ < CI^- < Cr³⁺ < Cr
Question 10
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Passage II

Accidently chewing of a stray fragment of aluminium foil can cause a sharp tooth pain if the aluminium comes in contact with an amalgam filling. This filling, an alloy of silver, tin and mercury acts as the cathode (of a tiny galvanic cell), the aluminium as anode and saliva (of mouth) serves as electrolyte. When the aluminium and the filling come in contact, an electric current passes from aluminium to the filling, which is sensed by a nerve in the teeth.

Q.

Net reaction taking place when amalgam is in contact with aluminium foil is

A

B
4AI + 3O₂ + 12H⁺ →4AI³⁺ + 6H₂ O

C
4AI + 3O₂ →2AI₂ O₃

D
2H₂ + O₃ →2H₂ O

Solution
Question 11
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Passage II

Accidently chewing of a stray fragment of aluminium foil can cause a sharp tooth pain if the aluminium comes in contact with an amalgam filling. This filling, an alloy of silver, tin and mercury acts as the cathode (of a tiny galvanic cell), the aluminium as anode and saliva (of mouth) serves as electrolyte. When the aluminium and the filling come in contact, an electric current passes from aluminium to the filling, which is sensed by a nerve in the teeth.

Q.

Standard emf experienced by the person with dental filling is

A
2.89 V

B
-2.89 V

C
-0.93 V

D
+0.43 V

Solution
Question 12
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Passage III

When suspected drunk drivers are tested with a breathalyser, the alcohol (ethanol) in the exhaled breath is oxidised to ethanoic acid with an acidic solution of potassium dichromate.

Q.

Under standard state, E^°_cell of the breathalyser would be

A
1.39V

B
1.27V

C
-1.39V

D
-1.51V

Solution

CH₃ CH₂ OH is oxidised to CH₃ COOH and is reduced to Cr³⁺ .
E °_cell - E °CH₃ CH₂ OH/CH₃ COOH + E ° /Cr³⁺
= -0.06+1.33
= 1.27 V
Reaction quotient
Question 13
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Passage III

When suspected drunk drivers are tested with a breathalyser, the alcohol (ethanol) in the exhaled breath is oxidised to ethanoic acid with an acidic solution of potassium dichromate.

Q.

Under the condition of unit activity of each species,emf recorded in the breathalyser at pH 4 would be

A
0.31 V

B
-1.01 V

C
1.64 V

D
0.95 V

Solution

CH₃ CH₂ OH is oxidised to CH₃ COOH and is reduced to Cr³⁺ .
E °_cell - E °CH₃ CH₂ OH/CH₃ COOH + E ° /Cr³⁺
= -0.06+1.33
= 1.27 V
Reaction quotient
Question 14
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E °values of Mⁿ⁺ + ne^- →M are given below
Fe²⁺ /Fe -0.44 V, Mg²⁺ /Mg -2.37 V, Pt(H₂ )/H⁺ 0.00 V,
AI³⁺ /AI -1.66 V , Zn²⁺ /Zn - 0.76 V, Cu²⁺ /Cu +0.34 V,
Ag⁺ /Ag + 0.80 V, Ni²⁺ /Ni -0.26 V

How many of the metals can be used to prevent corrosion of Ni to Ni²⁺ ?

A
4

B
3

C
2

D
1

Solution

Fe, Al, Zn and Mg can be used to protect Ni from being oxidised to Ni²⁺ since, E °_cell >0 and ΔG °<0.