Electrochemistry(12th Grade > Chemistry ) Questions and answers for exam Preparation

12th Grade > Chemistry

ELECTROCHEMISTRY MCQs

Total Questions : 30 | Page 1 of 3 pages

K M n O_{4}

acts as an oxidising agent in the neutral medium and gets reduced to

M n O_{2}

.The equivalent weight of

K M n O_{4}

in neutral medium

mol. wt/2
mol. wt/3
mol. wt/4
mol .wt/7

Discuss Question

Answer: Option B. -> mol. wt/3
:
B
In neutral medium

M n^{+ 7}

oxidation state changes into +4 oxidation state, hence equivalent weight of

K M n O_{4} = \frac{M}{3}

Question 2. The rusting of iron takes place as follows

2 H^{+} + 2 e^{-} + \frac{1}{2} O_{2} \to H_{2} O (l); E^{\circ} = + 1.23 V

F e^{2 +} + 2 e^{-} \to F e (s); E^{\circ} = - 0.44 V

Calculate

Δ G^{\circ}

for the net process

−322 kJ mol−1
−161 kJ mol−1
−152 kJ mol−1
76 kJ mol−1

Discuss Question

Answer: Option A. -> −322 kJ mol−1
:
A

\begin{matrix} F e (s) & ⟶ & F e^{2 +} & + & 2 e^{-}; & Δ G_{1}^{\circ} 2 H^{+} & + & 2 e^{-} & + & \frac{1}{2} O_{2} & ⟶ & H_{2} O (I); & Δ G_{2}^{\circ} F e (s) & + & 2 H^{+} & + & \frac{1}{2} O_{2} & ⟶ & F e^{2 +} & + & H_{2} O & ; Δ G_{3}^{\circ} \end{matrix}

Applying,

Δ G_{1}^{\circ} + Δ G_{2}^{\circ} = Δ G_{3}^{\circ}

Δ G_{3}^{\circ} = (- 2 F \times 0.44) + (- 2 F \times 1.23)

Δ G_{3}^{\circ} = - (2 \times 96500 \times 0.44 + 2 \times 96500 \times 1.23)

Δ G_{3}^{\circ} = - 322310 J

∴ Δ G_{3}^{\circ} = - 322 k J

Question 3. Corrosion of iron is essentially an electrochemical phenomenon where the cell reactions are

Fe is oxidised to Fe2+ and dissolved oxygen in water is reduced to O2−
Fe is oxidised to Fe3+ and H2O is reduced to O2−2
Fe is oxidised to Fe2+ and H2O is reduced to O−2
Fe is oxidised to Fe2+ and H2O is reduced to O2

Discuss Question

Answer: Option A. -> Fe is oxidised to Fe2+ and dissolved oxygen in water is reduced to O2−
:
A

A n o d e : F e (s) \to F e^{2 +} (a q) + 2 e^{-}

C a t h o d e : 4 H^{+} + O_{2} + 4 e^{-} \to 2 H_{2} O

Question 4. It has been observed that gaseous hydrogen chloride is a very poor conductor of electricity but a solution of hydrogen chloride gas in water is a good conductor of electricity. This is due to the fact that

Water is good conductor of electricity
Hydrogen chloride gas ionizes in water
A gas is an electronic conductor but a liquid is always an electrolytic conductor
Gas does not obey Ohm's law whereas a solution does

Discuss Question

Answer: Option B. -> Hydrogen chloride gas ionizes in water
:
B
HCl gas ionizes in water solution and the ions so formed act as charge carriers.

Question 5. Which colorless gas evolves, when

N H_{4} C l

reacts with zinc in a dry cell battery

Discuss Question

Answer: Option C. -> H2
:
C

2 N H_{4} C l + Z n \to 2 N H_{3} + Z n C l_{2} H_{2} ↑

Question 6. The limiting molar conductivities

λ^{\circ}

for

N a C l, K B r

and

K C l

are

126, 152

and

150 S c m^{2} m o l^{- 1}

respectively. The

λ^{\circ}

for

N a B r

278 S cm2mol−1
176 S cm2mol−1
128 S cm2mol−1
302 S cm2mol−1

Discuss Question

Answer: Option C. -> 128 S cm2mol−1
:
C

(126 s c m^{2}) \land_{N a C l}^{\circ} = \land_{N a +}^{\circ} + \land_{C l^{-}}^{\circ}

. . . . . . . (1)

(152 s c m^{2}) \land_{K B r}^{\circ} = \land_{K^{+}}^{\circ} + \land_{B r^{-}}^{\circ}

. . . . . . . (2)

(150 s c m^{2}) \land_{K C l}^{\circ} = \land_{K^{+}}^{\circ} + \land_{C l^{-}}^{\circ}

. . . . . . . (3)

By equation

(1) + (2) - (3)

∵ \land_{N a B r}^{\circ} = \land_{N a +}^{\circ} + \land_{B r^{-}}^{\circ} = 126 + 152 - 150 = 128 S c m^{2} m o l^{- 1}

Question 7. What is wrongly stated about electrochemical series

It is the representation of element in order of increasing or decreasing standard electrode reduction potential
It does not compare the relative reactivity of metals
It compares relative strengths of oxidizing agents
H2 is centrally placed element

Discuss Question

Answer: Option B. -> It does not compare the relative reactivity of metals
:
B
Electrochemical series compare the relative reactivity of metals.

Question 8. Assertion : Salts like

K C l, K N O_{3}

i.e., inert electrolytes are used in salt bridge.
Reason : An inert electrolyte can easily be filled in the U-tube.

If both assertion and reason are true and the reason is the correct explanation of the assertion.
If both assertion and reason are true but reason is not the correct explanation of the assertion.
If assertion is true but reason is false.
If the assertion and reason both are false.

Discuss Question

Answer: Option C. -> If assertion is true but reason is false.
:
C
Assertion is true but reason is false.
Ions of inert electrolytes are not involved in any electrochemical change until they react chemically with the electrolytes in the two half-cells.
The number of cations transferred = the number of anions transferred

Question 9.

4.5 g

of aluminum (at. mass 27amu) is deposited at cathode from

A l^{3 +}

solution by a certain quantity of electric charge. The volume of hydrogen produced at STP from

H +

ions in solution by the same quantity of electric charge will be

22.4 L
44.8 L
5.6 L
11.2 L

Discuss Question

Answer: Option C. -> 5.6 L
:
C

E q o f A l = e q o f H_{2}

\frac{4.5}{\frac{27}{3}} = e q o f H_{2}; \frac{4.5}{9} = e q o f H_{2}

2 H^{+} + 2 e^{-} \to H_{2}

eq. of

H_{2}

= Number of moles

\times n

factor

0.5 = n_{H_{2}} \times 2

V_{H_{2}} = \frac{0.5}{2} \times 22.4; V_{H_{2}} = 5.6 L

Question 10. In the experiment set up for the measurement of EMF of a half cell using a reference electrode and a salt bridge, when the salt bridge is removed, the voltage

Does not change
Decreases to half the value
Increase to maximum
Drops to zero

Discuss Question

Answer: Option D. -> Drops to zero
:
D
The salt bridge is required to maintain electrical neutrality. When the salt bridge is removed, charges would accumulate at the electrodes which prevents production of electricity.