Tag: chemical changes

Questions Related to chemical changes

A gas which does not conduct electricity in the liquid state but conducts electricity when dissolved in water is :

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. $HCl$.

  2. $H _2$.

  3. $SO _2$.

  4. $Cl _2$.

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Correct Option: A
Explanation:
It is HCl having a unique property which does not conduct electricity when it is in gas phase since the gas phase of HCl is in the form of molecules and not in ionic form, the anhydrous HCl gas does not conduct electricity.
             But HCl solution or Aquous $HCl$ conducts electricity since HCl completely dissociates in solution to form hydronium $H _3O^+$ and $Cl^-$ ions which are getting transported into opposite direction when potential electric junction is created by applying differential voltage, thus migration of these ions in opposite directions to each other allows electricity and thus becomes conductor of electricity.

Which metal is deposited at the cathode during the electrolysis of the solution of Copper [II] Sulphate:

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. $S$

  2. $Cl$

  3. $Cu$

  4. $Na$

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Correct Option: C
Explanation:

At cathode:
$Cu^{+2} +2e^- \rightarrow Cu$
At anode:
$2 H _2O \rightarrow O _2 + 4H^+ + 4 e^-$

The metal which is deposited at the cathode during the electrolysis of the solution of zinc sulphate is :

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. Zn

  2. Mg

  3. Na

  4. H

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Correct Option: A

Assertion: $NH _3$ gas conduct electricity.
Reason: $NH _3$ dissociates into ions in the gas phase.

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion

  2. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion

  3. Assertion is true but Reason is false

  4. Assertion is false but Reason is true

  5. Both Assertion and Reason are false

Show answer
Correct Option: E
Explanation:

$NH _3$ doesnt conducts electricity in gaseous form but in solution form due to formation of $NH _4^+,  OH^-$ ions, it conducts electricity.

Assertion: Sodium cannot be obtained by the electrolysis of aqueous solution of NaCl using Pt electrodes.
Reason: In the aqueous NaCl solution, $H _2O$ will get reduced during electrolysis.
chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion

  2. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion

  3. Assertion is true but Reason is false

  4. Assertion is false but Reason is true

  5. Both Assertion and Reason are false

Show answer
Correct Option: A
Explanation:

Correction : in question sodium cannot be obtained. In aqueous solution , it is $H _2O$ which is reduced.
$2H _2O + 2e^{-} \rightarrow 2H _2 + 2\overset {\ominus}{O} H$

In Down's process for the manufacture of sodium, $CaCl _2$ is added to:

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. decrease the melting point

  2. increase the melting point

  3. decrease the interionic interactions

  4. increase the interionic interactions

Show answer
Correct Option: A
Explanation:

$CaCl _2$ is added to lower the melting point from 1080 K (melting point of NaCl) to 850 K. This is due to following reasons.
a. At such high temperature, it is very difficult to maintain Na in the molten state during electrolysis.
b. Moreover, Na is voltile at this temperature and a part of it would vapourize.
c. Molten Na, gets dispersed in molten NaCl to form metallic fog at high temperature.
d. Both Na and $Cl _2$ produced in the electrolysis have a corrosive action on the vessel used.
The over all reactions nare as follows :
At cathode : $Na^{\oplus} + e^{-} \rightarrow Na$
At anode : $Cl^{\ominus} \rightarrow \displaystyle \frac{1}{2} Cl _2 + e^{-}$

Assertion: In Hall's process for the preparation of Al, graphite anode has a long life.
Reason: $O _2$ is liberated in the process at the graphite anode.
chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion

  2. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion

  3. Assertion is true but Reason is false

  4. Assertion is false but Reason is true

  5. Both Assertion and Reason are false

Show answer
Correct Option: D
Explanation:

Molten $Al _2O _3 + Na _3AiF _6 (Cryolite) + CaF _2 (Flurospar) + AlF _3$ is used.
The electrolysis of this mixture is carried out using carbon (graphite) electrodes. The $O _2$ liberated at the anode reacts with the graphite anode needs to be replaced periodically.
Thus, graphite anodes do not have long life.
The overall reactions are as follows :
Cathode : $Al^{+3}(melt) + 3e^{-} \rightarrow Al (l)$.
Anode : $C(s) + O^{2-} (melt) \rightarrow CO(g) + 2e^{-}$
$C(s) + 2O^{2-} (melt) \rightarrow CO _2 (g) + 4e^{-}$

______ is prepared by the electrolysis of molten $KHF _2$ and anhydrous $HF$.

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. $H _2$

  2. Solid $K$

  3. $F _2$

  4. none of the above

Show answer
Correct Option: C
Explanation:

Moissan in 1896 prepared $F _2$ as :
$KHF _2 \rightleftharpoons KF + HF$
$KF \rightleftharpoons  K^{\oplus} + F^{\ominus}$
At anode : $2F^{\ominus} \rightarrow F _2 + 2e^{-}$
At cathode : $K^{\oplus} + e^{-} \rightarrow K$
$K + 2HF \rightarrow 2KF + F _2$

Assertion: The electrolysis of molten sodium hydride liberates hydrogen gas at the anode.


Reason: At the cathode, hydrogen ions having lower discharge potential than sodium ions will be liberated.

Choose the correct option.

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion

  2. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion

  3. Assertion is true but Reason is false

  4. Assertion is false but Reason is true

  5. Both Assertion and Reason are false

Show answer
Correct Option: B

During electrolysis of aqueous $CuBr _{2}$ using Pt electrode:

chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis

  1. $Br _{2}(g)$ is evolved at anode.

  2. Cu(s) is deposited at cathode

  3. $Br _{2}(g)$ is evolved at anode and $H _{2}(g)$ at cathode

  4. $H _{2}(g)$ is evolved at anode

Show answer
Correct Option: A,B
Explanation:

Aqueous $CuBr _{2}$ :
At cathode:

$Cu^{2+}\, +\, 2e^{-}\, \rightarrow\, Cu$;  
At anode :
$2Br^{\ominus}\, \rightarrow\, Br _{2}\, +\, 2e^{-}$

$Br _{2}(g)$ is evolved at anode and Cu(s) is deposited at cathode.