Tag: chemistry

Questions Related to chemistry

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.

During electrolysis of fused calcium hydride, the hydrogen is produced at:

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

  1. cathode

  2. anode

  3. hydrogen is not liberated at all

  4. $H _2$ produced reacts with oxygen to form water

Show answer
Correct Option: B
Explanation:

$CaH _2\, \xrightarrow {Electrolysis}\, Ca^{2+}\, +\, 2H^{-}$


At cathode (reduction) :
$Ca^{2+}\, +\, 2e^-\, \rightarrow\, Ca$

At anode (oxidation) :
$2H^{-}\, \rightarrow\, H _2\, +\, 2e^-$

At the anode, hydrogen is produced. 

Hence, option $B$ is correct.

The gas evolved at the anode when $K _2SO _4\, (aq)$ is electrolyzed between Pt electrodes is:

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

  1. $O _2$

  2. $H _2$

  3. $SO _2$

  4. $SO _3$

Show answer
Correct Option: A
Explanation:

Here,
Electrolysis of $H _2O$ takes place:

$2H _2O\, \rightarrow\, O _2\, +\, 4H^{\oplus}\, +\, 4e^-$

and $O _2$ evolved.

Which of the following statements is/are correct ?

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

  1. The electrolysis of concentrated $H _{2}SO _{4}$ at $0 - 5^{\circ}C$ using a Pt electrode produces $H _{2}S _{2}O _{8}$

  2. The electrolysis of a brie solution produces $NaClO _{3}$ and NaClO.

  3. The electrolysis of a $CuSO _{4}$ solution using Pt electrodes causes the liberation of $O _{2}$ at anode and the deposition of copper at cathode.

  4. All electrolytic reactions are redox reactions.

Show answer
Correct Option: A,C,D
Explanation:

Electrolysis of concentrated $H _{2}SO _{4}$ at $0 - 5^{\circ}C$ using Pt electrode produces $H _{2}S _{2}O _{8}$.

At anode : $2H _{2}SO _{4}\, \rightarrow\, H _{2}S _{2}O _{8}\, +\, 2H^{\oplus}\, +\, 2e^{-}$

Statement (b) is wrong. Electrolysis of brine (aq. NaCl) gives $H _{2}(g)$ at cathode and $Cl _{2}(g)$ at anode.

Statements (c) and (d) are factual statements.

Hence, A, B and D are correct options.

When an aqueous solution of $CaCl _{2}$ is electrolyzed using inert electrodes, which of the following is(are) true ?

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

  1. Calcium deposits on cathode

  2. Calcium deposits an anode

  3. Chlorine is liberated on anode

  4. Calcium hydroxide precipitates near cathode on prolonged hydrolysis.

Show answer
Correct Option: C,D
Explanation:

During electrolysis of aqueous $CaCl _2$

We have,
At anode: $2Cl^- \rightarrow Cl _2 + 2e^-$
At anode: $2H^+ + 2e^- \rightarrow H _2$
Electrolyte = $Ca^{2+} + OH^-$

Which of the following does not evolve oxygen at anode when the electrolysis is carried out of:

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

  1. Dilute $H _{2}SO _{4}$ with $Pt$ electrodes

  2. Fused sodium hydroxide with $Pt$ electrodes

  3. Acidic water with $Pt$ electrodes

  4. Dilute sulphuric acid using $Cu$ electrodes

Show answer
Correct Option: D
Explanation:

$E^{\ominus} _{ox}$ of $Cu$ =$ - 0.34 $
Using copper electrodes, copper will oxidize : 
$Cu\, \rightarrow\, Cu^{2+}\, +\, 2e^{-}$
So oxygen is not evolved.

A dilute aqueous solution of sodium fluoride is electrolyzed, the products at the anode and cathode are:

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

  1. $O _{2},\, H _{2}$

  2. $F _{2},\, Na$

  3. $O _{2},\, Na$

  4. $F _{2},\, H _{2}$

Show answer
Correct Option: A
Explanation:

Ion with higher potential will go to corresponding electrode and discharge.
So,
$NaF$:
Cathode : $2H^{\oplus}\, +\, 2e^{-}\, \rightarrow\, H _{2}$
Anode : $4\overset{\ominus}{O}H\, \rightarrow\, O _{2}\, +\, 2H _{2}O\, +\, 4e^{-}$