Tag: chemistry

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Multiple choice chemistry chemical changes electrochemical reactions electrolysis and its applications electrolytic cells and electrolysis
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.

  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
Reveal answer Fill a bubble to check yourself
D Correct answer
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 : $C(s) + O^{2-} (melt) \rightarrow CO(g) + 2e^{-}$
$C(s) + 2O^{2-} (melt) \rightarrow CO _2 (g) + 4e^{-}$

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

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

  1. $H _2$
  2. Solid $K$
  3. $F _2$

  4. none of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
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 : $K^{\oplus} + e^{-} \rightarrow K$
$K + 2HF \rightarrow 2KF + F _2$

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

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.

  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
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In molten sodium hydride (NaH), electrolysis involves hydride ions (H-) migrating to the anode where they are oxidized to release hydrogen gas. The reason states that hydrogen ions have a lower discharge potential, which is incorrect contextually since NaH contains H- ions acting as the source of hydrogen at the anode, not H+ ions at the cathode in this specific molten setup. Thus, the assertion is true and the reason is false.

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

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

  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
Reveal answer Fill a bubble to check yourself
A,B Correct answer
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.

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

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

  1. cathode

  2. anode

  3. hydrogen is not liberated at all
  4. $H _2$ produced reacts with oxygen to form water
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

Which of the following statements is/are correct ?

  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.
Reveal answer Fill a bubble to check yourself
A,C,D Correct answer
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.

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

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

  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.
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

During electrolysis of aqueous $CaCl _2$

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

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

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

  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
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

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

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

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

  1. $O _{2},\, H _{2}$
  2. $F _{2},\, Na$
  3. $O _{2},\, Na$
  4. $F _{2},\, H _{2}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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