Tag: chemical changes

Questions Related to chemical changes

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

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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. $SO _3$

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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 : $4\overset{\ominus}{O}H\, \rightarrow\, O _{2}\, +\, 2H _{2}O\, +\, 4e^{-}$

A solution of sodium sulphate was electrolyzed using some inert electrode. The products at the electrodes are:

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

  1. $O _2,\, H _2$

  2. $O _2,\, Na$

  3. $O _2,\, SO _2$

  4. $O _2,\, S _2O _8^{2-}$

Show answer
Correct Option: A
Explanation:

Since the reduction potential of $H _2O$ is greater than reduction potential of $Na^{\oplus}\, so\, H _2O$ undergoes reduction to give
$H _2(g)$ at cathode.

$\displaystyle H _2O\, +\, e^-\, \rightarrow\, \overset {\ominus}O H +\, \frac{1}{2} H _2(g)$

Similarly, the oxidation potential of $H _2O$ is greater than the oxidation potential of $SO _4^{2-}$ ion. So, $H _2O$ undergoes oxidation to give $O _2(g)$ at anode.

$\displaystyle H _2O\, \rightarrow\, 2H^{\oplus}\, +\, 2e^-\, +\, \frac{1}{2} O _2(g)$

Which one is wrong if electrolysis of $CH _3COONa (aq)$ is made using $Pt$ electrodes?

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

  1. $pH$ of solution increases

  2. Molar ratio of gases at anode and cathode is $ 3 : 1.$

  3. $[CH _3COO^{\ominus}]$ in solution decreases.

  4. The molar ration of gases at anode and cathode is $2 : 1.$

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Correct Option: D
Explanation:

Sodium acetate undergoes electrolysis to form ethane gas, carbondioxide and hydrogen gas.

Hydrogen is liberated at cathode and ethane at anode.

Select the wrong statement.

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

  1. The electrolysis of molten $CaH _2$ liberates $H _2$ at cathode

  2. During the discharge of lead storage battery, sulphuric acid is consumed.

  3. Sulphur acts as a polymerizing agent in the vulcanization of rubber.

  4. Galvanization of iron denotes coating with Zn.

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Correct Option: A

In Hall's process, in the production of Al, carbon is used as the anode material. The reactions are:

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

  1. Anode: $Al^3+3e^-\, \rightarrow\, Al$

    Cathode: $C(s)+2O^{2-}\, \rightarrow\, CO _2(s)\, +\, 4e^-$

  2. Anode: $C(s)+2O^{2-}\, \rightarrow\, CO _2(s)\, +\, 4e^-$

    Cathode: $Al^3+3e^-\, \rightarrow\, Al$

  3. Anode: $Al\, \rightarrow\, Ae^{3+}\, +\, e^-$

    Cathode: $CO _2\, +\, 4e^-\, \rightarrow\, C(s)\, +\, 2O^2$

  4. None of the above

Show answer
Correct Option: B
Explanation:

In the production of Al (in Hall's process), reactions are:
Cathode:
$Al^{3+}\, +\, 3e^-\, \rightarrow\, Al$
Anode:
$2O^{-2}\, \rightarrow\, O _2\, +\, 4e^-$
$C\, +\, O _2\, \rightarrow\, CO _2$