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Chapter 5

Electrolysis

Class 10 - Dalal Simplified ICSE Chemistry Solutions


Equations Worksheet

Question 1

1. ElectrolysisComplete and balance the electrode reactions
a. Electrolysis of fused lead bromide   
1. Dissociation of lead bromidePbBr2 ⇌ ............... + ............... [ions present Pb2+, Br1-] ]
2. Reaction at cathode [inert-graphite]Pb2+ + ............... ⟶ ............... [product Lead metal]
3. Reaction at anode [inert-graphite]Br1- - ............... ⟶ ...............
Br + Br ⟶ ............... [product Bromine vapours]
b. Electrolysis of acidified water   
4. Dissociation of acidified waterH2SO4 ⇌ ............... + ...............
   H2O ⇌ ............... + ............... [ions H1+, SO42-, OH1-]
5. Reaction at cathode [inert-platinum]H1+ + ............... ⟶ ...............
   H + H ⟶ ............... [product Hydrogen gas]
6. Reaction at anode [inert-platinum]OH1- - ............... ⟶ ...............
   4OH ⟶ ............... + ............... [product oxygen gas]
c. Electrolysis of aq. copper sulphate [Active-copper electrode] 
7. Dissociation of aq. copper sulphateCuSO4 ⇌ ............... + ............... [ions present — Cu2+, H1+, SO42-, OH1-]
H2O ⇌ ............... + ...............
8. Reaction at cathode [copper (or Pt. or C)]Cu2+ + ............... ⟶ ............... [product copper metal]
9. Reaction at anode [active copper]Cu - ............... ⟶ ............... [product nil - Cu2+ ions]
[Inert-platinum electrode]
Dissociation of aq. copper sulphate
CuSO4 ⇌ ............... + ............... [ions present — Cu2+, H1+, SO42-, OH1-]
H2O ⇌ ............... + ...............
Reaction at cathode [Pt. or Carbon or Cu]Cu2+ + ............... ⟶ ............... [product copper metal]
Reaction at anode [inert-Pt. or C]OH1- - ............... ⟶ ...............
4OH ⟶ ............... + ............... [product oxygen gas]

Answer

1. ElectrolysisComplete and balance the electrode reactions
a. Electrolysis of fused lead bromide   
1. Dissociation of lead bromidePbBr2Pb2+ + 2Br1- [ions present Pb2+, Br1-] ]
2. Reaction at cathode [inert-graphite]Pb2+ + 2e-Pb [product lead metal]
3. Reaction at anode [inert-graphite]Br1- - 1e-Br
Br + Br ⟶ Br2 [product Bromine vapours]
b. Electrolysis of acidified water   
4. Dissociation of acidified waterH2SO42H1+ + SO42-
   H2O ⇌ H1+ + OH1- [ions H1+, SO42-, OH1-]
5. Reaction at cathode [inert-platinum]H1+ + 1e-H x 4
   2H + 2H ⟶ 2H2 [product Hydrogen gas]
6. Reaction at anode [inert-platinum]OH1- - 1e-OH x 4
   4OH ⟶ 2H2O + O2 [product oxygen gas]
c. Electrolysis of aq. copper sulphate [Active-copper electrode]
7. Dissociation of aq. copper sulphateCuSO4Cu2+ + SO42- [ions present — Cu2+, H1+, SO42-, OH1-]
H2O ⇌ H1+ + OH1-
8. Reaction at cathode [copper (or Pt. or C)]Cu2+ + 2e-Cu [product copper metal]
9. Reaction at anode [active copper]Cu - 2e-Cu2+ [product nil - Cu2+ ions]
[Inert-platinum electrode]
Dissociation of aq. copper sulphate
CuSO4Cu2+ + SO42- [ions present — Cu2+, H1+, SO42-, OH1-]
H2O ⇌ H1+ + OH1-
Reaction at cathode [Pt. or C or Cu)]Cu2+ + 2e-Cu [product copper metal]
Reaction at anode [inert-pt. or C]OH1- - 1e-OH x 4
4OH ⟶ 2H2O + O2 [product oxygen gas]

Question 2A

2. A : ElectroplatingComplete and balance the electrode reactions
Electroplating of an article with nickle   
10. Dissociation of aq. nickle sulphateNiSO4 ⇌ ............... + ...............
   H2O ⇌ ............... + ...............
11. Reaction at cathode [article to be plated]Ni2+ + ............... ⟶ ............... [Ni deposited on article]
12. Reaction at anode [block of active-nickle]Ni - ............... ⟶ ............... [product nil - Ni2+ ions]
Electroplating of an article with silver   
13. Dissociation of sodium silver cyanideNa[Ag(CN)2] ⇌ ............... + ............... + ...............
   H2O ⇌ ............... + ...............
14. Reaction at cathode [article to be plated]Ag1+ + ............... ⟶ ............... [Ag deposited on article]
15. Reaction at anode [block of active-silver]Ag - ............... ⟶ ............... [product nil - Ag1+ ions]

Answer

2.A : ElectroplatingComplete and balance the electrode reactions
Electroplating of an article with nickle   
10. Dissociation of aq. nickle sulphateNiSO4Ni2+ + SO42-
   H2O ⇌ H1+ + OH1-
11. Reaction at cathode [article to be plated]Ni2+ + 2e-Ni [Ni deposited on article]
12. Reaction at anode [block of active-nickle]Ni - 2e-Ni2+ [product nil - Ni2+ ions]
Electroplating of an article with silver   
13. Dissociation of sodium silver cyanideNa[Ag(CN)2] ⇌ Na1+ + Ag1+ + 2CN1-
   H2O ⇌ H1+ + OH1-
14. Reaction at cathode [article to be plated]Ag1+ + 1e1-Ag [Ag deposited on article]
15. Reaction at anode [block of active-silver]Ag - 1e1-Ag1+ [product nil - Ag1+ ions]

Question 2B

2. B Electrorefining of copperComplete and balance the electrode reactions
16. Dissociation of aq. copper sulphateCuSO4 ⇌ ............... + ...............
   H2O ⇌ ............... + ...............
17. Reaction at cathode [pure thin sheet of Cu]Cu2+ + ............... ⟶ ............... [Cu deposited on thin sheet]
18. Reaction at anode [impure block of active Cu]Cu - ............... ⟶ ............... [product nil - Cu2+ ions]
Electrometallurgy of sodium   
19. Dissociation of sodium chloride fusedNaCl ⇌ ............... + ...............
20. Reaction at cathode [inert - electrode]Na1+ + ............... ⟶ ............... [product Na metal]
21. Reaction at anode [inert - electrode]Cl1- - ............... ⟶ ...............
   Cl + Cl ⟶ ............... [product Cl2 gas]
Electrometallurgy of aluminium   
22. Dissociation of pure aluminaAl2O3 ⇌ ............... + ...............
23. Reaction at cathode [inert - electrode]Al3+ + ............... ⟶ ............... [product Al metal]
24. Reaction at anode [inert - electrode]O2- - ............... ⟶ ............... ⟶ 3O2 [product O2 gas]

Answer

2. B Electrorefining of copperComplete and balance the electrode reactions
16. Dissociation of aq. copper sulphateCuSO4Cu2+ + SO42-
   H2O ⇌ H1+ + OH1-
17. Reaction at cathode [pure thin sheet of Cu]Cu2+ + 2e-Cu [Cu deposited on thin sheet]
18. Reaction at anode [impure block of active Cu]Cu - 2e-Cu2+ [product nil - Cu2+ ions]
Electrometallurgy of sodium   
19. Dissociation of sodium chloride fusedNaCl ⇌ Na1+ + Cl1-
20. Reaction at cathode [inert - electrode]Na1+ + 1e-Na [product Na metal]
21. Reaction at anode [inert - electrode]Cl1- - 1e-Cl
   Cl + Cl ⟶ Cl2 [product Cl2 gas]
Electrometallurgy of aluminium   
22. Dissociation of pure aluminaAl2O32Al3+ + 3O2-
23. Reaction at cathode [inert - electrode]2Al3+ + 6e-2Al [product AL metal]
24. Reaction at anode [inert - electrode]3O2- - 6e-3[O] ⟶ 3O2 [product O2 gas]

Questions

Question 1(2008)

During the electrolysis of molten lead bromide, which of the following takes place ?

  1. Bromine is released at the cathode
  2. Lead is deposited at the anode
  3. Bromine ions gain electrons
  4. Lead is deposited at the cathode

Answer

Lead is deposited at the cathode

Reaction at cathode : Pb2+ + 2e- ⟶ Pb

Question 2.1(2008)

Here is an electrode reaction:

Cu ⟶ Cu2+ + 2e-. [i.e. Cu – 2e- ⟶ Cu2+]

At which electrode (anode or cathode) would such a reaction take place ? Is this an example of oxidation or reduction ?

Answer

Cu ⟶ Cu2+ + 2e-

This reaction takes place at anode.

This is an example of oxidation.

Question 2.2(2008)

A soln. contains Mg2+ ions, Fe2+ ions and Cu2+ ions. On passing an electric current through this soln. which ions will be the first to be discharged at the cathode? Write the equation for the cathode reaction.

Answer

Cu2+ (Copper ions will get discharged at cathode)

Reaction at Cathode:
Cu2+ + 2e- ⟶ Cu

Question 1(2009)

(A) Sulphur

(B) Silver chloride

(C) Hydrogen chloride

(D) Copper [II] sulphate

(E) Graphite

State which from A to E fits the description — 'A pink metal is deposited at the cathode during the electrolysis of the soln. of this salt.

Answer

(D) Copper [II] sulphate

At cathode : Cu2+ + 2e- ⟶ Cu

Question 2(2009)

Select the correct answer — The aqueous solution of the compound which contains both ions and molecules is:

  1. Sulphuric acid
  2. Hydrochloric acid
  3. Nitric acid
  4. Acetic acid

Answer

Acetic acid
Reason — Acetic acid is a weak electrolyte and particles in a weak electrolyte are ions and unionized molecules.

Question 3(2009)

Correct the following statements — Lead bromide conducts electricity.

Answer

Molten lead bromide conducts electricity.
Reason — Solid lead bromide is a non-conductor of electricity since it's ions are not free but held together by an electrostatic force of attraction. In molten state the ions become free and hence conduct electricity.

Question 4(2009)

A metal article is to be electroplated with silver. The electrolyte selected is sodium argentocyanide.

(i) What kind of salt is sodium argentocyanide.

(ii) Why is it preferred to silver nitrate as an electrolyte?

(iii) State one condition to ensure that the deposit is smooth, firm and long lasting.

(iv) Write the reaction taking place at the cathode.

(v) Write the reaction taking place at the anode.

Answer

(i) It is a Complex Salt.

(ii) Migration of Ag1+ ions from above complex salt soln. is slow compared to that from silver nitrate. Hence, an even deposition of metal silver is obtained on the article. Therefore, the electrolyte sodium argentocyanide soln. is preferred over silver nitrate solution.

(iii) A low current for a longer time should be used to ensure that the deposit is smooth, firm and long lasting.

(iv) Ag1+ + 1e- ⟶ Ag [deposited]

(v) Ag - 1e- ⟶ Ag1+ [Anode diminishes in mass]

Question 5(2009)

Aqueous solution of nickel sulphate contains Ni2+ and SO42- ions

(i) Which ion moves towards the cathode?

(ii) What is the product at the anode?

Answer

(i) Nickel ion moves towards the cathode.

(ii) Product at anode - Nil [Ni2+ are formed]

Question 1(2010)

Select the correct answer — A compound which during electrolysis in it's molten state liberates a reddish brown gas at the anode :

  1. Sodium chloride
  2. Copper (II) oxide
  3. Copper (II) sulphate
  4. Lead (II) bromide

Answer

Lead (II) bromide
Reason — During electrolysis of Lead [II] bromide, Br-1 ions are discharged at the anode. Reddish brown fumes are due to bromine vapours.

Question 2(2010)

During electroplating of an article with nickel —

(i) Name — (a) The electrolyte (b) The cathode (c) The anode

(ii) Give the reaction of the electrolysis at (a) The cathode (b) The anode

Answer

(i) (a) The electrolyte — Aq. soln. of nickel sulphate (b) The cathode — Cleaned article to be electroplated (c) The anode — Plate or block of nickel metal

(ii) the reaction of the electrolysis at:
(a) At Cathode: Ni2+ + 2e ⟶ Ni
(b) At Anode : Ni – 2e ⟶ Ni2+

Question 3(2010)

A, B and C are three electrolytic cells, connected in different circuits. Cell 'A' contains NaCl soln., and the bulb in the circuit glows brightly, when the circuit is completed. Cell 'B' contains acetic acid and the bulb glows dimly. Cell 'C' contains sugar soln., and the bulb does not glow. Give reasons for each observation.

Answer

When the switch is 'On' the electrolyte starts dissociating. Current remaining the same, the glow of the bulb indicates whether the electrolyte is a strong, weak or a non-electrolyte.

As NaCl is a strong electrolyte (i.e., cell A), therefore it allows large amount of electricity to flow through it. Hence, the bulb glows brightly.

Acetic acid is a weak electrolyte (i.e., cell B) and it allows a small amount of electricity to flow through them hence, the bulb glows dimly.

Sugar soln. is a non-electrolyte (i.e., cell C), therefore it does not conduct electricity and do not undergo chemical decomposition due to the flow of current through it. Hence, the bulb does not glow.

Question 1(2011)

Give reasons – The electrolysis of acidulated water is considered to be an example of catalysis.

Answer

Pure water is almost a non-electrolyte and will not normally conduct electricity. It consists almost entirely of molecules. It can be electrolytically decomposed by addition of traces of dil. H2SO4 which dissociates as: H+1 and SO42- and help in dissociating water into H+1 and OH- ions, water being a polar solvent. Hence, electrolysis of acidulated water is considered to be an example of catalysis.

Question 2(2011)

During electrolysis of CuSO4 solution using platinum [cathode] and carbon [anode]:

(i) State what you observe at the cathode and anode.

(ii) State the change noticed in the electrolyte

(iii) Write the reactions at the cathode and anode.

Answer

(i) At cathode : Cu2+ ions and H1+ ions migrate to the cathode. Cu2+ ions are discharged in preference to H1+.

At anode : SO42- and OH1- ions migrate to the anode but OH1- ions are discharged since they are lower in the electrochemical series.

(ii) The blue colour of CuSO4 solution fades since the blue Cu2+ ions which are discharged at the cathode are not replaced at the anode.

(iii) Cathode : Cu2+ + 2e- ⟶ Cu

Anode: OH1- - 1e- ⟶ OH x 2

[2OH ⟶ H2O + [O]]

4OH ⟶ 2H2O + O2

Question 3(2011)

Differentiate between electrical conductivity of – copper sulphate solution and of copper metal.

Answer

Electrical conductivity of
copper sulphate solution
Electrical conductivity of
copper metal
The flow of electricity takes place by flow of ions which are denser compared to electrons.The flow of electricity takes place by flow of electrons which have negligible mass.
There is decomposition of the copper sulphate solution and thus the chemical properties of electrolyte are altered.There is no decomposition of copper metal and thus the chemical properties of copper are intact.
Good conductors of electricity in aq. soln. or molten state but not in solid state.Good conductor of electricity in solid and in molten state.
During conduction through copper sulphate solution, there is transfer of ions.During conduction through copper metal, there is no transfer of matter.
The flow of electricity decomposes the electrolyte and new products are formed.The flow of electricity only produces heat energy and no new products are formed.

Question 1(2012)

Rewrite the correct statement with the missing word/s: Cations migrate during electrolysis.

Answer

Cation migrates to the cathode during electrolysis.

Question 2(2012)

Identify the weak electrolyte from the following:

  1. Sodium Chloride solution
  2. Dilute Hydrochloric acid
  3. Dilute Sulphuric acid
  4. Aqueous acetic acid.

Answer

Aqueous acetic acid

Question 3(2012)

Match the following in column A with the correct answer from the choices given in column B.

Column AColumn B
1. Ammonium hydroxide soln.Contains only ions.
2. Dilute hydrochloric acidContains only molecules
3. Carbon tetrachlorideContains ions and molecules

Answer

Column AColumn B
1. Ammonium hydroxide soln.Contains ions and molecules
2. Dilute hydrochloric acidContains only ions
3. Carbon tetrachlorideContains only molecules

Question 4(2012)

Give reasons: An aqueous solution of sodium chloride conducts electricity.

Answer

The ions of sodium chloride are not free but held together by strong electrostatic force of attraction. However in aqueous solution of sodium chloride the ions break lose and move freely. Hence, an aqueous solution of sodium chloride conducts electricity.

Question 5(2012)

Select the correct answer from the list in brackets:

(i) An aqueous electrolyte consists of the ions mentioned in the list. The ion which could be discharged most readily during electrolysis. [Fe2+, Cu2+, H+].

(ii) The metallic electrode which does not take part in an electrolytic reaction. [Cu, Ag, Pt, Ni].

(iii) The ion which is discharged at the anode during the electrolysis of copper sulphate solution using copper electrodes as anode and cathode. [Cu2+, OH1-, SO42-, H+].

(iv) When dilute sodium chloride is electrolysed using graphite electrodes, the cation which is discharged at the cathode most readily. [Na1+, OH1-, H1+, Cl1-].

(v) During silver plating of an article using potassium argentocyanide as an electrolyte, the anode material should be [Cu, Ag, Pt, Fe].

Answer

(i) An aqueous electrolyte consists of the ions mentioned in the list. The ion which could be discharged most readily during electrolysis. — Cu2+.

(ii) The metallic electrode which does not take part in an electrolytic reaction. — Pt.

(iii) The ion which is discharged at the anode during the electrolysis of copper sulphate solution using copper electrodes as anode and cathode. — Cu2+.

(iv) When dilute sodium chloride is electrolysed using graphite electrodes, the cation which is discharged at the cathode most readily. — H1+

(v) During silver plating of an article using potassium argentocyanide as an electrolyte, the anode material should be — Ag.

Question 1(2013)

State one appropriate observation for : Electricity is passed through molten lead bromide.

Answer

Solid lead bromide is a non conductor of electric current since it's ions are not free and held together by strong electrostatic force of attraction. The ions become free in molten state. Hence, electricity is passed through molten lead bromide.

Question 2(2013)

State which of these will act as a non-electrolyte ?

  1. Liquid carbon tetrachloride
  2. Acetic acid
  3. Sodium hydroxide aqueous solution acid.
  4. Potassium chloride aqueous solution.

Answer

Liquid carbon tetrachloride is a non-electrolyte

Question 3(2013)

Copper sulphate soln. is electrolysed using copper electrodes as seen in diagram.

Copper sulphate soln. is electrolysed using copper electrodes as seen in diagram. Which electrode to your left or right is known as the oxidising electrode and why? Write the equation representing the reaction that oc­curs. State two appropriate observations for the above elec­trolysis reaction. Electrolysis, Simplified Chemistry Dalal Solutions ICSE Class 10.

(i) Which electrode to your left or right is known as the oxidising electrode and why?

(ii) Write the equation representing the reaction that oc­curs.

(iii) State two appropriate observations for the above elec­trolysis reaction.

Answer

(i) The electrode connected to the positive terminal of the battery (i.e., electrode on the left) is the oxidizing electrode as anions donate excess electrons to the anode and are oxidized to neutral atoms.

(ii) Cu – 2e ⟶ Cu2+

(iii) Copper anode shows a loss in mass. The blue colour of aq. copper sulphate soln. remains unchanged.

Question 4(2013)

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

Answer

Hydrogen chloride gas

Question 1(2014)

The observation seen when fused lead bromide is electrolysed is:

  1. a silver grey deposit at anode and a reddish brown deposit at cathode.
  2. a silver grey deposit at cathode and a reddish brown deposit at anode.
  3. a silver grey deposit at cathode and reddish brown fumes at anode.
  4. silver grey fumes at anode and reddish brown fumes at cathode.

Answer

a silver grey deposit at cathode and reddish brown fumes at anode.
Reason — silver grey colour is due to lead metal deposited at cathode and reddish brown fumes at anode is due to bromine vapours.

Question 2(2014)

During electroplating an article with silver, the electrolyte used is:

  1. silver nitrate solution
  2. silver cyanide solution
  3. sodium argentocyanide solution
  4. nickel sulphate solution

Answer

sodium argentocyanide solution

Question 3(2014)

Give one word or phrase for: Electrolytic deposition of a superior metal on a base metal.

Answer

Electroplating

Question 4(2014)

State your observation: At the cathode when acidified aq. CuSO4 soln. is electrolysed with copper electrodes.

Answer

Copper, a brownish pink metal is deposited at the cathode when acidified aq. CuSO4 soln. is electrolysed with copper electrodes

Question 5(2014)

State which electrode: anode or cathode is the oxidizing electrode. Give a reason for the same.

Answer

Anode is the oxidizing electrode.
Reason — The anions donate the excess electrons to the anode and are oxidized to neutral atoms. Hence, the anode is the oxidizing electrode by which the electrons leave the electrolyte.

Question 6(2014)

Name the kind of particles present in:

(i) Sodium hydroxide soln.

(ii) Carbonic acid.

(iii) Sugar soln.

Answer

(i) Strong electrolyte — Sodium (Na+) ions and hydroxide (OH-) ions.

(ii) Weak electrolyte — Ions (H+, HCO3, CO32- ) and molecules (H2CO3)

(iii) Non-electrolyte — Molecules of sugar soln.

Question 7(2014)

M2O is the oxide of a metal 'M' which is the above hydrogen in the activity series. M2O when dissolved in water forms the corresponding hydroxide which is a good conductor of electricity.

(i) State the reaction taking place at the cathode.

(ii) Name the product at the anode.

Answer

Reaction at cathode : M+ + e- ⟶ M

Product at anode : Oxygen

Question 1(2015)

State one observation for. 'At the anode when aqueous copper sulphate solution is electrolysed using copper electrodes'.

Answer

Copper anode shows a loss in mass.

Question 2(2015)

Give scientific reasons :

(i) During electrolysis of molten lead bromide, graphite anode is preferred to other electrodes.

(ii) Electrical conductivity of acetic acid is less in comparison to that of dil. H2SO4 – at a given concentration.

(iii) Electrolysis of molten lead bromide is considered to be a redox reaction.

Answer

(i) As graphite is unaffected by the reactive bromine vapours released at the anode hence, a graphite anode is preferred during the electrolysis of molten lead bromide.

(ii) Dil. H2SO4 is a strong electrolyte and acetic acid is a weak electrolyte. Therefore, dil. H2SO4 allows large amount of electricity to flow through them and are good conductors of electricity whereas acetic acid allows small amount of electricity to flow through them and are poor conductors of electricity. Hence, electrical conductivity of acetic acid is less in comparison to that of dil. H2SO4 – at a given concentration.

(iii) Electrolysis of molten lead bromide involves oxidation and reduction reactions and hence is a redox reaction.

Reduction reaction at cathode:
Pb2+ + 2e- ⟶ Pb

Oxidation reaction at the anode:
Br1- - 1e- ⟶ Br
Br + Br ⟶ Br2

Question 3(2015)

Differentiate between the terms strong electrolyte and weak electrolyte. (stating any two differences)

Answer

Strong electrolytesWeak electrolytes
They are electrolytes which allow a large amount of electricity to flow through them and hence are good conductors of electricity.They are electrolytes which allow small amount of electricity to flow through them and hence are poor conductors of electricity.
They are almost completely dissociated in fused or aqueous solution state.They are partially dissociated in fused or aqueous soln. state.

Question 4(2015)

During purification of copper – State:

(i) The anode used

(ii) The electrolyte used.

Write the equation taking place at the anode in the above.

Answer

(i) Impure block of copper

(ii) Aq. copper sulphate solution [acidified]

Equation at the anode: Cu – 2e- ⟶ Cu2+

Question 1(2016)

The particles present in strong electrolytes are:

Select the correct answer from A, B, C and D:

A: only molecules
B: mainly ions
C: ions and molecules
D: only atoms

Answer

mainly ions

Question 2(2016)

Write equations for the reactions taking place at the two electrodes during the electrolysis of:

(i) Acidified CuSO4 solution with copper electrodes.

(ii) Molten PbBr2 with inert electrodes. (mention clearly the name of the electrode in each case)

Answer

(i) Electrode — Cathode : Copper, Anode : Copper

Dissociation of aq. CuSO4 :

CuSO4 ⇌ Cu2+ + SO42-

H2O ⇌ H1+ + OH1-

H2SO4 ⇌ 2H1+ + SO42-

Reaction at cathode [pure thin sheet of Cu]:
Cu2+ + 2e- ⟶ Cu [Cu deposited on thin sheet]

Reaction at anode [impure block of active Cu]:
Cu - 2e- ⟶ Cu2+ [product nil - Cu2+ ions]

(ii) Electrode — Cathode : Iron or Graphite, Anode : Graphite

Dissociation of molten PbBr2 : PbBr2 ⇌ Pb2+ (cathode) + 2Br1- (anode)

Reaction at cathode:
Pb2+ + 2e- ⟶ Pb [product lead metal]

Reaction at anode:
Br1- - 1e-⟶ Br
Br + Br ⟶ Br2

Question 3(2016)

Name:

(i) the product formed at the anode – during electrolysis of acidified water using platinum electrodes.

(ii) the metallic ions that should be present – in the electrolyte when an article made of copper is to be electroplated with silver.

Answer

(i) Oxygen

(ii) Silver ions

Question 4(2016)

Give reasons why:

(i) Sodium Chloride will conduct electricity only in fused or aq. soln. state.

(ii) In the electroplating of an article with silver, the electrolyte sodium argentocyanide soln. is preferred over silver nitrate solution.

(iii) Although copper is a good conductor of electricity, it is a non-electrolyte.

Answer

(i) The ions Na+ and Cl are not free but held together by strong electrostatic force of attraction. In fused or molten state the ions break free and move. Hence, NaCl will conduct electricity only in fused state or aq. soln. state.

(ii) Migration of Ag1+ ions from sodium argentocyanide solution is slow compared to that from silver nitrate. Hence, an even deposition of metal silver is obtained on the article. Therefore, the electrolyte sodium argentocyanide soln. is preferred over silver nitrate solution.

(iii) Copper does not undergo chemical decomposition due to flow of electric current through it. Hence, copper is a good conductor of electricity but it is a non-electrolyte.

Question 1(2017)

Identify the substance underlined, in each of the following —

(i) The electrolyte used for electroplating an article with silver.

(ii) The particles present in a liquid such as kerosene, that is a non-electrolyte.

Answer

(i) Sodium argentocyanide

(ii) Molecules only

Question 2(2017)

State the observations at the anode & at the cathode during the electrolysis of –

(i) Fused lead bromide using graphite electrodes.

(ii) Copper sulphate solution using copper electrodes.

Answer

(i) The observations at the anode & at the cathode are:
At cathode — silvery grey deposit of lead metal.
At anode — reddish brown fumes of bromine vapours.

(ii) The observations at the anode & at the cathode are:
At cathode — Brownish pink copper metal is deposited at cathode during electrolysis of copper sulphate.
At anode — Copper ions are formed. Copper anode diminishes in mass.

Blue colour of CuSO4 remains unchanged.

Question 3(2017)

Select the ion in each case, that would get selectively discharged from the aqueous mixture of the ions listed below:

(i) SO42-, NO3- and OH- ;

(ii) Pb2+, Ag+ and Cu2+.

Answer

(i) OH- ion will get discharged in preference to SO42- or NO3- ions.

(ii) Ag+ ions will get discharged in preference to Pb2+ or Cu2+ ions.

Question 1(2018)

Choose the correct answer from the options given : The electrolysis of acidified water is an example of :

  1. Reduction
  2. Oxidation
  3. Redox reaction
  4. Synthesis

Answer

Redox reaction
Reason — Reaction at cathode: Hydrogen ions, H+ being the only positively charged ions migrate to the cathode. Since, the cathode is a reservoir of electrons , H+ ions gain an electron and become a neutral hydrogen atom. Therefore reduction takes place at cathode.

H+ + e- ⟶ H

H + H ⟶ H2

Reaction at anode : SO42- and OH- ions both migrate to the anode. OH-being lower in electrochemical series is discharged preferentially.

OH- ions loses an electron to the anode and becomes neutral OH. The combination of OH forms water with the liberation of oxygen, given off at anode.

OH- ⟶ OH + e-

OH + OH ⟶ H2O + O

O + O ⟶ O2

Therefore, oxidation takes place at anode. Hence, electrolysis of acidified water is an example of redox reaction

Question 2(2018)

State one relevant observation : At the anode when molten lead bromide is electrolysed using graphite electrodes

Answer

At the anode, brown fumes of bromine vapours are observed.

Question 3(2018)

Give a reason for : Conductivity of dilute hydrochloric acid is greater than that of acetic acid.

Answer

Dil. H2SO4 is a strong electrolyte and acetic acid is a weak electrolyte. Therefore, dil. H2SO4 allows large amount of electricity to flow through it and is a good conductor of electricity whereas acetic acid allows small amount of electricity to flow through it and is a poor conductor of electricity. Hence, electrical conductivity of dilute hydrochloric acid is greater than that of acetic acid.

Question 4(2018)

Name the gas that is produced at the anode during the electrolysis of acidified water.

Answer

Hydrogen gas

Question 5(2018)

Fill in the blank with the correct choice given in the bracket - Electrolysis of aqueous sodium chloride solution will form ............... at the cathode. [hydrogen gas / sodium metal]

Answer

Electrolysis of aqueous sodium chloride solution will form hydrogen gas at the cathode.

Question 6(2018)

Copy and complete the following table which refers to the conversion of ions to neutral particles.

ConversionIonic equationOxidation/Reduction
Chloride ion to chlorine molecule
Lead [II] ion to lead

Answer

ConversionIonic equationOxidation/Reduction
Chloride ion to chlorine moleculeCl1- - 1e- ⟶ Cl [Cl2]
Cl + Cl ⟶ Cl2
Oxidation
Lead [II] ion to leadPb2+ + 2e- ⟶ PbReduction

Question 7(2018)

For the electro-refining of copper —

(i) State what the cathode is made up of.

(ii) Write the reaction that takes place at the anode.

Answer

(i) Pure thin sheet of copper.

(ii) Cu - 2e- ⟶ Cu2+

Question 1(2019)

Choose the correct answer from the options A, B, C, D given. An electrolyte which completely dissociates into ions is

  1. Alcohol
  2. Carbonic acid
  3. Sucrose
  4. Sodium hydroxide

Answer

Sodium hydroxide
Reason — Sodium hydroxide is a strong electrolyte hence, it completely dissociates into ions.

Question 2(2019)

State one observation for the following : Copper sulphate solution is electrolysed using copper electrodes.

Answer

A brownish pink deposit of copper metal is seen at the cathode when copper solution is electrolyzed using copper electrodes. The blue colour of Copper Sulphate solution does not face and Copper anode diminishes in mass.

Question 3(2019)

Arrange the following according to the instruction given in the bracket : Mg2+, Cu2+, Na1+, H1+ [In the order of preferential discharge at the cathode. ]

Answer

Cu2+, H1+, Mg2+, Na1+

Lower the position of the ion, greater the tendency to be liberated at the cathode (or respective electrode).

Question 4(2019)

Name the particles present in :

(i) Strong electrolyte

(ii) Non-electrolyte

(iii) Weak electrolyte

Answer

(i) Ions only

(ii) Molecules only

(iii) Ions and Molecules

Question 1(2020)

Choose the correct answer from the options given : The inert electrode used in the electrolysis of acidified water is :

  1. Nickle
  2. Platinum
  3. Copper
  4. Silver

Answer

Platinum

Question 2(2020)

State one relevant reason for : Graphite anode is preferred to platinum in the electrolysis of molten lead bromide.

Answer

As graphite is unaffected by the reactive bromine vapours released at the anode hence, a graphite anode is preferred to platinum in the electrolysis of molten lead bromide.

Question 3(2020)

Give one word or a phrase for the statement : Electrode used as cathode in electrorefining of impure copper.

Answer

Pure thin sheet of copper is used as cathode.

Question 4(2020)

Fill in the blanks from the choices give in bracket : An alkali which completely dissociates into ions is ............... [ammonium hydroxide, calcium hydroxide, lithium hydroxide]

Answer

An alkali which completely dissociates into ions is lithium hydroxide

Question 5(2020)

Choose the correct word which refers to the process of electrolysis from A to E, to match the description (i) to (iv): A: Oxidation, B: Cathode, C: Anode , D: An electrolyte , E : Reduction

(i) Conducts electricity in aq. or in molten state.

(ii) Loss of electrons takes place at anode.

(iii) A reducing electrode

(iv) Electrode connected to the positive end or terminal of the battery.

Answer

(i) Conducts electricity in aqueous or in molten state — D: An electrolyte

(ii) Loss of electrons takes place at anode — A: oxidation

(iii) A reducing electrode — B: Cathode

(iv) Electrode connected to positive end or terminal of the battery. — C: Anode

Question 6(2020)

Differentiate between the following pairs based on the information given in the bracket. Conductor and electrolyte [conducting particles]

Answer

ConductorElectrolyte
The flow of electricity takes place by flow of electrons which have negligible mass.The flow of electricity takes place by flow of ions which are denser compared to electrons.

Question 7(2020)

Identify the underlined substance: The electrode that increases in mass during the electro-refining of silver.

Answer

Cathode
Reason — Pure copper is deposited on the thin sheet of pure copper placed at the cathode.

Question 8(2020)

An aqueous solution of nickle [II] sulphate was electrolyzed using nickle electrodes. Observe the diagram and answer the questions that follow:

An aqueous solution of nickle [II] sulphate was electrolyzed using nickle electrodes. What do you observe at the cathode and anode respectively. Name the cation that remains as a spectator ion in the solution. Which equation for the reaction at the anode is correct. Electrolysis, Simplified Chemistry Dalal Solutions ICSE Class 10.

(i) What do you observe at the cathode and anode respectively.

(ii) Name the cation that remains as a spectator ion in the solution.

(iii) Which equation for the reaction at the anode is correct.

  1. Ni ⟶ Ni2+ + 2e-
  2. Ni + 2e- ⟶ Ni2+
  3. Ni2+ ⟶ Ni + 2e-
  4. Ni2+ + 2e- ⟶ Ni

Answer

(i) The observations at cathode and anode are:
At cathode: Ni2+ ions move towards cathode and are discharged as neutral Ni atoms by gain of electrons. Thus, Ni is deposited at cathode.
At anode: Diminishes in mass. Ni2+ ions are formed.

(ii) H+ ions.

(iii) Ni ⟶ Ni2+ + 2e-

Additional Questions

Question 1

Define :

(i) Electrolysis

(ii) Electrodes

(iii) Ions

(iv) Electrolytic dissociation.

Answer

(i) Electrolysis — It is the decomposition of a chemical compound [electrolyte] in the aqueous or fused [molten] state by the passage of direct electric current resulting in discharge of ions as neutral atoms at the respective electrodes.

(ii) Electrodes — They allow the electric current to enter or leave the electrolytic solution. The electrodes are two in number and are made of metal or carbon.

(iii) Ions — They are atoms which carry a positive or negative charge and become free and mobile when an electric current is passed through an aqueous solution of a chemical compound.

(iv) Electrolytic dissociation — The process due to which an ionic compound in the fused [molten] or in aqueous solution state dissociates into ions by passage of electric current through it is called electrolytic dissociation.

Question 2

Differentiate between

(i) Electrolytes and Non-electrolytes

(ii) Strong and Weak electrolytes

(iii) Anode and Cathode

(iv) Electrolytic dissociation and ionization with suitable examples.

Answer

(i) Differences between Electrolytes and Non-electrolytes are:

ElectrolytesNon-electrolytes
Chemical compound which conduct electricity in the fused or in aqueous solution state and undergo chemical decomposition due to the flow of current through it.Chemical compound which do not conduct electricity in the fused or aqueous solution state and do not undergo chemical decomposition due to the flow of current through it.
Electrolytes are ionic compoundsNon-electrolytes are covalent compounds
Particles in Electrolytes – ions only or Ions and molecules onlyParticles in non-electrolytes – Molecules only
Examples :
Acids — dil. HCl, HNO3 H2SO4.
Alkalis — KOH, NaOH solutions.
Ionic salts — PbBr2 [molten], CuSO4 [aq.]
Examples :
Pure or distilled water, Alcohol, Kerosene, Carbon disulphide, liquid carbon tetrachloride, sucrose, glucose, sugar solution.

(ii) Differences between Strong and Weak electrolytes are:

Strong electrolytesWeak electrolytes
They are electrolytes which allow a large amount of electricity to flow through them and hence are good conductors of electricity.They are electrolytes which allow small amount of electricity to flow through them and hence are poor conductors of electricity.
They are almost completely dissociated in fused or aqueous solution state.They are partially dissociated in fused or aqueous soln. state.
Particles in strong electrolytes are mainly ions onlyParticles in weak electrolytes are ions and unionized molecules.
Examples :
Generally all strong acids and bases and most salts of strong acids.
Acids: dil. HCl, H2SO4, HNO3, HBr, HI.
Bases : NaOH, KOH, LiOH soln.
Salts: NaCl, NaNO3, Na2SO4, CuCl2
Examples :
Generally all weak acids and bases and most salts of weak acids.
Acids: Carbonic, Acetic, Oxalic, Formic.
Bases: NH4OH, Ca(OH)2, Mg(OH)2.
Salts: Sodium carbonate, bicarbonate, oxalate and formate aq. soln.

(iii) Differences between Anode and Cathode are:

AnodeCathode
It is the electrode connected to the positive terminal of the batteryIt is the electrode connected to the negative terminal of the battery.
Anions migrate to anodeCations migrate to cathode.
The anions donate excess electrons to the anode and they are oxidised to neutral atoms. Hence, Anode is the Oxidising Electrode.The cations gain excess electrons from the cathode and they are reduced to neutral atoms. Hence, Cathode is the Reducing Electrode.

(iv) Differences between Electrolytic dissociation and ionization are:

Electrolytic DissociationIonization
It is a process which takes place in electrovalent compoundsIt is a process which takes place in covalent compounds.
It involves separation of ions which are already present in an ionic compounds.It involves formation of charged ions from molecules which are not in the ionic state.
PbBr2 (ionic) ⇌ Pb2+ + 2Br1-HCl [aq.] (covalent) ⇌ H1+ + Cl1-.
Ionization may also involve atoms changing into ions [e.g. Mg ⟶ Mg2+ + 2e-]

Question 3

Compare the flow of electricity through a nickel wire and nickel sulphate solution.

Answer

Electricity flow through Nickel wire (metal)Electricity flow through Nickel Sulphate Solution (electrolyte)
It is due to the flow of electrons which have negligible massIt is due to the flow of ions which are denser compared to electrons.
There is no decomposition of parent metal and thus the chemical properties of the metal are intact.There is decomposition of the electrolytic solution and thus the chemical properties of electrolyte are altered.
Good conductor of electricity in solid and in molten state.Good conductors of electricity in aqueous solution or molten state but not in solid state.
During conduction, there is no transfer of matter.During conduction there is transfer of ions.
The flow of electricity only produces heat energy and no new products are formed.The flow of electricity decomposes nickel sulphate solution and new product — Nickel is formed.

Question 4

Name three organic compounds and one neutral liquid which are non-electrolytes.

Answer

Organic compounds which are non-electrolyte — sucrose, glucose, sugar solution

Neutral liquid which are non-electrolytes — liquid carbon tetrachloride

Question 5

State which of the following solutions are weak electrolytes — dil. HCI; carbonic acid; NH4OH; dil. H2SO4; AgNO3; Na2CO3; PbBr2; KOH; HI; oxalic acid, NaHCO3; sodium acetate; Na2SO4; NaOH.

Answer

Weak Electrolytes — Carbonic acid, NH4OH, Na2CO3 oxalic acid.

Question 6

State which of the following solutions contain:

(i) molecules only

(ii) ions only

(iii) both molecules and ions — CS2; CH3COOH ; NH4OH ; NaOH ; dil. HNO3 ; Na2CO3 ; CuCl2 ; oxalic acid ; pure H2O, kerosene ; HI.

Answer

(i) molecules only — pure H2O, kerosene, CS2

(ii) ions only — dil. HNO3, NaOH, CuCl2, HI

(iii) both molecules and ions — CH3COOH; NH4OH; Na2CO3; oxalic acid

Question 7

State giving reasons, in which state or medium does (i) NaCl (ii) HCl gas (iii) NH3 gas conduct electricity.

Answer

(i) NaCl will conduct electricity only in fused state or aq. soln. state. This is because the ions Na+ and Cl are not free but held together by strong electrostatic force of attraction. Hence, it is a bad conductor of electricity in solid state.

(ii) HCl conducts electricity only when it is dissolved in water. When HCl (polar solvent) is added to water, the slightly negatively charged oxygen atom of water exerts an electrostatic pull on positively charged hydrogen ion of HCl. Thus, H+ ions combine with the water, forming hydronium ion and the residual chloride ions remain in water and these ions help in conduction of electricity.

(iii) NH3 gas conducts electricity only when it is dissolved in water. When NH3 gas is dissolved in water it forms NH4OH. As NH4OH is a weak electrolyte, so it ionizes to a small extent to give (NH4+ and OH-) ions and these ions help in conduction of electricity.

Question 8

State on what basis are acids, bases and salts classified as strong or weak electrolytes.

Answer

Acids, bases and salts that almost completely dissociate in fused or in aqueous solution state and contain mainly ions only are classified as strong electrolytes.

Acids, bases and salts that partially dissociate in fused or in aqueous solution state and ions as well as unionised molecules are classified as weak electrolytes.

Question 9

Explain the terms :

(i) metal activity or electro chemical series

(ii) selective discharge of ions.

Answer

(i) Depending on the ease with which the metals lose their electrons and form ions, they are arranged in a series known as metal activity series or electro chemical series.
The arrangement is so done that the elements that ionize most readily [discharged with great difficulty] are placed at the top of the series and other elements in the descending order.

(ii) The preferential discharge of ions present in an electrolyte at the respective electrodes is known as selective discharge of ions.

Question 10

From the ions –

(i) SO42– and OH1–

(ii) Cu2+ and H1+

(iii) Ag1+ and H1+

state giving reasons which ion is discharged at the respective electrode in each case.

Answer

(i) OH1- ions are discharged since they are lower in electrochemical series.

(ii) Cu2+ ions are discharged in preference to H1+, as Cu is lower in the electro chemical series than H

(iii) Ag1+ will be discharged because Ag is lower in the electro chemical series than H.

Question 11

With reference to nature of electrodes — name three inert and three active electrodes.

Answer

Three Inert electrodes:

  1. Iron
  2. Graphite
  3. Platinum

Three Active electrodes:

  1. Copper
  2. Nickel
  3. Silver

Question 12

State the reason for difference in product formed at the anode during electrolysis of aq. CuSO4 using :

(i) active electrode — copper anode

(ii) inert electrode — platinum anode.

Answer

(i) Electrolysis of aq. CuSO4 using copper anode — Product at anode is nil. [Copper ions are formed]

Reaction at anode: Cu - 2e- ⟶ Cu2+

SO42- and OH- ions migrate to anode but neither of them are discharged due to the nature of anode, (copper loses electrons more easily than SO42- and OH-) because copper anode itself ionises to give Cu2+ ions.

(ii) Electrolysis of aq. CuSO4 using inert platinum anode — Product at anode is oxygen gas.

SO42- and OH- ions migrate to anode but OH- ions are discharged since they are lower in the electrochemical series.

Oxidation of the OH- ions gives unstable hydroxyl radical which forms water with the liberation of oxygen.

Question 13

Give the electrode reactions for formation of

(i) Lead metal and bromine vapours from molten PbBr2 using inert electrodes

(ii) H2 and O2 gas [2:1] from acidified water using inert Pt electrodes.

Answer

(i) Formation of Lead metal and bromine vapours from molten PbBr2 using inert electrodes

At cathode:
Pb2+ + 2e ⟶ Pb

At anode:
2Br – 2e ⟶ Br
Br + Br ⟶ Br2

(ii) Formation of H2 and O2 gas [2:1] from acidified water using inert Pt electrodes.

At cathode:
4H+ + 4e ⟶ 4H
2H + 2H ⟶ 2H2

At anode:
4OH+ – 4e ⟶ 4OH
4OH ⟶ 2H2O + O2

Question 14

Starting from aq. copper (II) sulphate solution, give equations for the reactions at the cathode and anode during electrolysis of aq. CuSO4 using active copper electrodes.

Answer

Dissociation of aq. CuSO4 :

CuSO4 ⇌ Cu2+ + SO42-

H2O ⇌ H1+ + OH1-

Electrode reaction:

Reaction at Cathode:
Cu2+ + 2e- ⟶ Cu [Product : Copper metal]

Reaction at anode:
Cu – 2e- ⟶ Cu2+ [Product : Nil , Cu2+ ions ]

Question 15

Give reasons for the following changes —

(i) pure water a non-electrolyte — becomes an electrolyte on addition of dil. H2SO4

(ii) Blue colour of aq. CuSO4 — turns almost colourless on it's electrolysis using Pt electrodes.

Answer

(i) Pure water is almost a non-electrolyte and will not normally conduct electricity. It consists almost entirely of molecules. It can be electrolytically decomposed by addition of traces of dil. H2SO4 which dissociates as: H+1 and SO42- and help in dissociating water into H+1 and OH- ions, water being a polar solvent. Hence, pure water a non-electrolyte, becomes an electrolyte on addition of dil. H2SO4

(ii) The blue colour of CuSO4 solution is due to the Cu2+ ions. During it's electrolysis using Pt electrodes, at the cathode Cu2+ ions are discharged as neutral copper atoms by accepting electrons. These are not replaced by Cu2+ ions from the Anode because at the Anode, OH- ions are discharged. Hence, the blue colour of CuSO4 solution fades and it becomes almost colourless on it's electrolysis using Pt electrodes.

Question 16

'Iron is electroplated with silver' —

(i) define the term in italics

(ii) state two reasons for electroplating

(ii) state why the iron is not placed at the anode and silver at the cathode during electroplating.

Answer

(i) Electroplating — It is the electrolytic process of deposition of a superior metal [i.e., nickle, silver, chromium, gold] on the surface of a baser metal or article [e.g., iron, copper, brass]

(ii) Reasons for electroplating:

  1. Prevents corrosion or rusting — of the base metal.
  2. Makes the article attractive and gives it an expensive appearance.

(iii) The article to be electroplated (i.e., iron) is always placed at the cathode because during electrolytic reaction the metal is always deposited at the cathode by gain of electrons.
The metal to be plated on the article (i.e., silver) is always made the anode because the metal anode continuously dissolves as ions in solution and is hence replaced periodically.

Question 17

Draw a diagram for —

(i) electroplating an article with silver;

(ii) electrorefining or purification of copper.

Answer

(i) Below diagram shows electroplating an article with silver:

Draw a diagram for electroplating an article with silver. Electrolysis, Simplified Chemistry Dalal Solutions ICSE Class 10.

(ii) Below diagram shows electrorefining or purification of copper:

Draw a diagram for electrorefining or purification of copper. Electrolysis, Simplified Chemistry Dalal Solutions ICSE Class 10.

Question 18

State the (i) electrolyte (ii) cathode used (iii) anode used (iv) electrode reaction at cathode (v) electrode reaction at anode (vi) product at cathode and anode

during

(a) electroplating an article with nickel (b) electroplating a spoon with silver (c) purification of impure copper.

Answer

(a) Electroplating an article with nickel :

  1. Electrolyte — Aqueous solution of Nickle Sulphate
  2. Cathode used — Cleaned article to be electroplated
  3. Anode used — Active electrode. Plate or block of nickle metal
  4. Electrode reaction at cathode — Ni2+ + 2e- ⟶ Ni [deposited]
  5. Electrode reaction at anode — Ni - 2e- ⟶ Ni2+
  6. Product at cathode — Nickel. (It is deposited on the article.)
    Product at anode — Nil (Ni2+ ions formed)

(b) Electroplating a spoon with silver :

  1. Electrolyte — Sodium argentocyanide Na[Ag(CN)2]
  2. Cathode used — Cleaned article to be electroplated
  3. Anode used — Active electrode. Plate or block of Silver metal
  4. Electrode reaction at cathode — Ag1+ + 1e- ⟶ Ag [deposited]
  5. Electrode reaction at anode — Ag - 1e- ⟶ Ag1+
  6. Product at cathode — Silver. (It is deposited on the article.)
    Product at anode — Nil (Ag1+ ions formed)

(c) Purification of impure copper :

  1. Electrolyte — Aqueous copper sulphate solution [acidified]
  2. Cathode used — Pure thin sheet of copper
  3. Anode used — Impure block of copper
  4. Electrode reaction at cathode — Cu2+ + 2e- ⟶ Cu [deposited]
  5. Electrode reaction at anode — Cu - 2e- ⟶ Cu2+ [cation]
  6. Product at cathode — Pure Copper. (It is deposited over thin sheet of pure Copper.)
    Product at anode — Nil (Cu2+ ions formed)

Question 19

Give a reason why the metals – copper, silver and lead are electrorefined but K, Na and Ca are not.

Answer

Method of extraction of metals depends on position of the metal in the activity series. Extraction of K, Na and Ca are done by electrolysis (Electrometallurgy). They are higher in electrochemical series and are extracted by electrolysis of their fused salts. On electrolysis metal is deposited at the cathode. Thus, they are already deposited at the cathode in pure state and hence need not be refined further.
On the other hand, lead and copper are extracted by using reducing agents and silver through thermal decomposition. Hence, they contain impurities and need to be generally electro-refined by electrolysis.

Question 20

Explain the term 'electrometallurgy'. At which electrode is the extracted metal always deposited ?

Answer

The process of extraction of metals by electrolysis is called electrometallurgy. Metals higher in the electrochemical series are extracted by electrolysis. During this process, the extracted metal is always deposited at the cathode.

Question 21

State how activity series of metals plays a role in extraction of metals from oxides.

Answer

Depending on the ease with which the metals lose their electrons and form ions they are arranged in metal activity series or electrochemical series.

The arrangement is so done that the elements that ionize most readily [discharged with great difficulty] are placed at the top of the series and other elements in the descending order.

Metals at the top of the series (eg. K, Na, Ca, Mg, Al) ionize most readily. Being highly reactive, they cannot be reduced by common reducing agent like C, CO, H2.

Metals at the middle of the activity series (eg. Zn, Fe, Pb, Cu), being less reactive, can be extracted from their ores by reduction with common reducing agents like C, CO, H2.

Metals near the bottom of the activity series (eg. Hg, Ag), due to their very low reactivity, can be extracted from their ores, by heating only.

Metal at the bottom of the activity series (Au, Pt) exist in native state .

Question 22

State the electrode reaction at the respective electrodes during extraction of Al from Al2O3.

Answer

Reaction:
Al2O3 ⇌ 2Al3+ + 3O2-

Reaction at cathode:
2Al3+ + 6e- ⟶ 2Al

Reaction at anode:
3O2- – 6e- ⟶ 3[O]
3[O] + 3[O] ⟶ 3O2

Unit Test Paper 5 — Electrolysis

Question 1

Match the statements 1 to 5 with their answers selected from A to J.

A: Cathode, B: Sucrose soln., C: Cl1-, D : Formic acid, E: Electrometallurgy, F: Ammonia, G: Mg2+, H: Electrorefining, I: Sulphur dioxide, J: Anode.

  1. A compound containing molecules only.
  2. A compound which ionizes in soln. state but not in gaseous state.
  3. The ion which accepts electrons from the cathode and gets reduced to neutral atoms.
  4. The electrode to which the cyanide ions of aq. Na[Ag(CN)2] migrate during electrolysis.
  5. An application of electrolysis in which the anode does not generally diminish in size.

Answer

The matched statements are given below:

Sl. No.StatementAnswer
1A compound containing molecules only.B: Sucrose soln.
2A compound which ionizes in soln. state but not in gaseous state.F: Ammonia
3The ion which accepts electrons from the cathode and gets reduced to neutral atoms.G: Mg2+
4The electrode to which the cyanide ions of aq. Na[Ag(CN)2 ] migrate during electrolysis.J: Anode.
5An application of electrolysis in which the anode does not generally diminish in size.E: Electrometallurgy

Question 2

Complete the table given below:

   Nature of AnodeNature of CathodeIons present in electrolyteIon/s discharged at Cathode / Anode
1. Electroplating an iron rod with silver
2. Electroplating a copper sheet with nickel
3. Electrorefining of silver
4. Extraction of potassium from KCl
5. Extraction of aluminium from Al2O3

Answer

The completed table is given below:

   Nature of AnodeNature of CathodeIons present in electrolyteIon/s discharged at Cathode / Anode
1. Electroplating an iron rod with silverSilverIron rodNa+, CN-, Ag+Cathode : Ag+
Anode : Nil
2. Electroplating a copper sheet with nickelNickleCopper sheetNi2+, SO42-Cathode : Ni2+
Anode : Nil
3. Electrorefining of silverImpure silverPure silverAg+, NO3-Cathode : Ag+
Anode : Nil
4. Extraction of potassium from KClGraphiteIronK+, Cl-Cathode : K+
Anode : Cl-
5. Extraction of aluminium from Al2O3GraphiteGraphiteAl3+, O2-Cathode : Al3+
Anode : O2-

Question 3

Select the correct word from the words in bracket to complete the sentence:

  1. The electrode at which anions donate excess electrons and are oxidized to neutral atoms is the ............... [anode/cathode]

  2. On electrolysis, Ag1+ and H1+ ions migrate to the ............... [cathode/anode] and ............... [Ag1+/H1+] are discharged.

  3. Electrolysis is a / an ............... [oxidation/reduction/redox] reaction in which reduction reaction takes place at the ............... [cathode/anode].

  4. According to Arrhenius's theory the amount of electricity conducted by the electrolyte depends on the ............... [nature/concentration] of the ions in solution.

  5. Salts ionize in aq. soln. on passage of electric current to give ............... [negative/positive] ions other than H+ ions.

Answer

  1. The electrode at which anions donate excess electrons and are oxidized to neutral atoms is the anode

  2. On electrolysis, Ag1+ and H1+ ions migrate to the cathode and Ag1+ are discharged.

  3. Electrolysis is a redox reaction in which reduction reaction takes place at the cathode .

  4. According to Arrhenius's theory the amount of electricity conducted by the electrolyte depends on the concentration of the ions in solution.

  5. Salts ionize in aq. soln. on passage of electric current to give positive ions other than H+ ions.

Question 4

Give balanced equations for the electrode reactions involved in the following conversions at the respective electrodes:

  1. Aluminium oxide → Oxygen gas ← Copper [II] Sulphate
  2. Copper metal → Copper ions → Copper metal
  3. Lead [II] chloride → Chlorine gas ← Hydrochloric acid
  4. Hydroxyl ions ← Acidified water → Oxygen gas
  5. Potassium bromide → Bromine gas ← Lead bromide

Answer

  1. Aluminium oxide → Oxygen gas ← Copper [II] Sulphate

    Electrolysis of molten Al2O3:
    Al2O3 ⇌ 2Al3+ + 3O2-
    Reaction at anode:
    3O2- - 6e-⟶ 3[O]
    3[O] + 3[O] ⟶ 3O2 [product O2 gas]

    Electrolysis of copper (II) sulphate:
    CuSO4 ⇌ Cu2+ + SO42- [ions present]
    H2O ⇌ H1+ + OH1-
    Reaction at anode:
    OH1- - 1e-⟶ OH x 4
    4OH ⟶ 2H2O + O2 [product oxygen gas]
  2. Copper metal → Copper ions → Copper metal

    Reaction at anode:
    Cu – 2e- ⟶ Cu2+
    Reaction at cathode:
    Cu2+ + 2e- ⟶ Cu
  3. Lead [II] chloride → Chlorine gas ← Hydrochloric acid

    Electrolysis of molten PbCl2:
    PbCl2 ⇌ Pb2+ + 2Cl-
    Reaction at anode:
    2Cl- – 2e- ⟶ 2[Cl]
    Cl + Cl ⟶ Cl2

    Electrolysis of hydrochloric acid:
    HCl ⇌ H+ + Cl-
    Reaction at anode:
    2Cl- – 2e- ⟶ 2[Cl]
    Cl + Cl ⟶ Cl2
  4. Hydroxyl ions ← Acidified water → Oxygen gas

    Electrolysis of acidified water
    H2SO4 ⇌ 2H1+ + SO42-
    H2O ⇌ H1+ + OH1-
    Reaction at anode:
    OH1- - 1e- ⟶ OH x 4
    4OH ⟶ 2H2O + O2 [product oxygen gas]
  5. Potassium bromide → Bromine gas ← Lead bromide

    Electrolysis of molten KBr:
    KBr ⇌ K1+ + Br1-
    Reaction at anode:
    Br- – 1e- ⟶ Br
    Br + Br ⟶ Br2

    Electrolysis of molten PbBr2:
    PbBr2 ⇌ Pb2+ + 2Br-
    Reaction at anode:
    Br- – 1e- ⟶ Br
    Br + Br ⟶ Br2

Question 5

Select the correct answer from the list in bracket:

  1. The cation discharged at the cathode most readily.
    [Fe2+, Cu2+, Pb2+, H1+]
  2. The anion discharged at the anode with most difficulty.
    [SO42-, Br1-, NO31-,OH1-]
  3. The metallic electrode which does not take part in an electrolytic reaction.
    [Cu, Ag, Fe, Ni]
  4. The ion/s which is/are discharged during electrolysis of aq. CuSO4 using Cu electrode.
    [Cu2+, SO42-, H1+, OH1-]
  5. A covalent compound which in aqueous state conducts electricity.
    [CCl4, CS2, NH3, C2H4]

Answer

  1. The cation discharged at the cathode most readily — Cu2+
  2. The anion discharged at the anode with most difficulty — SO42-
  3. The metallic electrode which does not take part in an electrolytic reaction —Fe
  4. The ion which is discharged during electrolysis of aq. CuSO4 using Cu electrode — Cu2+
  5. A covalent compound which in aqueous state conducts electricity — NH3

Question 6.1

Give reason for the following :

Electrolysis of molten lead bromide is considered a redox reaction.

Answer

Electrolysis of molten lead bromide involves oxidation and reduction reaction and hence is a redox reaction. The reactions are as follows :

Reaction at the cathode:
Pb2+ + 2e- ⟶ Pb [Reduction reaction]

Reaction at anode:
Br1- - 1e- ⟶ Br
Br + Br ⟶ Br2 [Oxidation reaction]

Question 6.2

Give reason for the following :

Lead bromide undergoes electrolytic dissociation in the molten state but is a non-electrolyte in the solid state.

Answer

Solid lead bromide is a non-electrolyte since it's ions are not free but held together by an electrostatic force of attraction. These ions become free when lead bromide is in fused or molten state, hence it undergoes electrolytic dissociation in the fused or molten state.

Question 6.3

Give reason for the following :

The blue colour of aq. copper sulphate does not change when it is electrolysed using copper electrodes.

Answer

For every copper ion (Cu2+) discharged at the cathode as neutral copper atom (Cu), a copper ion (Cu2+) is added to the solution at the anode. Hence, total number of Cu2+ remains same. Therefore, the blue colour of CuSO4 which is due to the presence of Cu2+ ions does not fade [SO42- and OH1- ions are spectator ions].

Question 6.4

Give reason for the following :

During electrolytic dissociation of sodium chloride, the sodium ions are discharged at the cathode.

Answer

Cathode is an electrode connected to the negative terminal of the battery, the electrode hence acquires a negative charge and positively charged sodium ions (Na+) migrate to negative electrode, i.e., cathode.

Question 6.5

Give reason for the following :

In the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid.

Answer

As nitric acid is a volatile acid, it may decompose and nitrate radical (NO31-) may tend to interfere with the electrolytic reaction. Hence, in the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid.

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