Electrolysis

Fill in the blanks:

(a) Powdered sodium chloride (common salt) does not conduct an electric current, but it does so when ............... or when ............... .

(b) Molten lead bromide conducts electricity. It is called an ................ It is composed of lead ............... and bromide ................ The lead ions are ............... charged and are called ................ The bromide ............... are ............... charged and are called ................

(c) Substances which conduct electricity in the solid state are generally ................

(d) The electron releasing tendency of zinc is ............... than that of copper.

(e) A solution of HCl gas in water conducts electricity because ..............., but a solution of HCl gas in toluene does not conduct an electric current because ................

(f) Pure water consists entirely of ............... (ions/molecules).

(g) We can expect that pure water ............... (will/will not) normally conduct electricity.

(h) Electrolysis is the passage of ................ (electricity/electrons) through a liquid or a solution accompanied by a ................ (physical/chemical) change.

Answer

(a) Powdered sodium chloride (common salt) does not conduct an electric current, but it does so when in aqueous state or when in molten state .

(b) Molten lead bromide conducts electricity. It is called an electrolyte. It is composed of lead ions and bromide ions. The lead ions are positively charged and are called cations. The bromide ions are negatively charged and are called anions.

(c) Substances which conduct electricity in the solid state are generally metals .

(d) The electron releasing tendency of zinc is more than that of copper.

(e) A solution of HCl gas in water conducts electricity because it ionizes , but a solution of HCl gas in toluene does not conduct an electric current because it does not ionize.

(f) Pure water consists entirely of molecules

(g) We can expect that pure water will not normally conduct electricity.

(h) Electrolysis is the passage of electricity through a liquid or a solution accompanied by a chemical change.

Define the following terms:

(a) Electrolysis

(b) Non-electrolyte

(c) Cation and anion

(d) Weak electrolyte

Answer

(a) Electrolysis — It is the process of decomposition of a chemical compound in aqueous solution or in molten state accompanied by a chemical change using direct electric current.

(b) Non-electrolyte — It is a compound which neither in solution nor in the molten state allows an electric current to pass through it.

(c) Cation — Ions carrying positive charge are called cations.
     Anion — Ions carrying negative charge are called anions.

(d) Weak electrolyte — Electrolytes which allow small amount of electricity to flow through them and are partially dissociated in aqueous solution are called weak electrolytes.

What is the difference between Modern explanation and Arrhenius explanation for the theory of electrolysis

Answer

What is the difference between Electrolytic dissociation and ionisation

Answer

What is the difference between A cation and an anion

Answer

What is the difference between Electrolytic dissociation and thermal dissociation

Answer

What is the difference between Strong electrolyte and weak electrolyte

Answer

Name:

(a) a salt which is a weak electrolyte

(b) a base which is a weak electrolyte

(c) an inert electrode and an active electrode

(d) a positively charged non-metallic ion

(e) the electrode at which reduction occurs

(f) a non-metallic element which is a conductor of electricity

Answer

(a) Lead acetate

(b) Ammonium hydroxide

(c) Inert electrode: Graphite and Active electrode: Copper

(d) H⁺

(e) Cathode

(f) Graphite

Electrolysis is a redox process. Explain.

Answer

During electrolysis, the reaction at the cathode involves reduction of cations as they gain electrons to become neutral atoms while that at anode involves oxidation of anions as they lose electrons to become neutral. As redox reactions are reactions where oxidation and reduction takes place simultaneously. Hence, electrolysis is a redox process.

Example : Dissociation of sodium chloride during electrolysis.

NaCl ⇌ Na⁺ + Cl^-

At cathode : Na⁺ + e^- ⟶ Na (reduction)

At anode : Cl^- - e^- ⟶ Cl (oxidation)

Cl + Cl ⟶ Cl₂

Overall reaction: 2NaCl ⟶ 2Na + Cl₂

Classify the following substances under three headings:

(a) strong electrolytes

(b) weak electrolytes

(c) non-electrolytes

Acetic acid, ammonium chloride, ammonium hydroxide, carbon tetrachloride, dilute hydrochloric acid, sodium acetate, dilute sulphuric acid.

Answer

(a) Strong electrolytes — dilute hydrochloric acid, dilute sulphuric acid, sodium acetate, ammonium chloride

(b) Weak electrolytes — acetic acid, ammonium hydroxide

(c) Non-electrolytes — carbon tetrachloride

Explain why:

(a) Cu, though a good conductor of electricity, is a non electrolyte.

(b) Solid sodium chloride does not allow electricity to pass through.

Answer

(a) 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.

(b) In solid sodium chloride, ions Na⁺ and Cl^- are not free to move due to the strong electrostatic forces. Hence, solid sodium chloride does not allow electricity to pass through it.

Choose A, B, C or D to match the descriptions (i) to (v) below. Some alphabets may be repeated.

A. non-electrolyte

B. strong electrolyte

C. weak electrolyte

D. metallic conductor

(i) Molten ionic compound

(ii) Carbon tetrachloride

(iii) An aluminium wire

(iv) A solution containing solvent molecules, solute molecules and ions formed by the dissociation of solute molecules.

(v) A sugar solution with sugar molecules and water molecules.

Answer

(i) Molten ionic compound — B (Strong electrolyte)

(ii) Carbon tetrachloride — A (Non-Electrolyte)

(iii) An aluminium wire — D (Metallic conductor)

(iv) A solution containing solvent molecules, solute molecules and ions formed by the dissociation of solute molecules — C (Weak electrolyte)

(v) A sugar solution with sugar molecules and water molecules — A (Non-Electrolyte)

An electrolyte which completely dissociates into ions is:

a. Alcohol

b. Carbonic acid

c. Sucrose

d. Sodium hydroxide

Answer

Sodium hydroxide

NaOH ⇌ Na⁺ + OH^-

Name two substances in each case:

(a) Contain only molecule

(b) Contain only ions

(c) Contain ions as well as molecules

Answer

(a) Distilled water, alcohol

(b) NaOH, KOH

(c) Carbonic acid , acetic acid

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

(a) SO₄^2-, NO₃^-, and OH^-

(b) Pb²⁺, Ag⁺, and Cu²⁺

Answer

(a) OH^-, Lower the position of the anion in the series, more easily it gets discharged at anode.

(b) Ag⁺, Elements lower in the series gets discharged more easily at the cathode during electrolysis because their cations can easily gain electrons.

(a) Among Zn and Cu, which would occur more readily in nature as metal and which as ion?

(b) Why cannot we store AgNO₃ solution in copper vessels?

(c) Out of Cu and Ag, which is more active?

Answer

(a) Zn has more tendency to release electrons, thus Zn occurs more readily as ion while Cu as metal.

(b) As copper lies above silver in electrochemical series, hence, Cu is more reactive than silver and it displaces Ag from silver nitrate.

Cu + AgNO₃ ⟶ Cu(NO₃)₂ + 2Ag

(c) Cu is more reactive than Ag hence, Cu lies above Ag in the electrochemical series.

(a) How would you change a metal like Cu into it's ions?

(b) How would you change Cu²⁺ ions to Cu?

Answer

(a) By the method of electrolysis, metal like Cu can be changed into it's ions.

Using —

electrolyte : A solution of copper sulphate and dil. sulphuric acid

Cathode : Thin strip of pure copper

Anode: Impure copper

When the current is passed through the electrolyte, the copper block loses electrons and passes into solution as soluble copper ions.

Cu - 2e^- ⟶ Cu²⁺

(b) By the same process as above, when current is passed through the electrolyte, the copper ions of the copper sulphate solution are attracted to the cathode where they gain electrons and gets deposited on the copper strip.

Cu²⁺ + 2e^- ⟶ Cu

A solution of caustic soda (NaOH) in water or when fused, conducts an electric current. What is the similarity in these two cases?

Answer

When caustic soda is dissolved in water or fused, the forces of attraction between their ions are broken and the ions then move about and conduct electricity.

During electrolysis of an aqueous solution of sulphuric acid between platinum electrodes, two types of anions migrate towards the anode but only one of them is discharged.

(a) Name the two anions.

(b) Name the main product of the discharge of anion at the anode and write the anode reaction.

(c) Name the product at the cathode and write the reaction.

(d) Do you notice any change in colour? State why?

(e) Why this electrolysis is considered as an example of catalysis?

Answer

(a) SO₄^2- and OH^-

(b) SO₄^2- and OH^- both migrate to the anode. OH^- being lower in the electrochemical series is discharged preferentially.

OH^- loses one electron to the anode and becomes neutral OH.

OH^- ⟶ OH + e^-

The combination of OH forms water with the liberation of oxygen, which is given off at the anode.

OH + OH ⟶ H₂O + O

O + O ⟶ O₂

(c) The product formed at cathode is hydrogen.

The reaction is : H⁺ gains an electron and become neutral hydrogen atom.

H⁺ + e^- ⟶ H

Hydrogen atoms combine to form molecule and this comes out as hydrogen gas.

H + H ⟶ H₂

(d) No change in colour is observed.

(e) Water in pure state consists almost entirely of molecules. It is a polar covalent compound and can form ions when traces of dilute sulphuric acid is added. As dilute sulphuric acid catalyses this ionisation, hence this electrolysis of acidified water is considered as an example of catalysis.

Copper sulphate solution is electrolysed using a platinum anode. Study the diagram given below and answer the following questions:

(a) Give the names of the electrodes A and B.

(b) Which electrode is the oxidising electrode?

Answer

(a) A = Platinum anode, B = Platinum or copper cathode

(b) A = Platinum anode as oxidation of non-metal ions always takes place at the anode.

To carry out the so-called 'electrolysis of water', sulphuric acid is added to water. How does the addition of sulphuric acid produce a conducting solution?

Answer

Water in pure state consists almost entirely of molecules. Hence it is a non-electrolyte that will not conduct electricity.
Water is a polar covalent compound. It can be electrolytically decomposed by addition of traces of dil. H₂SO₄ which dissociates as: H⁺¹ and SO₄^2- and help in dissociating water into H⁺¹ and OH^- ions. Thus, dilute sulphuric acid catalyses the ionisation of water to produce a conducting solution.

(a) Choosing only words from the following list, write down the appropriate words to fill in the blanks (i) to (v) below:

anions , anode, cathode, cations , electrode, electrolyte, nickel , voltameter.

The electroplating of an article with nickel requires an (i) ............... which must be a solution containing (ii) ............... ions. The article to be plated is placed as the (iii) ............... of the cell in which the plating is carried out. The (iv) ............... of the cell is made from pure nickel. The ions that are attracted to the negative electrode and discharged are called (v) ...............

(b) When a molten ionic compound is electrolysed, the metal is always formed at ............... and the non-metal is formed at ...............

(c) Electrolysis of acidulated water is an example of ............... (Reduction/ oxidation/ redox reaction/synthesis).

Answer

The electroplating of an article with nickel requires an (i) electrolyte which must be a solution containing (ii) nickle ions. The article to be plated is placed as the (iii) cathode of the cell in which the plating is carried out. The (iv) anode of the cell is made from pure nickel. The ions that are attracted to the negative electrode and discharged are called (v) cations

(b) When a molten ionic compound is electrolysed, the metal is always formed at cathode and the non-metal is formed at anode

(c) Electrolysis of acidulated water is an example of redox reaction

Explain the following :

A solution of cane sugar does not conduct electricity, but a solution of sodium chloride is a good conductor.

Answer

A solution of cane sugar does not conduct electricity because it is composed of molecules that do not dissociate into ions when dissolved in water. In contrast, a solution of sodium chloride does conduct electricity because sodium chloride is an ionic compound consisting of sodium (Na⁺) and chloride ions (Cl^-). When dissolved in water, the sodium ions (Na⁺) and chloride ions (Cl^-) become free to move independently and can carry an electric current.

Explain the following :

Hydrochloric acid is a good conductor of electricity.

Answer

HCl is polar covalent in nature, i.e., shows charge distribution in its molecules such that the hydrogen atom has a slight positive charge and the chlorine atom has a slight negative charge.

H^δ+ — Cl^δ-

When hydrogen chloride is added to water [a polar covalent solvent], the slightly negative charged oxygen atom of water exerts an electrostatic pull on the slightly positively charged hydrogen ions present in the molecule of HCl.

Thus, H⁺ ions combine with water to form hydronium ions [H₃O]⁺.

The presence of hydronium and chloride ions makes hydrochloric acid a good conductor of electricity.

Explain the following :

During the electrolysis of an aqueous solution of NaCl, hydrogen ion is reduced at the cathode and not the sodium ion though both Na⁺ and H⁺ ions are present in the solution.

Answer

Na⁺ and H⁺ migrate to cathode, but as H⁺ has lower position in electrochemical series than Na⁺, so hydrogen ion is reduced at the cathode and not the sodium ion.

H⁺ + e^- ⟶ H

Explain the following :

On electrolysis of dilute copper (II) sulphate solution, copper is deposited at the cathode but no hydrogen gas evolves there. Explain why.

Answer

In the electrochemical series, copper is placed below hydrogen. Thus, copper ions discharge at the cathode over H⁺ ions. That's why copper is deposited at the cathode but no hydrogen gas evolves there.

Explain the following :

When a dilute aqueous solution of sodium chloride is electrolysed between platinum electrodes, hydrogen gas is evolved at the cathode but metallic sodium is not deposited. Why?

Answer

In the electrochemical series, hydrogen is placed much below sodium, hence, hydrogen is discharged at the cathode in preference to sodium.

Explain the following :

Zinc can produce hydrogen on reacting with acids but copper cannot. Explain.

Answer

Zinc is more reactive than hydrogen and is placed above hydrogen in the reactivity series, so it displaces hydrogen from acids, but copper is less reactive than hydrogen and is placed below hydrogen in the reactivity series, so it does not liberate hydrogen from acids.

Give reason for the following:

Electrolysis of molten lead bromide is considered to be a reaction in which oxidation and reduction go side by side i.e, a redox reaction.

Answer

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:
Br^1- - 1e^- ⟶ Br
Br + Br ⟶ Br₂

Give reason for the following:

The blue colour of aqueous copper sulphate fades when it is electrolyzed using platinum electrodes.

Answer

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. As Cu²⁺ ions decrease, the blue colour of CuSO₄ solution fades and it becomes almost colourless as soon as Cu²⁺ ions are finished.

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 strong electrostatic forces 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.

Give reasons for the following:

Aluminium is extracted from it's oxide by electrolytic reduction and not by conventional reducing agents.

Answer

As aluminium is higher in the electrochemical series, hence it has a strong affinity for oxygen and its oxides are highly stable. It is not possible to reduce the oxides by common reducing agents like carbon monoxide and hydrogen. Hence, electrolytic reduction is used.

Give reasons for the following:

The ratio of hydrogen and oxygen formed at the cathode and anode is 2:1 by volume.

Answer

In the electrolysis of acidified water:

At anode OH^- loses one electron and becomes neutral OH.

OH^- ⟶ OH + e^-

The combination of OH forms water with the liberation of oxygen.

2OH + 2OH ⟶ 2H₂O + O₂

Since, 4 OH neutral particles are involved in the equation so 4 electrons are lost in order to get 4 OH neutral particles.

4OH^- ⟶ 4OH + 4e^-

Thus, the formation of 1 molecule of oxygen at the anode releases 4 electrons and to ensure that there is no build-up of electrons in any part of the circuit, the reaction of the cathode must take up 4 electrons,

i.e., 4H⁺ + 4e^- ⟶ 2H₂

This shows that the number of molecules of hydrogen is twice that of oxygen.

According to Avogadro's Law, molecules can be substituted by volumes. Hence, electrolysis of water gives 2 volumes of H₂ and 1 volume of O₂.

Give reasons for the following:

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

Answer

Dilute sulphuric acid is non-volatile while dilute nitric acid is a volatile acid. It may decompose and nitrate radical (NO₃^1-) may tend to interfere with the electrolytic reaction. Hence, in the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid.

Give reasons for the following:

Ammonia is unionized in the gaseous state but in the aqueous solution, it is a weak electrolyte.

Answer

NH₃ is polar covalent in nature, i.e., shows charge distribution in its molecules such that the 3 hydrogen atoms have a slight positive charge and Nitrogen atom has a slight negative charge.

The nitrogen atom in ammonia has a lone pair of electrons. In aqueous solution, the ammonia molecule combines with a hydrogen atom H⁺ by sharing the lone pair of electrons of nitrogen atom to form ammonium ion (NH₄⁺). Thus, in water, ammonium hydroxide (NH₄OH) dissociates into NH₄⁺ (ammonium ion) and OH^- (hydroxide ion) as follows:

NH₄OH ⟶ NH₄⁺ + OH^-

Due to this ionisation, aqueous solution of ammonia (NH₄OH) behaves as a weak electrolyte.

Give reasons for the following:

A graphite anode is preferred to other inert electrodes during electrolysis of fused lead bromide.

Answer

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

Give reasons for the following:

For electroplating with silver, silver nitrate is not used as electrolyte.

Answer

If silver nitrate solution is used directly instead of double cyanide of sodium and silver, the deposition of silver will be very fast and hence not very smooth and uniform .

Give reasons for the following:

Carbon tetrachloride is a liquid but does not conduct electricity.

Answer

Carbon tetrachloride (CCl₄) is a liquid that does not conduct electricity because it is a non-polar covalent compound. In the case of CCl₄, carbon atom shares electrons with four chlorine atoms. The electrons are shared equally between the atoms, and the molecule has no permanent dipole moment, meaning there is no separation of charge. Therefore, there are no ions present in the compound, and no free electrons to carry an electrical current.

Give reasons for the following:

Potassium is not extracted by electrolysis of it's aqueous salt solution.

Answer

Potassium is not extracted from it's aqueous salt solution by electrolysis as aqueous solution will contain H⁺ ion along with metal ion. On the passage of electric current the H⁺ ion gets discharged in preference to metal. Thus, the product formed at cathode is hydrogen gas and not the metal.

Copy and complete the following table which refers to two practical applications of electrolysis.

Answer

Write the equation taking place at the anode for Q. 2(a).

Answer

(i) Ag - e^- ⟶ Ag⁺

(ii) Cu - 2e^- ⟶ Cu²⁺

(iii) Cl^- - e^- ⟶ Cl
      Cl + Cl ⟶ Cl₂

(a) Draw a labeled diagram to show how iron is electroplated with copper.

(b) Which solution is preferred as electrolyte, CuSO₄ or FeSO₄?

(c) Describe what happens to the iron object and the copper rod.

Answer

(a) Labeled diagram showing electroplating of Iron with Copper is shown below:

(ii) CuSO₄ is preferred as electrolyte must be a solution of a salt of metal to be electroplated.

(iii) The copper anode continuously dissolves as ions in solution and is replaced periodically. The electrolyte dissociates into Cu²⁺ ions which migrate towards the iron object taken as the cathode. Here, the Cu²⁺ ions gain electrons and become atoms of copper which deposit on the iron object as a firm coating. Thus, the iron object gets electroplated with copper.

Element X is a metal with a valency 2. Element Y is a non-metal with a valency 3.

(a) Write equations to show how X and Y form ions?

(b) If Y is a diatomic gas, write the equation for the direct combination of X and Y to form a compound.

(c) If the compound formed between X and Y is melted and an electric current passed through the molten compound, the element X will be obtained at the ............... and Y at the ............... of the electrolytic cell.(Provide the missing words)

Answer

(a) X ⟶ X²⁺ + 2e^-

Y + 3e^- ⟶ Y^3-

(b) 3X + Y₂ ⟶ X₃Y₂

(c) If the compound formed between X and Y is melted and an electric current passed through the molten compound, the element X will be obtained at the cathode and Y at the anode of the electrolytic cell.

Write two applications of electrolysis in which the anode diminishes in mass.

Answer

1. Electroplating of metals
2. Electrorefining of metals

(a) What kind of particles will be found in a liquid compound which is a non-electrolyte?

(b) If HX is a weak acid, what particles will be present in it's dilute solution apart from those of water?

(c) Cations are formed by ............... (loss/gain) of electrons and anions are formed by ............... loss/gain) of electrons. (Choose the correct word to fill in blank)

(d) What ions must be present in a solution used for electroplating a particular metal?

Answer

(a) Non-electrolyte contains molecules.

(b) Molecules of HX and H⁺ and X^- ions.

(c) Cations are formed by loss of electrons and anions are formed by gain of electrons.

(d) The electrolyte must contain ions of the metal which is to be electroplated on the article.

A strip of copper is placed in four different colourless salt solutions. They are KNO₃, AgNO₃, Zn(NO₃)₂ and Ca(NO₃)₂. Which one of the solutions will finally turn blue?

Answer

AgNO₃ solution will turn blue.

Reason — Copper is unable to displace potassium, calcium and zinc from their nitrate solution as they are higher than copper in the metal activity series.

But silver being lower than copper in metal activity series gets displaced by copper and blue coloured copper nitrate is formed.

2AgNO₃ + Cu ⟶ Cu(NO₃)₂ + 2Ag

Here is an electrode reaction:

Cu ⟶ Cu²⁺ + 2e^-.

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.

A soln. contains magnesium ions (Mg²⁺), iron (II) ions (Fe²⁺) and copper ions (Cu²⁺). 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) as between magnesium, iron and copper, copper is the lowest in electrochemical series.

Reaction at Cathode:
Cu²⁺ + 2e^- ⟶ Cu

Why is carbon tetrachloride, which is a liquid, a non-electrolyte?

Answer

Carbon tetrachloride (CCl₄) is a liquid that does not conduct electricity because it is a non-polar covalent compound. In the case of CCl₄, carbon atom shares electrons with four chlorine atoms. The electrons are shared equally between the atoms, and the molecule has no permanent dipole moment, meaning there is no separation of charge. Therefore, there are no ions present in the compound, and no free electrons to carry an electrical current.

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

A. Bromine is released at the cathode

B. Lead is deposited at the anode

C. Bromine ions gain electrons

D. Lead is deposited at the cathode

Answer

Lead is deposited at the cathode

Reaction at cathode : Pb²⁺ + 2e^- ⟶ Pb

Select the correct answer:

The aqueous solution of the compound which contains both ions and molecules is

A. H₂SO₄

B. HCl

C. HNO₃

D. CH₃COOH

Answer

Acetic acid [CH₃COOH]

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

Correct the following statement: Lead bromide conducts electricity.

Answer

Molten lead bromide conducts electricity.

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 silver metal 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]

Aqueous solution of nickel sulphate contains Ni²⁺ and SO₄^2- 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 [Nickel anode loses electrons to give Ni²⁺ ions in solution]

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

(i) Sodium chloride

(ii) Copper (II) oxide

(iii) Copper (II) sulphate

(iv) 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.

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⁺

A, B and C are three electrolytic cells connected in different circuits. Cell 'A' contains NaCl solution. 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 solution, and the bulb does not glow. Give reason for each observation.

Answer

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.

Give reason: The electrolysis of acidulated water is considered to be an example of catalysis.

Answer

Water in pure state consists almost entirely of molecules. It is a polar covalent compound and can form ions when traces of dilute sulphuric acid is added. As dilute sulphuric acid catalyses this ionisation, hence this electrolysis of acidified water is considered as an example of catalysis.

During the electrolysis of copper (II) sulphate solution using platinum as a cathode and carbon as an anode,

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

(ii) State the change noticed in the electrolyte

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

Answer

(i) At cathode — Cu²⁺ ions and H¹⁺ ions migrate to the cathode. Cu²⁺ ions are below H¹⁺ ions in the activity series, thus Cu²⁺ ions are discharged in preference to H¹⁺, to form neutral copper atoms. Thus, copper atoms deposit themselves on the cathode.

At anode — SO₄^2- and OH^1- ions migrate to the anode but OH^1- ions are discharged since they are lower in the electrochemical series. The neutral (OH) radicals reunite to form water and oxygen.

(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: 4OH^1- - 4e^- ⟶ 4OH

2OH + 2OH ⟶ 2H₂O + O₂

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

Answer

Identify the weak electrolyte from the following:

(i) Sodium Chloride solution

(ii) Dilute Hydrochloric acid

(iii) Dilute Sulphuric acid

(iv) Aqueous acetic acid.

Answer

Aqueous acetic acid.

Match the following in Column A with the correct answer from the choices given in Column B:

Answer

Give reason: 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.

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 platinum electrodes as anode and cathode. [Cu²⁺, OH^-, SO₄^2-, H⁺].

(iv) When dilute sodium chloride is electrolysed using graphite electrodes, the cation which is discharged at the cathode most readily. [Na¹⁺, OH^1-, H¹⁺, Cl^1-].

(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 platinum electrodes as anode and cathode. — OH^-.

(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.

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

Answer

When electricity is passed through molten lead bromide solution, dark reddish brown fumes of bromine evolve at the anode.

Br^- - e^- ⟶ Br

Br + Br ⟶ Br₂

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

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

Answer

Liquid carbon tetrachloride is a non-electrolyte

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

(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.

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

Answer

Hydrogen chloride gas

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.

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

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

Answer

Electroplating

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

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.

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, CO₃^2- ) and molecules (H₂CO₃)

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

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

State observation at the anode when aqueous copper sulphate solution is electrolysed using copper electrodes.

Answer

Copper anode shows a loss in mass.

During electrolysis of molten lead bromide, graphite anode is preferred to other electrodes. Give reason.

Answer

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.

Electrolysis of molten lead bromide is considered to be a redox reaction. Give reason.

Answer

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:
Br^1- - 1e^- ⟶ Br
Br + Br ⟶ Br₂

Give reasons why:

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

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

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

Answer

(a) 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.

(b) 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.

(c) 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.