Electrolysis

Electrolysis

Q1. Rewrite the following statements by selecting the correct option:

1. When conduction of electricity occurs by movement of ions it is called __________ conduction.
a.       Gaseous
b.      Non – metallic
c.       Electrolytic
d.      Electronic.

2. Aqueous solution of _________ cannot conduct electricity.
a.       Common salt
b.      Sugar
c.       Sulphuric acid
d.      Sodium hydroxide

3. Anode is ___________________
a.       Any terminal of a battery.
b.      The negative terminal of a battery
c.       The electrode connected to positive terminal
d.      The electrode connected to negative terminal.

4. An electrolytic cell does not include __________________
a.       an electrode
b.      an electrolyte
c.       a vessel
d.      a battery

5. At the cathode in an electrolytic cell __________________
a.      The electrolyte accepts electrons.
b.      The electrolyte loss electrons.
c.       The metal dissolves
d.      None of the above.


Q2. Rewrite the second column so as to match the first column
Column I
Column II
Electronic conductor
Non – electrolyte
Nerve signal
Electrolytic cell
Electroplating
i.        Protection from corrosion
ii.      Electrodes and electrolyte
iii.    Electrons as charge carrier
iv.    Urea
v.      Iron transport across cell membrane.

Answer.( 1 – c ), ( 2 – d ), ( 3 – e ), ( 4 – b ), ( 5 – a )



Q3. State whether the following statements are true or false.

a.       Dilute hydrochloric acid is an electrolyte.
Answer. True
b.      Anode is the electrode at which electrons leave the electrolyte.
Answer. True
c.       The cathode dissolves during electroplating.
Answer. False: - The anode dissolved during electroplating.
d.      An electrolyte contains only covalent bonds.
Answer. False: - A non – electrolyte contains only covalent bonds.
e.       Ions play important role in life processes.
Answer. True.

Q4. Fill in the blanks:

1.      Copper wire is an example of metallic conductor.
2.      On testing the electrical conduction of aqueous solution it is learnt that sodium chloride is an electrolyte.
3.      During electrolysis of NaCl solution in electrolytic cell sodium ions will move towards cathode.
4.      In the process of electroplating metal with which the article is to be plated is used as Cathode.
5.      During the electrolysis of aqueous copper chloride, copper ions are converted into metallic copper at the cathode.

Q5. Give scientific reasons.

Textual Questions

1. Solution of an electrolyte can conduct electricity between two electrodes.
Answer.
(i)                 Most of the electrolytes are electrovalent compounds made up of positive ions and negative ions.
(ii)               In the aqueous solution, these ions get separated (dissociation) from each other and move freely.
(iii)             When an electric potential is applied across the electrodes in contact with the electrolyte the positively charged ions (cations) move towards the cathode and negatively charged ions (anions) move towards the anode.
(iv)             These movements of cations and anions of an electrolyte are responsible for conduction electricity through the electrolyte. Hence solution of electrolyte can conduct electricity between two electrodes.

2. Copper is found to get deposited at the cathode when current is passed through aqueous copper chloride.
Answer.
1.      Decomposition of an electrolyte by passage of electric current through it, is called electrolysis.
2.      During electrolysis of aqueous copper chloride the positively charged copper ions (Cu2+) move towards the cathode and accept electrons from the cathode to form copper atoms.
3.      This can be represented by cathode reaction


4.      These copper atoms get deposited on cathode turning it reddish in colour. Hence, copper is found to get deposited at the cathode when current is passed through aqueous copper chloride.


3. During the electrolysis of aqueous copper chloride using carbon electrodes the colour of the electrolyte gets faded.
Answer.
1.      Decomposition of an electrolyte by passage of electric current through it is called electrolysis.
2.      During electrolysis of aqueous copper chloride the positively charged copper ions (Cu2+) move towards the cathode and accept electrons from cathode to form copper atoms.
3.      This can be represented by cathode reaction


4.      These copper atoms get deposited on cathode turning it reddish in colour thus the concentration of the solution goes on decreasing, fading the blue colour of the solution.
5.      Hence, during the electrolysis of aqueous copper chloride using carbon electrodes the colour of the electrolyte gets faded.

4. When a brass article is silver plated, it is used as cathode.
Answer.
1.      Electroplating is a process in which an object made up of a conducting material is coated with a less actual metal using electrolysis technique.
2.      The article to be electroplated is used as cathode, the rod of metal to be coated is used as anode and the electrolyte is a solution having cations of the metal to be coated.
3.      In the cathode reaction of the process, the metal ions move towards the cathode and accept electrons from cathode to form metal atoms which get deposited on the object.
4.      Hence, when a brass article is silver plated, it is used as cathode.

5. When aluminium article is anodised its surface gets blackened.
Answer.
1.      Anodising is a process used to make the aluminium surface resistant to corrosion and abrasion.
2.      Electrolyte used is dilute sulphuric acid and the aluminium article to be anodised is used as anode.
3.      The anode reaction results in formation of a black coloured thin film of aluminium oxide on the surface of aluminium anode.
4.      Hence, when aluminium article is anodised its surface gets blackened.

Non – textual questions

1. Pure liquid hydrogen chloride is a bad conductor of electricity, but when dissolved in water it conducts electricity.
Answer.
1.      An electric current flows through a solution due to the motion of oppositely charged ions.
2.      Pure liquid hydrogen chloride is covalent compound, being covalent compound it does not contain ions.
3.      When dissolved in water, hydrogen chloride undergoes ionization to form H+ Cl- ions. The movement of these ions under the influence of electric potential conducts electricity through the solution.
4.      Hence, pure liquid hydrogen chloride is a bad conductor of electricity, but when dissolved in water it conducts electricity.

2. An electric current cannot be passed through a sugar solution or a urea solution in water.
Answer.
1.      An electric current flows through a solution due to the motion of oppositely charged ions.
2.      Sugar and urea are covalent compounds.
3.      They do not form ions in their aqueous solution.
4.      Since they cannot form ions in their aqueous solution, an electric current cannot be passed through a sugar solution or a urea solution in water.

3. A solution of HCl in benzene is a non – electrolyte.
Answer.
1.      An electric current flows through a solution due to the motion of oppositely charged ions.
2.      Pure HCl is a covalent compound, being covalent compound it does not contains ions.
3.      In water it undergoes ionization to form H+ and Cl- ions and thus conduct electric current.
4.      But it does not undergo ionization in benzene.
5.      Hence, a solution of HCl in benzene is a non – electrolyte.

4. A few drops of sulphuric acid are added to water during the electrolysis of distilled water.
Answer.
1.      Distilled water is purely a covalent compound and does not undergo dissociation. Thus it is a bad conductor of electricity.
2.      In order to make it a good conductor of electricity a few drops of sulphuric acid (H2SO4) are added do it.
3.      Sulphuric acid dissociates in water to form H+ and SO24- ions, which carry the electric current.
4.      Hence, a few drops of sulphuric acid are added to water during the electrolysis of distilled water.

5. Iron tools and instruments are often plated with nickel or chromium.
Answer.
1.      Electroplating is a process in which an object made up of conducting material, is coated with less active metal using electrolysis technique.
2.      It is done to protect objects of reactive metals.
3.      Iron is very reactive metal, therefore iron tools and instruments rust when exposed to air.
4.      An electroplating of nickel or chromium protect it against rusting.
5.      Hence ions tools and instruments are plated with nickel and chromium.

6. Kitchen articles like pressure cooker, pan etc are anodised.
Answer.
1.      Anodising technique is used to make the aluminium surface of the article resistant to corrosion and abrasion.
2.      In this process the anode reaction results in formation of a black coloured thin film of aluminium oxide on the surface of article.
3.      Kitchen articles like pressure cooker, pan etc. are constantly subjected to heat, pressure and water causing corrosion and abrasion.
4.      To avoid this, kitchen articles like pressure cooker, pan etc are anodised.

7. Electric current can flow through a common salt solution(NaCl).
Answer.
  1. When common salt (NaCl) is dissolved in water, it dissociates to form Na+ ions and Cl- ions.
  2. When the electrodes are connected a cell, the Cl- ions move towards the anode (positive terminal) while the Na+ ions move towards the cathode (negative terminal).
  3. This motion of the ions towards the respective electrodes constitutes and electric current.

Q6. Distinguish between the following pairs: (Give two points)

Textual questions.

1. Electrolyte and non – electrolyte.

Electrolyte
Non – electrolyte
1.      A substance whose aqueous solution can conduct electric current is called electrolyte.
2.      Electrolytes are electrovalent compounds.
3.      All the acids, bases and salts are electrolytes.
1.      A substance whose aqueous solution cannot conduct electric current is called non – electrolyte.
2.      Non – electrolytes are covalent compounds.
3.      Organic compounds such as glucose, urea, cane sugar and alcohol are non – electrolytes.


2. Cathode and anode

Cathode
Anode
1.      The electrode connected to the negative terminal of a battery is called cathode.
2.      During electrolysis cations move towards cathode.
3.      Cathode is that electrode at which electrons enter the electrolyte.
1.      The electrode connected to the positive terminal of a battery is called anode.
2.      During electrolysis anions move towards anode.
3.      Anode is that electrode at which electrons leave the electrolyte.


3. Electric current through metals and electric current through electrolyte.
Electric current through metals
Electric current through electrolyte
1.      The conduction of electric current through metals is due to flow of free electrons.
2.      In metals free electrons flow from point of lower potential to point of higher potential.
3.      Conduction of electric current through metals is possible for unlimited time period.
4.      e.g. electric bulbs, electric motor, electrical devices etc.
1.      The conduction of electric current through electrolytes conductors is due to movements of ions (cations and anions)
2.      In electrolytes, the electrons enter the electrolyte through cathode and leave the electrolyte through anode.
3.      Conduction of electric current through electrolyte is possible only for limited time period.
4.      e.g. dry cells, car batteries, techniques such as electrolysis, electroplating and anodizing.


4. Cathode reaction and Anode reaction
Cathode reaction
Anode reaction
1.      During electrolysis, the chemical changes occurring at cathode are represented by cathode reaction.
2.      In cathode reaction cations accept electrons available at cathode.
3.      Electroplating of an article is the result of cathode reaction.
1.      During electrolysis, the chemical changes occurring at anode are represented by anode reaction.
2.      In anode reaction, anions lose electrons to the anode.
3.      Anodising of an article is the result of anode reaction.

5. Electroplating and Anodising
Electroplating
Anodising
1.      Electroplating is a process in which an object made up a conducting material, is coated with a less active metal using electrolysis technique.
2.      Electroplating can be done on metals and alloys such as ion, steel, brass, silver etc.
3.      The article to be electroplated is used as a cathode.
4.      Electroplating of an article is the result of cathode reaction.
5.      Electrolyte used is a solution having cations of the metal which is to be plated.
1.      Anodising is a process used to make the aluminium surface resistant to corrosion and abrasion using electrolysis technique.
2.      Anodising can be done only on aluminium.
3.      The article to be anodised is used as an anode.
4.      Anodising of an article is the result of anode reaction.
5.      Electrolyte used is dilute sulphuric acid.


Q7. Define the following

Textual Questions

i.                    Electrolyte: - A substance whose aqueous solution can conduct electric current is called electrolyte.
ii.                  Electrolysis: - Decomposition of an electrolyte, by passage of electric current through it is called electrolysis.
iii.                Anode: - Anode is that electrode at which electrons leave the electrolyte. Anode is connected to the positive terminal of a battery.
iv.                Cathode: - Cathode is that electrode at which electrons enter the electrolyte. Cathode is connected to the negative terminal of a battery.

Non – Textual Questions

i.                    Non electrolyte: - A substance whose aqueous solution cannot conduct electric current is called non – electrolyte.
ii.                  Electrodes: - The points at which electric current enters or leaves and electrolyte are called electrodes.
iii.                Electroplating: - Electroplating is a process in which an object made up of a conducting material is coated with a less active metal using electrolysis technique.
iv.                Anodising: - Anodising is a process which is used to make the aluminium surface resistant to corrosion and abrasion by electrolysis technique.


Q8. Draw labelled diagram for the following:

Textual questions

i. Electroplating assembly.


ii. Electrolysis of copper chloride using carbon electrodes.



Non – Textual Questions

i. Testing Electric conduction of solutions.



Q9. Write notes on the following:

Textual questions

1. Electrolysis of copper chloride


Construct an electrolytic cell using 150 ml aqueous solution of 2g copper chloride as electrolyte and two carbon rods as electrodes. Connect a 6 volt battery, a switch and a bulb in the electric circuit properly. Switch on the current in the circuit. The bulb starts glowing indicating that the beaker contains an electrolyte. Pass the current through the solution for some more time (about 15 min) and observe the changes. Two important observations can be noted in this demonstration.

i.                    Changes at the Electrodes: - The portion of the cathode dipped in the electrolyte becomes reddish. Bubbles of some gas are formed at the anode.
ii.                  Colour of the electrolyte: - The blue colour of the copper chloride solution in the cell appears to fade out over a period of about 15 minutes. The reddish colour acquired by the cathode is typical copper red colour. Chemical test confirm that it is metallic copper. This tells us that metallic copper is deposited at the cathode. Smell and chemical tests of the gas collected at the anode indicate that it is chlorine gas.
Fading of the blue colour of copper chloride solution is indicative of decreasing concentration of the solution with the passage of electric current through it. It means that copper is formed at the cathode and chlorine is formed at anode by decomposition of copper chloride.



Thus, electrolysis of aqueous copper chloride using carbon electrodes results in deposition of copper at the cathode and evolution of chlorine at the anode.

2. Electroplating


Electroplating is a process in which an object made up of a conducting material, is coated with less active metal using electrolysis technique.

The process: - Surface of the article to be electroplated is cleaned. An electrolytic cell is constructed as follows.
1.      The article to be electroplated is used as cathode.
2.      A rod or a plate of the less active metal to be coated on the article is used as anode.
3.      These electrodes are immersed in a solution of an electrolyte having cations of the same metal which is used as anode.
4.      An electric current is passed through the electrolyte for a designated (chosen) period of time.
5.      The article used as cathode, gets coated with metal used as anode.

Uses: - Electroplating is done for two purposes.
  1. To protect objects of reactive metals from corrosion. E.g. iron or steel articles are electroplated with chromium or nickel to prevent rusting.
  2. To improve the look of the object. E.g. Brass article are electroplated with silver to make them more attractive. Electroplating of silver ornaments with gold (gliding) elevates their rich appearance.


3. Anodising Technique
Answer. Anodising technique is one of the applications of electrolysis.
i.                    This technique is used to make the aluminium surface resistant to corrosion and abrasion.
ii.                  In this technique aluminium article is used as an anode. Electrolyte used is dilute sulphuric acid.
iii.                The anode reaction results in formation of a black coloured thin film of aluminium oxide on the surface of aluminium anode. 
iv.                Kitchen articles like anodised pressure cooker, anodised pan are applications of anodising technique, anodised name plates are another very popular application.
v.                  By putting appropriate dyes in the electrolytic bath coloured surface with decorative finish can be achieved. This finds applications in gift articles, home appliances and architectural decoration.



4. Electrolytic cell

Decomposition of an electrolyte, by passage of electric current through it, is called electrolysis. The process of electrolysis takes place in an electrolytic cell.
An arrangement in which cathode and anode are in contact with an electrolyte (placed in a vessel) so as to pass electric current through it is called an electrolytic cell.
In the above diagram, the beaker holding an electrolyte and two electrodes is an electrolytic cell.
An electrolyte is a substance whose aqueous solution can conduct electric current. The points at which electric current enters or leaves an electrolyte are called electrodes.
Electrodes are connected to the terminals of a source of electric power (like a battery). The electrode connected to the positive terminal of a battery is called anode, while that connected to the negative terminal is called cathode. Cathode is that electrode at which electrons enter the electrolyte while anode is that electrode at which electrons leave the electrolyte. Metal plates, metal rods and carbon rods are some examples of commonly used electrodes.

Non – textual questions.
1. Electrolysis:
Answer. A substance whose aqueous solution can conduct electric current is called electrolyte. All the acids, bases and salts are electrolytes. Decomposition of an electrolyte, by passage of electric current through it, is called electrolysis. The process of electrolysis takes place in an electrolytic cell.
An arrangement in which cathode and anode are in contact with electrolyte (placed in a vessel) so as to pass electric current through it is called an electrolytic cell.
In this process of electrolysis, when an electric potential is applied across the electrodes in contact with an electrolyte, the positively charged ions(cations) move towards the cathode and the negatively charged ions(anions) move towards the anode where they are picked up from the electrolyte.
These movements of cations and anions of an electrolyte in opposite directions under the influence of electric potential are responsible for conduction of electricity through the electrolyte. During electrolysis chemical changes occur at the electrodes, this changes are called as electrode reactions and are represented as cathode reaction and anode reaction.
There are many industrial applications of electrolysis. The two important applications are electroplating and anodising.



2. ‘Ions are charge carriers in living system’ Explain with examples.
Answer. In metallic conductors the charge carriers are electrons while in electrolytic conductors, it is the ions which carry the charge. The phenomena which are similar to the movement of ions in an electrolyte within an electrolytic cell are also found in living systems.


A living cell contains about seventy percent by weight of water and about one percent by weight of inorganic ions. Water in a cell is in the form of cell – sap. Simply speaking cell – sap plays the role of aqueous solvent and the inorganic ions the role of electrolytes. The source of inorganic ions is the mineral component of diet of organisms. For example, calcium, iron, phosphorus are taken up by organisms mainly in the form of cations and anions when minerals are absorbed by living cells, the ions are transported across the cell membrane.
Proper functioning of living cell requires high concentration K+ and very low concentration of Na+ inside the cell. A healthy heart requires proper balance in the body. Many phenomena in living organisms are controlled by ion transport across cell membrane. Some examples initiated by transport of certain ions across the cell membrane are:
  1. A leaf closing response in mimosa plant.
  2. Muscle contraction and transmittance of nerve signal in animals.
  3. The passage of high electric current through the body of living organism causes damage to it, as in the case of a bat hanging itself on a pair of overhead electrical wires leading to its death.

Q10. Answer the following.

Non textual questions

1. What are the three types of conductors?
Answer. The three main types of electric conductors are
i.                    Gaseous conductors
ii.                  Metallic or electronic conductors and
iii.                Electrolytic conductors.

2. Classify the following substances into electrolyte and non – electrolyte.
Benzene, Copper chloride, Sulphuric acid, Sugar solution, Ethyl alcohol, Acetic acid, Blue vitriol solution and sodium hydroxide solution.
Answer. Electrolytes: - Copper chloride, sulphuric acid, Blue vitriol solution, sodium hydroxide solution, Acetic acid.
Non – Electrolytes: - Benzene, Sugar Solution, Ethyl alcohol.

3. State the anode reaction and cathode reaction take place in electroplating of (Text book pg. no. 18)
  1. Nickel plating
  2. Chromium plating
  3. Silver plating
  4. Gold plating

Types of electroplating
Anode reaction
Cathode reaction
Nickel Plating



Chromium Plating



Silver Plating



Gold Plating