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Chemical Effects of Current: Short Questions and Answers of Physics

1. What is electrolysis?

Electrolysis is the process of decomposition of salt in a molten or solution state into its constituent ions when an electric current is passed through it. Electrolysis is carried out in a vessel called a Voltameter which consists of electrodes & electrolytes. 

2. What do you mean by electroplating?

Electroplating is a process of depositing a thin layer of a metal (gold, silver) over another metal by using the principle of electrodeposition. The electrode to be electroplated is made the cathode & the metal to be deposited over it is the anode. The electrolyte used is a soluble salt of the metal to be deposited. 

3. State Faraday’s law of electrolysis.

Faraday’s First law [HSEB 2070 Set D]

It states that the mass of a substance deposited at an electrode during electrolysis is directly proportional to the quantity of charge passed through the electrolyte. If m is the mass of ions deposited when q amount of charge is passed through the electrolyte, then

m ∝ q

or m=Zq

Where Z is the constant of proportionality called Electrochemical Equivalent of the substance.

Faraday’s second law states that if the same quantity of charge is passed through different electrodes the masses of ions deposited at the electrolytes are proportional to their respective chemical equivalents. If m1 and m2 are the masses of ions deposited on different electrodes when the same amount of charge is passed and E1 & E2 be their respective chemical equivalents, then

M1/M2=E1/E2

4. What do you mean by Faraday’s constant?

Faraday’s constant is defined by the ratio E/Z and is equal to the amount of charge required to deposit the mass of a substance equal to its chemical equivalent in grams at an electrode during electrolysis. it is denoted by the symbol F and has a value of 96500 Cm/mole. It implies that 96500C of charge is required to deposit one gram equivalent of a substance. 

5. Chemical Effects of Current 103 What do you mean by the electrochemical equivalent of a substance?

From Faraday’s first law of electrolysis, the mass of ions deposited at an electrode when a charge q is passed is given by m = Zq = Zit Where Z is called the Electrochemical Equivalent of the substance. The mass of ions in gram liberated by one coulomb of electric charge is passed through an electrolyte is called Electrochemical Equivalent of the substance.

E.C.E.(Z)= mass of ions deposited /(current x time)

6. How do metals differ from electrolytes in terms of electric conduction?

OR Why is the conductivity of an electrolyte low as compared to that of a metal? 

OR How do you distinguish the passage of electric current through metal and that through electrolyte? [HSEB 2053] 

OR The conductivity of an electrolyte is low as compared to metal. Why? [HSEB 2070, 2071, 2072]

In metallic conductors, the current is due to the free electrons drifting hither and thither; whereas in electrolytes, the current is due to both positive and negative ions, with the former moving towards anodes and the later towards cathodes. Since electrons are lighter, their flow is easier and the resistance of metals is less. The positive and negative ions are both comparatively heavier, so their flow is comparatively difficult, so the resistance is higher. In addition, the number of free charges (electrons) is very high in metals whereas those (freed positive and negative ions) in electrolytes are very less in number. 

7. Why are separate electrodes used in electrolysis when the metal wires used for conduction could have done the job of?

The main job of electrodes is conduction, which can also be done by simple wires used in the electric connection. When they are used alone, they might react unnecessarily with the electrolytes and be finished off. So, electrodes are used as a connection between the wires and the electrolytes. They are selected in such a way that they are good conductors and they do not react with electrolytes (unless used for special purposes), ensuring a long life of the operation. 

Chemical Effects of Current: Short Questions and Answers of Physics

8. Is electrolysis possible with alternating current? Why?

Electrolysis by definition is the breaking of salt in a molten or solution form into its constituents by means of electric current. If this is to be done, the cations should be attracted to the cathode and the anions t to the other. This is only possible if there is a dc supply.

Had there been ac supply, the electrode would alternately change; because of which there would be no preferential direction for the ions even if they break, making the process impossible.

10. A secondary cell should not be short-circuited. Why?

A secondary cell, by design, has very little internal resistance. So, when it is short-circuited, there is no external resistance and the amount of current that flows in the circuit is given by I = E/r. Since ‘r` is very less, the current will be very large, causing excessive heat and thus causing damage to cell as well as wire and other components.

11. What is the function of charcoal and manganese dioxide used in the porous pot of Lechlanche cell? [HSEB 2060]

In the Lechlanche cell, there is the production of a lot of hydrogen ions (H+ which can combine with themselves to form hydrogen molecules (H2). Such molecules often aggregate around the electrodes preventing their full contact with the electrolytes. This reduces current which is called ‘Polarization’. When Manganese Dioxide is used, it reacts with the hydrogen ions as they are formed to form water as evident from the following reaction 

2H+ + 2MnO2-> Mn203 + H2O + 2e+

This prevents layer formation and making the cell workable. So manganese dioxide acts as the depolarizer. The function of charcoal is to act as a solid conductor.

12. What are the differences between primary and secondary cells?

The following are the differences between the primary and secondary cells: 

Primary cellsSecondary cells
1. A chemical reaction is irreversible.1. Chemical reaction is reversible.
2. They cannot be recharged (i.e. once they are used up, they have to be thrown away).2. They can be recharged (i.e. once they are used up, they can be reused again by recharging).
3. They have high internal resistance.3. They have low internal resistance.
4. High current cannot be obtained.4. High current can be obtained. 

13. What are Ionization and Electrolysis different from each other?

When an electrolyte is dissolved in water, it splits up into its constituent ions (+ve and -ye) because of the high permittivity (= dielectric constant) of water. This process is called Ionization. If a unidirectional electric field is applied across the electrolyte by placing proper electrodes, the current is carried by the ions and this causes permanent segregation of its constituents. This process is known as Electrolysis. So ‘Ionization‘ is just ‘Splitting‘ whereas ‘Electrolysis‘ is proper ‘posting‘ of ions to where they should belong.

14. What is the difference between a voltameter and a voltmeter?

A voltameter is a vessel where electrolysis is performed. A voltmeter is an instrument to measure potential difference which works on the principles of the magnetic effect of electric current. Though the name is similar, their working procedures are totally different and are used for completely unrelated purposes.

15. What are the uses of electrolysis?

Electrolysis is used in:

(a) Electroplating.

(b) Purification of metals.

(c) Printing purposes.

(d) Chemical Analysis.

(e) Chemical Manufacturing. 

16. When an electrolyte is dissolved in water, it dissociates into its constituent negative and positive ions. Then, why don’t they (the opposite ions) recombine?

The force of attraction between two charge bodies Q1 and Q2 at a distance of ‘r’ in a medium of permittivity ‘ε‘ is given by

F = 1/4πε[Q1Q2/r^2] where ‘ε‘ is the permittivity of the medium and given by ε = εrelε0 = Kε0

(Where ‘k’ is the dielectric constant of the medium and εo is the permittivity of vacuum).

This gives F = 1/4πkε0[Q1Q2/r^2]

An electrolyte molecule has positive and negative ions which are attracted according to the above relation. When they are placed in water, the high value of ‘K’ makes the force of attraction very less and the molecules thus drift away. So they cannot recombine in water. 

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