11th Class Chemistry Chapter 10 Electro Chemistry Short Questions Answer
11th Class Chemistry Chapter 10 Electro Chemistry Short Question Answer
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1. The oxidation number of some of the elements vary from compound to compound. Justify it?
Oxidation number is the apparent change positive or negative or zero of an atom of an element. Oxygen has its oxidation number of ―2 I most of its compounds. Well, in per oxides the oxidation number of oxygen is ―1 and in superoxide, it is ―1/2 while in OF2 it is +2.
2. Calculate the oxidation number of Mn in KMnO4 and MnO2?
The oxidation number is the apparent charge positive or negative which an atom has in a molecule
KMnO4
K +Mn +40 = 0
+1 + Mn +4 (-2) = 0
Mn – 7 = 0
Mn = +7
MnO2
Mn + 20 = 0
Mn + 2(-2) = 0
Mn – 4 = 0
Mn = +4
3. What is electrolyte? Give examples of fused NaCl?
When electrical current is passed through the solution of an electrolyte or the electrolyte in the molten stete, then the migration of the ions takes place, towards the respective electrodes. This process is called electrolysis. Molten NaCl is electrolysed in Down’s cell to prepare Na on commercial scale. This cell is called electrolytic cell. PbCl2 in molten state gives Pb and Cl2 at respectice electrodes.
4. Why the positively charged ions moves towards the cathode and negatively charged ions go to the anode to complete the oxidation reduction process in an electrolytic cell?
anode has a deficiency of electrons so the negatively charged ions will move towards it. Cathode has efficiency of electrons, so the positively charged ions move towards it and get the electron from it. In this way, circuit is completed and electrolysis takes place. In this way, oxidation happens at the anode and reduction at the cathode.
5. Why the products of electrolysis in molten electrolyte are different from the products of electrolysis in the solution state?
In the molten state the ions of the electrolyte are deposited at the electrodes after discharging. In solution state the molecules of the solvent or the ions of solvent are also present. These ions feel competition with the ions of electrolyte, so the products of electrolysis may be different from the simple molten electrolyte.
6. How the electrolysis of aqueous solutions of KNO3 gives hydrogen and oxygen at cathode and anode, respectively?
KNO3 gives K+ and NO3-1 along with the very little dissociation of water to give H+ and OH– . H+ and OH– and discharge at cathode and anode, respectively to give H2 gas at cathode and O2 at anode. K+ and NO3– are not preferably discharge.
7. The electrolyte in the presence of inert electrodes gives different results than the situation when the electrodes also take part in the reaction. Why?
The elements which act as inert electrodes do not take part in the redox reactions and simply carry the electrical current. When CuSO4 id electrolysed by using Pt or graphite electrodes,then Cu is deposited at the cathode and oxygen at the anode. When CuSO4 is electrolysed in H2SO4 by using Cu electrides, then the Cu is transferred fron anode to the cathode. In this way the anode dissolves and cathode is built up.
8. How copper can be purified electrolytically?
The arrangement is done in such a way that the impure copper is made the anode and pour copper at cathode. These electrodes are dipped in CuSO4 solution. Copper from the anode is removed in the form of Cu+2 ions and goes to the cathode and deposits there by accepting the electrons. In this way it is purified.
9. How is Al anodized in an electrolytic cell?
For the preparation of anodized Al, Al metal is made the anode in an electrolytic cell having H2SO4 or H2Cr2O. In this way, a layer of oxide is produced at Al .This Al – oxide layer makes Al as an anodized Al.
10. How is the Daniel cell represented?
This cell is consisted of zinc as anode which loses the electrons and copper at cathode, which accepts the electrons. The solutions in the half cell are one molar each at 250C.
Zn(s)/Zn+2(aq) (1M) ‖ Cu+2(aq) (1M) │Cu(s)
Zn(s)/Zn+2(aq) (1M) ‖ Cu+2(aq) (1M) │Cu(s)
11. How the chemical energy is converted into electrical energy in a Galvanic cell or voltaic cell?
In a Galvanic cell of the substance is oxidized and other is reduced. They have their reduction protentials. The reduction potentials are due to inherent tendencies to be oxidized and reduced. So the tendency to do the chemical reaction can produce electrical current.
12. What is salt bridge?
It is consisted of a U- shaped tube and is filled with a jelly having dilute solution of a strong electrolyte. Its two ends are dipped in two half cells and ions current flows through it from cathode to the anode .
13. A salt bridge maintains the electrical neutrality in the cell. Justify it?
Salt bridge is a U- shaped glass tube having a saturated solution of some strong electrolyte like KCl, K2SO4 or KNO3. It prevents the physical contact between the two electrolytic solutions. At the time of electronic current in the outer circuit negative ions move from cathode compartment to anodic compartment. In this way, the solutions of both half cells remain natural.
14. How we can say that a voltaic cell is reversible cell?
When an external source of e.m.f. is applied in the reverse direction with greater voltage than that of cell, then the reverse reaction happens. The electrodes are regenerated and the concentrations of the original solutions are maintained. This is called reversibility of the cell.
15. How the galvanic cell can be reversed by an external source of e.m.f.?
When an external source of e.m.f is applied in the reverse direction, then copper electrode acts as anode and zinc as cathode
Zn+2(aq) + 2e-1 →Zn(s)
Cu(s) → Cu+2(aq)+2e-1
Zn+2(aq) + 2e-1 →Zn(s)
Cu(s) → Cu+2(aq)+2e-1
16.Sodium metal is softer than copper but both are very good electrical conductors. Explain why?
Both have free electrons in their structures, which are responsible for flow of electricity.
17. What is SHE?
It stand for standard hydrogen electrode. Its voltage is supposed to be zero. It is prepared by dipping a fall of platinum in 1 M HCl solution. Hydrogen gas at 1 atmospheric pressure is doubled into this solution at 250C
18. Standard hydrogen electrode (SHE) is the reference electrode to assign the reduction potential to the electrodes. Comment upon it?
In order to measure the relative tendency of the electrode to donate or accept the electrons, we need a reference electrode. S.H.E. has been selected as the reference electrode. The electrode potential and those which are below SHE have positive reduction potentials.
19. What are electrochemical series?
It is the systematic arrangement of various electrodes in the increasing order of reduction potentials. Hydrogen electrodes is almost in the middle of the series having electrode potential zero. It is called SHE. The electrodes above SHE have negative reduction potentials and those are which below SHE have positive reduction potentials.
20. The electrodes above SHE in electrochemical series undergo oxidation, when connected with SHE. The e.m.f. of the cell so obtained is equal to the standard reduction potential of the electrode. Why?
All those electrode which are above SHE have positive oxidation potential or negative reduction potentials. The electrode potential of SHE is zero. The electrodes above SHE should deliver electrons to SHE. When these electrodes are under standard eonditions, then emf of the cell obtained is equal to the oxidation potential of the electrodes.
21. How in electrochemical cells, SHE can act as anode and cathode both?
The electrodes above She in electrochemical series have negative reduction potential and positive oxidation potential. They have greater tendency to deliver the electrons to SHE.
Zn → Zn+2 + 2e– (oxidation at anode)
2H+ + 2e– → H2 (reduction at cathode)
The electrodes below SHE in electrochemical series have negative oxidation potential. They accept the electrons from SHE.
Cu+2 + 2e– → Cu0 (reduction at anode)
H2 → 2H+ + 2e– (oxidation at cathode)
22. How does electrochemical series tell us the distinction between the oxidation and reducing agents?
The electrodes below She are having high positive values of reduction potentials as F2, Br2etc. They are very good oxidizing agents. The elements like K, Ca, Na, Mg etc. have high negative values of reduction potentials. They are good reducing agent.
23. Na and K can displace hydrogen from acids but pt, pd and Cu cannot. Why?
The metals like pt, pd and Cu have sufficiently high positive value of reductions potentials. Therefore, they cannot liberate hydrogen from acids. On the other hand, Na and K are close to top of the electrochemical series and have very low reduction potentials and liberate hydrogen.
24. The standard oxidation potential of Zn is 0.76 V and its reduction potential is ―0.76 V Justify it?
The standard reduction potential of Zn for the reaction.
Zn2+ + 2e– → Zn0 , is – 0.76 V. When this reaction is reversed, then it is an oxidation half reaction.
Zn → Zn+2 + 2e– E0 = +0.76V
The value of oxidation potential will be the same but sign is reversed. i.e., + 0. 76V.
Zn2+ + 2e– → Zn0 , is – 0.76 V. When this reaction is reversed, then it is an oxidation half reaction.
Zn → Zn+2 + 2e– E0 = +0.76V
The value of oxidation potential will be the same but sign is reversed. i.e., + 0. 76V.
25. It is true that Mg+2 + Zn0 → Mg0 + Zn+2 is spontaneous reaction?
The reduction potential of Mg is more negative than Zn, So zinc has less tendency to loss the electrons than Mg. So the above reaction is non- spontaneous.
26. What are the electrode reactions in lead stage battery?
Lead is acting as anode and PbO2 is acting as cathode in lead storage battery.
At anode Pb(s) → Pb+2(aq) + 2e– (oxidation)
At cathode PbO2(s) + 4H+(aq) + 2e– → Pb+(aq) + 2H2O(2) (reduction)
At anode Pb(s) → Pb+2(aq) + 2e– (oxidation)
At cathode PbO2(s) + 4H+(aq) + 2e– → Pb+(aq) + 2H2O(2) (reduction)
27. Lead accumulator is a chargeable battery. Comment on it?
The lead accumulator is a secondary cell. It is charged by passage of a direct current through it. For this purpose, the anode and cathode of an external source are joined to the anode and the cathode of the cell, respectively. The redox Overall reaction during charges is:
2PbSO4(s) + 2H2O(1) → Pb(s) + PbO2(s) + 4H+(aq)+ 2SO42-(aq)
Hence lead accumulator is a chargeable battery.
2PbSO4(s) + 2H2O(1) → Pb(s) + PbO2(s) + 4H+(aq)+ 2SO42-(aq)
Hence lead accumulator is a chargeable battery.
28. A porous plate or a salt bridge is not required in lead storage cell. Why?
The oxidizing agents and reducing agents in lead storage cell are in the solid state, and they do not contact with each other in the absence of salt bridge. The internal resistance of the cell is very low.
29. What are electrode reactions of dry cell?
Zinc rod acts are anode and MnO2 as cathode. KOH is used is an electrolyte.
At anode Zn(s) + 2H2O(aq) + 2e– → Zn(oh)(s) + 2e– (oxidation)
At cathode 2MnO4 + 2H2O(aq) + 2e– → 2MnO(OH)(s) + 2OH–(aq) (reduction)
At anode Zn(s) + 2H2O(aq) + 2e– → Zn(oh)(s) + 2e– (oxidation)
At cathode 2MnO4 + 2H2O(aq) + 2e– → 2MnO(OH)(s) + 2OH–(aq) (reduction)
30. What are electrode reactions of silver oxide battery?
The electrolyte used in basic in character. Zinc acts as anode and Ag2O as cathode.
At anode Zn(s) + 2OH+(aq) →Zn (OH)2(s) + 2e– (oxidation)
At cathode Ag2O + H2O(aq) + 2e– → 2Ag(s) + 2OH–(aq) (reduction)
At anode Zn(s) + 2OH+(aq) →Zn (OH)2(s) + 2e– (oxidation)
At cathode Ag2O + H2O(aq) + 2e– → 2Ag(s) + 2OH–(aq) (reduction)
31. What is the difference between a cell bad a battery?
A cell can be electrochemical or electrolytic depending upon the fact, that whether the electrical energy is converted to chemical or chemical to convert electrical. In batteries we always get the electrical energy from the chemical energy.
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