Primary Cells And Secondary Cells - Day 1

[Q:-01] What is a cell ?

Ans:

      A cell is a single unit device for producing an electric current by converting chemical energy to electrical energy.

[Q:-02] What is a primary cell ?

Ans :

      The primary cell is a device for producing e.m.f by chemical reaction in an irreversible way which cannot be charged again when the cell once discharged.

[Q:-03] What are the characteristics of primary cells ?

Ans :

(i) The cell once discharged cannot be recharged.

(ii) Easily portable.

(iii) Initial cost is low.

(iv) Overall cost of supplying energy is high.

(v) Suitable for intermittent work such as bells, alarms, telephone, telegraph etc.

[Q:-04] What are the defects of a pt Unary cell ?

Ans :

Local action and polarisation.

[Q:-05] What is local action and how is it removed ?

Ans :

      Due to the impurities like iron, lead etc. being present within the electrode particularly in commercial zinc, zinc gets consumed, when the zinc rod is placed within the electrolyte even when it is, not connected to the external circuit. These impurities form some tiny cells which are short circuited by the main body of the zinc rod. This uncontrolled action of tiny cells, causing wastage of zinc is called local action.

      Local action can be removed by amalgamating the zinc rod or by rubbing the mercury over zinc rod.

[Q:-06] What is polarisation and how is it minimised ?

Ans :

      During operation of primary cell the forming of hydrogen gas bubbles on the copper or carbon plate increases the internal resistance of cell causing voltage drop and power loss. The effect produces a back e.m.f which reduces the current produced by the cell. This fact is known as polarisation.

       To minimise this effect depolarisers are used around the positive plate. In Leclanche cell manganese dioxide is used as depolariser.

[Q:-07] What are different primary cells ?

Ans :

(i) Simple or Voltaic cell.

(ii) Daniel cell.

(iii) Leclanche cell.

(iv) Dry cell.

(v) Standard cell (Weston cadmium cell).

[Q:-08] What are the essential parts of a Simple or Voltaic cell ?

Ans :

(i) A glass vessel,

(ii) A copper plate (positive),

(iii) A zinc plate (negative),

(iv) Diluted sulphuric acid (H₂SO₄) used as electrolyte.

[Q:-09] What chemical reaction takes place in a Voltaic cell ?

Ans :

Zn + H₂SO₄--> ZnSO4 + 2H

2H + 2e --> H2 (gas) [ where 'e' represents electron which comes from the zinc plate through the wire ]. 

[Q:-10] How can the polarisation be reduced in a Voltaic cell ?

Ans :

      Polarisation can be reduced either by manually brushing the electrode from time to time or by using some chemicals as depolariser which will oxidise the hydrogen atom into water or some other form before it can be accumulated around the copper plate.

[Q:-11] What are the component parts of a Daniel cell ?

Ans :

(i) A glass vessel,

(ii) A porous earthen pot,

(iii) A copper plate (positive)

(iv) A zinc plate (negative) and

(v) Electrolyte-

      a) Diluted sulphuric acid (H₂SO₄).

      b) Copper sulphate (CuSO₄).

[Q:-12] What chemical reaction takes place in Daniel cell ?

Ans :

Zn + H₂SO₄ --> ZnSO₄ + H₂

H₂ + CuSO₄ --> Cu + H₂SO₄

[Q:-13] What material is used as depolariser and how is it preserved in the Daniel cell ?

Ans :

      CuSO₄ solution is used as depolariser. CuSO₄ crystals are preserved in the shelves provided near the top of cell container but surrounding the porous pot.

[Q:-14] What is the e.m.f and Internal resistance of a Daniel cell ?

Ans :

      The e.m.f varies from 1.08 volts to 1.14 volts depending upon the concentration of the solution and its internal resistance is about 2.5 ohms.

[Q:-15] Where is Daniel cell used ?

Ans :

      Daniel cell is used for laboratory purpose, railway signals, telephones etc, where practically steady current is required.

[Q:-16] What are the advantages and disadvantages of a Daniel Cell ?

Ans :

Advantages :—

(i) The e.m.f remains nearly constant.

(ii) It will give a constant small current for some time without polarisation and with practically very little expense.

Disadvantages :—

(i) When it is left standing diffusion takes place through the porous pot in both direction and the amalgamation of the zinc tends to wear off.

(ii) Immediately after using, the cell should be dismantled and it should be reset every time before use.

(iii) It takes time in order to settle down and to ,give its final steady e.m,f after it is reset.

[Q:-17] What are the component parts of a Leclanche cell ?

Ans :

(i) A glass vessel.

(ii) A porous pot.

(iii) An amalgamated zinc rod (negative)

(iv) A carbon rod (positive)

(v) Concentrated solution of ammonium chloride (NH₄Cl) used as electrolyte.

(vi) A mixture of broken carbon and powdered manganese dioxide (Mn0₂)

[Q:-18] What is porous pot and what is its function ?

Ans :

      Porous pot is cylindrical pot made of a porous material placed at the centre of the glass jar which has in it a carbon rod surrounded by a mixture of manganese dioxide (Mn02) and powdered carbon. Porous pot allows the hydrogen gas only to pass inside but not the liquid electrolyte.

[Q:-19] What chemical reaction takes place in a Leclanche cell ?

Ans :

Zn + 2NH₄CI --> ZnCl₂ + 2NH₃ + H₂

H₂ + 2MnO₂ --> Mn₂0₃ + H₂O

[Q:-20] How does depolariser act in minimising the polarisation ?

Ans :

      The depolariser Mn0₂ reacts with the hydrogen particles to convert it into water in minimising the polarisation.

[Q:-21] What is the e.m.f and internal resistance of a Leclanche cell ?

Ans :

      The e.m.f is 1.45 volts and its internal resistance is about 1.0 ohm.

[Q:-22] Where is Leclanche cell used ?

Ans :

      It is used for small and intermittent currents e.g. for telephone, telegraph, electric bell, signalling works etc...

[Q:-23] What do you mean by dry cell ?

Ans :

      Dry cell means a modification of Leclanche type small primary cell containing no free liquid in which the electrolyte contents are in the form of paste.

[Q:-24] What are the component parts of a dry Cell ?

Ans :

(i) Zinc container (negative),

(ii) Carbon rod (positive),

(iii) A paste of carbon, black oxide of manganese, zinc chloride, amonium and water,

(iv) Muslin bag,

(v) A paste of plaster of paris, flour, zinc chloride salammoniac and water,

(vi) Brass cap,

(vii) Card Board case,

(viii) Bitumen filler.

[Q:-25] What chemical reactions take place in a dry cell ?

Ans :

      Its chemical reactions are similar to that of a Leclanche cell.

Zn + 2NH₄CI--> ZnCl₂ + 2NH₃+ H2

H₂ + 2MnO₂ --> Mn₂0₃ + H₂O