Cell and Battery notes

• Chemical effects of electric current -‘There are some liquids in which a passage of electric current is accompanied by chemical changes.’ This effect is known as chemical effect of electric current. The applications of chemical effect of electric current may be observed in daily life; e.g., nickel or copper plating on metallic articles, production of E.M.F by a cell, etc. If two leads taken from the positive and negative terminals of a battery are immersed in a salted water, then the production of bubbles can be seen at the lead ends; it is all due to chemical effect of electric current.

• Electrolysis- If an electric current is passed through different liquids or solutions in turn, then it is observed that the currrent passes through some of the solutions only and not through all of them. The liquid or a solution, through which an electric current can pass, is called a conductor-liquid such as ammonium chloride solution, silver nitrate solution etc.; and, through which an electric current can’t pass, is called an insulator-liquid such as distilled water, alcohol, oil etc. If some salt or acid is mixed in the distilled water, then it becomes conductive. Thus, ‘the process of chemical changes due to the passage of an electric current through a liquid or a solution is called electrolysis.

• Electrolyte - ‘The liquid or solution which undergoes a chemical change in it on account of the passage of an electric current, is called an electrolyte’; e.g., salted water, acidic or a basic solution etc.

• Electrodes (Anode and cathode) ‘Two conductor plates are immersed in the liquid to form a passage of current through it, they are known as electrodes’. The electrode through which the current enters the liquid, is called a positive electrode or anode, while the other through which it leaves the liquid (electrolyte) is called a negative electrode or cathode.

• Ions   During electrolysis, the molecules of the electrolyte split into their constituents whcih are called ions. When a p.d. is applied across the two electrodes, the positively charged ions (cat ions) move towards the cathode and the negatively charged ions (an ions) move towards the anode. On reaching at any electrode, an ion give up its charge and ceases to be an ion . The process of converting atoms into ions is called Ionization.

• Electrochemical equivalent: The mass of a substance liberated or deposited during electrolysis by one coulomb of electricity is termed as electrochemical equivalent (ECE) of that substance. The ECE of silver is 1.1182 milligram/coulomb.

• Coulomb: The coulomb (C) is the unit of electric charge (Q) or the quantity of electricity. The coulomb is the product of current in ampere and time in seconds.

• Faraday’s Law of Electrolysis

1.  First law: The mass of the substance liberated or deposited at any electrode during electrolysis is propotional to the quantity of eletricity passed through the electrolyte. The mass of the substance liberated at any electrode will be more, if more current is passed or a current for more time is passed through the electrolyte.If the mass liberated is m then                

                      m ∝ I  

                      m ∝ t  

                      m ∝ I × t  

                      m = Z × I × t

                       Where,

                       I = current, amperes

                       t = time, seconds

                     m = mass of the substance liberated, grams

                     Z = constant Here, the constant Z is known as electro-chemical equivalent (ECE).

• Second Law - ‘When the same quantity of electricity is passed through different electrolytes , then the quantites of elements liberated at the different electrodes are proportional to their electro-chemical equivalents.’ 

                        Mass ∝ E.C.E  

                        M ∝ Z

                     Where Z = electro-chemical equivalent

                        According to Faraday’s laws of electrolysis

                         m = Z × I × t

                         Where,

                        m = mass of substance liberated in grams

                        Z = electro-chemical equivalent

                         I = current in amperes

                         t = time in seconds.

• Application of electrolysis The principal applications of electrolysis are as follows:

                   1. Electroplating 

                   2. Electro-refining of metals 

                   3. Electrolytic capacitor 

                   4. Electrotyping

                   5. Extraction of metals

• Electroplating:-  The process of depositing a metal on the surface of another metal by electrolysis is known as electroplating. Electroplating is widely used in giving an attractive appearance and finish to all types of products. In this process inferior metals are coated with costly metals (such as silver, nickel, gold, chromium, etc.) to give an attractive shiny appearance and rust-proof suface
  

  
  
  
  

         Conditions for electroplating The following conditions must be fulfilled before electroplating                  an article. 

1. The article to be electroplated must have a chemically cleaned surface, i.e. it must not have any sort of dirt, rust and greasy surface.
2.  The article to be plated should form a cathode.
3. The anode must be of the metal to be deposited for maintaining the concentration of the solution constantly during electrolysis.
4. The metal to be coated has to be in the solution of an electrolyte.

• Current required for plating Low pressure direct current (DC) supply is always used for electroplating purposes. The pressure used varies from 1 to 16 V depending upon the rate of plating and the nature of the electrolyte. 
• Dynamo for electroplating The shunt dynamo is generally used for electroplating. It delivers large current at low pressure and this requires a large commutator and brush gear. Such types of dynamos are run by either an AC or a DC motor or the petrol engine, etc and the current required for plating is controlled by the current regulator. The generated voltage of the dynamo is controlled by the voltage regulator
• Cathodic protection in Eletroplating Cathodic protection (CP) is a technique used to control the corrossion of a metal surface by making it as the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded sacrifical metal to act as the anode.
• Type of cells Cell: A cell is an electrochemical device consisting of two electrodes made of different materials and an electrolyte. The chemical reaction between the electrodes and the electrolyte produces a voltage. Cells are classified as 

1. dry cells 
2. wet cells               

                A dry cell is one that has a paste or gel electrolyte. With newer designs and manufacturing techniques, it is possible to completely (hermetically) seal a cell. With complete seals and chemical control of gas build-up, it is possible to use liquid electrolytes in dry cells. Today the term `dry cell' refers to a cell that can be operated in any position without electrolyte leakage. 

              Wet cells are cells that must be operated in an upright position. These cells have vents to allow the gases generated during charge or discharge to escape. The most common wet cell is the lead-acid cell

                            Cells are further classified as primary and secondary cells.

• Primary cells:                                                                                                                                                                   Primary cells are those cells that are not rechargeable. That is, the chemical reaction that occurs during discharge is not reversed. The chemicals used in the reactions are all converted when the cell is fully discharged. It must then be replaced by a new cell. Types of primary cells:

1.  Voltaic cell 
2.  Carbon-zinc cell (Leclanche cell and Dry cell) 
3.  Alkaline cell 
4. Mercury cell 
5.  Silver oxide cell 
6.  Lithium cell 
7. Dry cell

•  Simple voltaic cell:                                                                                                                                   
  
                 A voltaic cell uses copper and zinc as the two electrodes and sulphuric acid as the electrolyte. When they are placed together a chemical reaction occurs between the electrodes and the sulphuric acid. This reaction produces a negative charge on the zinc (surplus of electrons) and a positive charge on the copper (deficiency of electrons). If an external circuit is connected across the two electrodes, electrons will flow from the negative zinc electrode to the positive copper electrode  The electric current will flow as long as the chemical action continues. In this type of cell the zinc electrode is eventually consumed as part of the chemical reaction.The voltaic cell is also known as a wet cell because it uses a liquid solution for the electrolyte. 
  
  
  

• Leclanche cell (Carbon-zinc cells) :                                                                                                             
  
                The container of this cell is a glass jar. The jar contains a strong solution of ammonium chloride (NH4 Cl). This solution is an alkali and acts as the electrolyte. A porous pot is placed at the centre of the glass jar. This porous pot has in it a carbon rod surrounded by a mixture of manganese dioxide (MnO2 ) and powdered carbon. The carbon rod forms the positive electrode of the cell and MnO2 acts as the de-polarizer. A zinc rod is dipped in the solution in the jar and acts as the negative electrode 
  
  
  

• Dry cell (Carbon-Zinc cell):                                                                                                                         
  
  
  The danger of spilling the liquid electrolyte from a Leclanche type of cell led to the invention of another class of cells called dry cells. The most common and least expensive type of a dry cell is the carbon-zinc type . This cell consists of a zinc container which acts as the negative electrode. In the centre is a carbon rod which is the positive electrode. The electrolyte takes the form of a moist paste made up of a solution containing ammonium chloride.   As with all primary cells, one of the electrodes becomes decomposed as part of the chemical reaction. In this cell the negative zinc container electrode is the one that is used up. As a result, cells left in equipment for long periods of time can rupture, spilling the electrolyte and causing damage to the neighbouring parts.                                                                                                                                                               Carbon-zinc cells are produced in a range of common standard sizes. These include 1.5 V AA,C and D cells .(AA Pen type cell, `C' medium size and 'D' large/economy size).

• Alkaline cells:                                                                                                                                             
  
  
     Alkaline cells use a zinc container for the negative electrode and a cylinder of manganese di-oxide for the positive electrode (Fig 7). The electrolyte is made up of a solution of potassium hydroxide or an alkaline solution.                                                                                                                  Alkaline cells are produced in the same standard sizes as carbon-zinc cells but are more expensive. They have the advantage of being able to supply large currents for a longer period of time. For example, a standard `D' type 1.5 V alkaline cell has a capacity of about 3.5 AH compared with about 2 AH for the carbon-zinc type. A second advantage is that the alkaline cell has a shelf life of about two and a half years as compared to about 6 to 12 months for the carbon-zinc type.
• Mercury cells:       
  
  Mercury cells are most often used in digital watches, calculators, hearing aids and other miniature electronic equipment. They are usually smaller and are shaped differently from the carbon-zinc type . The electrolyte used in this cell is alkaline and the electrodes are of mercuric oxide (cathode) and zinc (anode). 
• Silver oxide cells: 
  
  Silver oxide cells are much like mercury cells. However, they provide a higher voltage (1.5 V) and they are made for light loads. The loads can be continuous, such as those encountered in hearing aids and electronic watches. Like the mercury cell, the silver oxide cell has good energy-to-weight and energy-to-volume ratios, poor low-termperature response, and flat output voltage characteristics. The structures of the mercuric and silver oxide cells are very similar. The main difference is that the positive electrode of the silver cell is silver oxide instead of mercuric oxide. Fig shows the cross-section of a silver oxide cell.
  
  
  
  
• lithium cell                                                    

  

  The lithium cell is another type of primary cell. It is available in a variety of sizes and configurations. Depending on the chemicals used with lithium, the cell voltage is between 2.5 and 3.6 V. Note that this voltage is considerably higher than in other primary cells. Two of the advantages of lithium cells over other primary cells are  :                                                                                      1) longer shelf life                                                                                                                            2) )up to 10 years – higher energy-to-weight ratios up to 350 WH/Kg.   Lithium cells operate at temperatures ranging from 50 to +75oC. They have a very constant output voltage during discharge.

Defects of a simple cell:            With a simple voltaic cell, the strength of current gradually diminishes after some time. This defect is mainly due to two causes.

• Local action: In a simple voltaic cell, bubbles of hydrogen are seen to evolve from the zinc plate even on open circuit. This effect is termed local action. This is due to the presence of impurities like carbon, iron, lead, etc. in the commercial zinc. This forms small local cells on the zinc plate and reduces the strength of current of the cell.                                                                                  The local action is prevented by amalgamating the zinc plate with mercury. To do so, the zinc plate is immersed in dilute sulphuric acid for a short time, and afterwards, mercury is rubbed over its surface.

• Polarisation: As current flows, bubbles of H2 evolve at the copper plate on which they gradually form a thin layer. Due to this the current strength falls and finally stops altogether. This effect is called the polarization of the cell. Polarisation can be prevented by using some chemicals which will oxidize the hydrogen to water before it can accumulate on the plate.                                        The chemicals used to remove polarisation are called de-polarisers. We learnt that most of the primary cell except rechargeable ones are usable once only. It does not supply current continuously. The secondary cells overcome this disadvantage.