Electric Current and its Chemical Effects PDF

CHAPTER Electric Current and 16| Its Chemical Efects We have already learnt that materials, which allow electricity to pass through them easily, are known as conductors of electricity. Most of the metals are good conductors of electricity. Materials, which...

CHAPTER Electric Current and 16| Its Chemical Efects We have already learnt that materials, which allow electricity to pass through them easily, are known as conductors of electricity. Most of the metals are good conductors of electricity. Materials, which do not allow electricity to pass through them easily, are called poor conductors or insulators. Plastic, wood, bakellite are some of the better-known poor conductors of electricity. Conduction through Liquids Most of the well-known conductors are materials in the solid state. But what about liquids? Do liqulds conduct electricity? Can an electric current pass through a liquid? To check this, let us perform the following activity. Activity Connect atorch bulb (or LED) to abattery (of twocells of 1.5V each) using (copper) connecting wires as shown in the diagram.  tester. This arrangement is known as a continuity in a clean beaker. Now, dip the free (well-cleaned) ends of the Pour some lime juice wires of the tester (kept away from each other by about 2-3 cm) in the lime juice. two bottom of the beaker. Observe whether the Take care that the ends do not touch the bulb/LED glows or not. Take out the ends of the wire from the lime juice and dry and clean them. Now, dip them in a beaker containing ordinary tap water. Again observe the bulb/LED. Repeat your observation using distilled water. (This is available at medical stores or in the school chemistry laboratory). Next try repeating this activity using some other liquids (orange juice, tomato juice, and vinegar, vegetable oil, glycerine, hair oilcase? so on.) What do we observe in each Testing of conduction of electricity in Let us tabulate our observations in the form different liquids of a table. Bulb/LED glows/ Good conductor/ Liquid does not glow Poor conductor 1. Lime juice 2. Tap water 3. Distilled water 4. Orange juice 5. Tomato juice 6. Vinegar 7. Vegetable ol 8 Glycerine Hair oil 284)  We now realise that some liquids do conduct electricity: The above activity also shows that while some liquids are good conductors of electricity, there are many that are poor conductors of electricity. The water that we get from different sources, like taps, hand pumps, wells and ponds, generally contains several salts dissolved in it. This water is thus a reasonably good conductor of electricity. We should, therefore, never handle electrical appliances with wet hands, or while standing on a wet floor. Do you Know ? Air is apoor conductor of electricity. But during lightning, an electric current passes through ai. Actually, under appropriate conditions, most materials can conduct. It is, therefore, preferable to classfy materials as good conductors or poor conductors instead of classifying them as conductors and insulators. Cause of Conductivity of Liquids Let us first try to find out what happens when an electric current passes through a liquid. Activity 2 Take some distilled water in a clean and dry beaker. We now know that distilled water does not conduct electricity. Now, dissolve a small amount of common salt (sodium chloride) in this distilled water. What do we observe with the continuity tester now? The bulb/LED glows. We conclude that a solution of common salt in water is a good conductor. Repeat this activity using (i) dilute hydrochloric acid" () caustic soda* in place of common salt. Observe the bulb glowing in each case! Why do salt, acid, or caustic soda make the water more conductive to electricity? Common salt (NaCI) consists of (positively charged) sodium and (negatively charged) chlorine ions. The water not only breaks sodium chloride into (electrically charged) sodium and chlorine ions but also separates these ions. Caution: Use acld, or caustie soda, only under the supervision of your teacher or lab techniclan. It can cause harm If handled carelessly. 285  movement of these ions within the solution that makes distilled water It is the is electricity, in liquids, conductive to electricity. We thus realise that the flow of due to the movement of ions in them. dissolved in water, are known The molecules of salts, acids and bases, on being to break up into free(positive and negative) mobile ions. These free mobile ions making it conductive. allow the passage of electric current through the solution, We call such (conductive) solutions as electrolytes. Electrolytes molten (fused) or aqueous We say that : Acompound, that conducts electricity in decomposition (breaking (solution) state, and which simultaneously undergoes into ions) when an electric current passes through it, is an electrolyte. We classify electrolytes as strong electrolytes and weak electrolytes. This is into ions. done on the basis of the extent to which the electrolyte breaks 1. ionised completely to Strong electrolytes: These are electrolytes that get number of free mobile form free mobile ions in the solution. Hence a large ions are available in them to conduct electricity. pure Some of the examples of strong electrolytes are: sodium chloride, sulphuric acid and copper sulphate solution. to Weak electrolytes: These are electrolytes that get ionised only partiallyare 2. form free mobile ions. Hence only a small number of free mobile ions available in them to conduct electricity. ammonium Some of the examples of weak electrolytes are acetic acid, hydroxide and tap water. Some Examples of Strong and Weak Electrolytes Strong electrolyte Weak electrolyte Sea water Ordinary tap water Nitric acid Oxalic acid Copper sulphate solution Carbonic acid Sodium chloride Ammonium hydroxide Sulphuric Acid* Citric acid caution. Never, never, add water to Caution: Pure sulphuric acid needs to be handled with extreme care and sulphuric acid to water, Any sulphuric acid to dilute it. We get dilute sulphuric acld by adding (a few dropsorof)a senior laboratory assistant. such dilution, if needed, must be done under the supervision of the teacher 286  Do you Know A non-electrolyte does not provide ions in its solution and, therefore, current does not flow through such solutions. Some examples of non-electrolytes are distilled water, alcohol, carbon disulphide and carbon tetrachloride. However, these can ionise various other substances when they are put in them. Conversion of Chemical Energy into Electrical Energy We know that it is possible to convert one form of energy into another; however, energy can neither be created nor destroyed. The list of 'conversions, of one form of energy into another, is almost endless. The electric cells, we talked about in Class-VI, give us electrical energy at the expense of chemical energy. We can, therefore, say that the cells are an example of conversion of chemical energy into electrical energy. It was an Italian scientist, Alessandro Volta, who gave us the first practical 'source of such a conversion' in the year 1790. His arrangement is known as the voltaic cell. The general set-up of a voltaic cell is shown in the figure given below. When twO rods of zinc and copper (known as electrodes) are dipped in a solution of dilute sulphuric acid, there are chemical reactions within the system. It is these reactions that help us to get electrical -Copper rod energy from chemical energy. When we Zinc rod connect its two rods, to a (torch) bulb/ Dilute LED, using metal wires, a current flows and sulphuric acid the bulb/LED 'lights up. The voltaic cell The Voltalc Cell can, therefore, act as a source of electric current or electrical energy. Chemical Effects of Electric Current We now realise that chemical changes in a liquid can produce electric effect (electric current). Is the reverse also possible? Can an electric current, passing through a liquid, result in chemical changes? Michael Faraday, a well-known British experimental physiclst, began his experiments on the passage of electricity through liquids (electrolyte) in 1834. He observed interesting chemical changes (effects) taking place when an electric 287  current passes through an ionic solution. The resulting effects are known as chemical effects of current. Faraday called the phenomenon, resulting from the passage of electricity through liquids, as electrolysis. This is because it causes chemical changes within the electrolyte. Do you Krnow When an electric current passes through a pure metallic conductor, whether solid or liquid (e.g. mercury), there is no chemical effect, i.e. there is no change in the chemical composition of the conductor. There is only the usual heating effect of current. Let us now perform an activity to find out the kind of chemical effects (changes) that take place when an electric current passes through an ionic solution. Activity 3 To demonstrate the electrolysis of wate. Material required: A battery of 9V (six cells of 1.5 V), two graphite pencils, a beaker, tap water, connecting wires and tape. Circular cardboard piece to hold pencls Procedure: Sharpen both ends of both the pencils. Fill the beaker with tap water. Graphite pencils Make a simple circuit as shown below. It is Gas bubbles important to make good contact with the Tap water graphite in the pencils. Secure the wires with tape. Use a piece of cardboard to hold Passing current through water the pencils in a vertical position. Place the but are not exposed tips of the pencils in the water, such that tips are fully submerged here. Now, wait touching the bottom. We are using the two pencils as the electrodes for 3-4 minutes and observe the electrodes carefully. Can we call this Small gas bubbles are seen to be getting formed near the electrodes. change? change, taking place in the tap water, a chemical (with proper, and Note: We can add a few drops of an acid/small amount of salt conducting.] appropriate precautions), to make water more 288  It was a British Chemist, William Nicholson, who had shown that if electrodes were immersed in water anda steady current was passed of oxygen and hydrogen are produced. The oxygen through it, bubbles the electrode (called anode) connected to the bubbles are formed on positive terminal of the battery and hydrogen bubbles are formed on the other electrode (called cathode) connected to the negative terminal of the battery. We, thus, conclude that the passage of an electric solution, can cause chemical changes. current, through an ionic The chemical reactions taking place in the solution depend on the 1. nature of the electrodes. 2. nature and concentration of the solution. Some of the prominent effects of the chemical reactions, taking place at and within the solution, are electrodes 1.Metals may get deposited at the electrode surface. 2. Gaseous bubbles may get formed near the electrodes. 3. Change of colour of solution may occur due to ions. dissolution of different Do you Know ? The phenomenon of electrolysis, i.e. passage of current through an ionic the conduction of electricity through metals in two solution, is different from important ways: (1) Flow of current can cause chemical reactions. (i) Unlike conduction in metals matter, here, gets actually transported through the solutions. Applications The chemical effects of current (electrolysis) have some important scientific and industrial applications. We now enlist some of these applications: 1. Extraction of metals fromn ores: Certain metals are extracted from their ores by the process of electrolysis. For example, metals like aluminium. sodium, potassium, calcium are extracted by this method. 289  2. Electrorefining of metals: It is the process of obtaining pure metal from the impure one by the process of electrolysis. For example, in electrorefining of copper, a pure copper rod is made the cathode, and the impure copper rod as anode, with copper sulphate solution taken as the electrolyte. 3. Electroplating: The process of depositing a thin layer of any desired metal on another material, by the passage of electric current, is known as electroplating. We can demonstrate this process through the following activity. Activity 4 Take two conducting plates (10cm x 4cm), one of zinc and the other of copper. Take 250 ml of distilled water in a clean and dry beaker. Dissolve two table spoonful of copper sulphate in it. Add a few drops of dilute sulphuric acid (using appropriate precautions) to make it more conductive. Connect the copper electrode to the positive terminal and the zinc electrode to the negative terminal of the battery. Allow a steady current to pass through this electrolyte for 20-30 minutes. Now, carefully take out the plates from the solution. What do we observe? Do we find a coating on one of the plates? We will fnd a reddish copper coating on the zinc plate (cathode) connected to the negative terminal of the battery. It is copper, from copper sulphate solution, that has got deposited on this zinc electrode. We call this process of depositing a thin layer/ coating of any desired metal on another metallic object (by passing an electric current), as electroplating. Zinc plate (cathode) Copper plate (anode) Copper sulphate solution Try repeating this activity using carbon/copper plates/rods as cathode. List your findings.  How does this deposition on the electrode happen? To understand this, let us trace the flow of charges (ions). When electric current is passed through the copper sulphate solution, the copper sulphate solution dissociates into copper and sulphate ions. The free mobile copper ions are drawn to the cathode (zinc plate), get converted into copper atoms and then get deposited on it. Sulphate ions move to the cooper plate (anode) where they react with copper to form copper sulphate again. Thus, copper is effectively removed from the anode and deposited on the cathode. Process of electroplating: We now list some conditions that help in getting a smooth and firm deposit during electroplating. 1. The article, to be electroplated, should be made the cathode of an electrolytic cell. 2. The anode is made of that pure metal which is to be coated on the article. 3. Asuitable soluble salt of the anode metal is taken as the electrolyte. Practical Applications/Need Electroplating is a very useful process. It is widely used in industry for coating metal objects with a thin layer of different metals. We list below some practical reasons for which object/articles are electroplated. 1 To improve/alter their appearance: We all have seen the shining handles of bicycles and bath taps which are otherwise made of iron. This is done by electroplating them with chromium or nickel. Similarly, articles of base metals are often coated with precious metals to make them look more attractive. 2 To provide a protective/strengthening coating: We often find a chromium plating on bath taps, car bumpers, bicycle handles, towel rails, kitchen gas burners, wheel rims and so on. The chromium deposit not only provides a shiny appearance but also helps to avoid corrosion and effects of 'wear and tear' and scratches. Some coatings also help to increase the strength of the base metal.  3. To minimise the cost: We know that some of the metals, like gold, silver, platinum, nickel and chromium are very expensive. It would not be (generally) economical to make the whole object out of these metals. So the object/article is made from a less expensive metal and only a thin coating of the expensive metal is deposited over it. For example, artificial jewellery 0s often made by electroplating gold or silver on less expensive metals. Do you Know iron. As tÉn is The tin cans, we use for storage of food items, are made by electroplating tÉn onto protected from less reactive than iron, the food items do not come into contact with iron and are getting spoiled. Faraday's Discovery in 1831 by Michael Faraday. (A A very important phenomenon was discovered independently by Josheph Henry.) similar discovery, in that very year, was made of (almost) all large scale This phenomenon is, now, the basis of production reason that it is often said: electricity produced in the world. It is because of this been no electricity." "Had there been no Faraday, there would have already studied in Class-VIl, the magnetic effects of electric currents. We have energy into magnetic energy. These can be viewed as conversion of electrical effects of current. was, in a way, the reverse of these magnetic Faraday's discovery magnetism. To understand Faraday's we can get electricity from He showed that activity. discovery, let us perform the following Activity 5 and of well wind a large number of turnsbulb/LED Take a hollow cylindrical pipe of iron Clean the two ends of the wire and attach a torch insulated copper wire on it. move it rapidly towards the centre of the pipe magnet and to it. Now, take a strong bar we observe? We find that the bulb glows. However, it (without touching it). What do glows only for a while and stop glowing stopped. If we as soon as the magnet is away from Aaiau N now withdraw the magnet coil, we find that the bulb again the are, thus, glows up' momentarily. Wethis motion getting electricity through a coil. of the magnet near/through  Electro-Magnetic Induction The phenomenon, in which electric current can be generated by a changing magnetic field, is known as electro-magnetic induction. The discovery and understanding, of the phenomenon of electro-magnetic induction, is based on a series of experiments carried out by Faraday and Henry. Their experimental observations can be summed up as follows: 1. Whenever there is a relative motion between a magnet and a coil, that is part of an electric circuit, a current flows through the coil. 2. The flow of current stops as soon as the magnet and the coil are at rest with respect to each other. 3. The faster is the relative motion, between the magnet and the coil, the more is the current that flows through the coil. 4 The direction of current, obtained by moving the magnet away from the coil, is opposite to its direction when the magnet is moved towards the coil. The phenomenon of electro-magnetic induction forms the underlying principle of all modern day generators and transformers. The generators help us to convert mechanical energy into electrical energy. Today's civilisation owes its progress, to a very great extent, to the discovery of this phenomenon of electro-magnetic induction. Do you Know An artifcial satellite may have a long metallic chain/cable (=10 km long) attached to it. When the satellite orbits aparticular planet, it cuts the magnetic fkeld lines of that planet. An induced e.m.f. (motional e.m.f) gets developed in the metallic chain/cable. This (motional) e.m.f. can supply electrical power to the satellite. Michael araday, a great British Physicist (1791-1867), was the son of a blacksmith. He could attend a day school only where he learnt the basic ABCD', of reading, writing and arithmetic. From 14 years of age to 21, he worked as an apprentice in a book binder's shop. He would read books which came for binding. With this humble background, Faraday discovered a method of liquifying chlorine and benzene. He also discovered the phenomenon of electro-magnetic induction, magneto-optical efect and diamagnetism. He formulated the laws of electrolysis 1857. It is his discovery, of the phenomenon of electro-magnetic induction, that is as one of the one of the greatest 'gifts' of science to mankind. The world salutes him greatest geniuses of all times. (293)