Chemical Effects of Electric Current

"Electroplating is the process of depositing a thin layer of metal upon another metallic object in order to improve the appearance, or to change the surface properties of that object."

1.0Introduction

Conductors are materials which allow electric current to pass through them. That is, they are good conductors of electricity. Most metals, like copper, are good conductors. That is why the current-carrying part of a wire is made up of copper or aluminium. Insulators are materials which do not allow electric current to pass through them. That is, they are poor conductors of electricity. The outer part of a wire is an insulator, such as rubber or plastic. The insulator

An electric wire has conducting inner part and insulating outer part to prevent electric shock. protects your skin by blocking the current. The outside portion of electrical cords and many electrical devices are made of insulators for safety (see figure). If you touch a current carrying wire with broken insulation (exposing bare metal), it can be very dangerous as you may get an electric shock.

2.0Conductivity in solids

Among solids, metals are the good conductors of electricity. In a metal atom, a few electrons are not attracted as strongly to the nucleus as the other electrons, and are loosely bound to the atom. When metal atoms form a solid, the metal atoms can move only short distances. However, the electrons that are loosely bound to the atoms can move easily in the solid piece of metal. These electrons are called 'free electrons' or 'conduction electrons'. Among nonmetallic solids, graphite is a good conductor of electricity.

In an insulator, the electrons are bound tightly in the atoms that make up the insulator and therefore cannot move easily. Thus, insulators are poor conductors of electricity.

• The charged particles which flow in a particular direction to produce electric current are called 'current carriers'. In solid conductors, current carriers are 'free electrons'.
• Current carriers in solid conductors - free electrons.
• Current carriers in liquids - positive ions and negative ions. Current carriers in gases - positive ions and free electrons.
• Porcelain is used as an insulator in electrical applications such as power transmission and distribution.
• The above setup can be used to test whether a material is conducting or insulating. Thus, this setup is called 'electric tester'.

You may use an 'light emitting diode (LED)' in place of the electric bulb in the tester that is used in (active physics 1). LED glows even when a weak electric current flow through it. There are two wires (called leads) attached to an LED (see figure). One lead is slightly longer than the other. Remember that while connecting to a circuit, the longer lead is always connected to the positive terminal of the cell (or battery) and the shorter lead is connected to the negative terminal of the cell (or battery). This is because a light emitting diode is device which allows current to flow only in one direction.

LED's that are available in market produce red, green, yellow, orange blue or white light. The colour depends on the exact composition of their material. The diode is enclosed in a transparent cover so that light can come out from it. Gallium arsenide phosphide is preferred material for LED's of different colours.

• The longer lead of the LED is called anode and it is the positive terminal, similarly shorter lead of LED is called cathode and it is the negative terminal.

3.0Conductivity in liquids

An electric current flow due to movement of charged particles, they may be electrons or ions. In solids, flow of electric current is due to movement of electrons while in liquids, electric current flows because of movement of negative and positive ions. For example, table salt i.e., sodium chloride is made of sodium ions (+ve ions) and chloride ions (-ve ions) that are fixed in place and cannot move through the solid. However, when salt is dissolved in water, the sodium and chloride ions break apart and spread out evenly in the water, forming a solution. Now the positive and negative ions are free to move. Thus, liquids containing ions can conduct electricity.

• Ions are charged atoms or electrified atoms which may contain positive or negative charges. A neutral atom has no net charge on it. Negative ions are called 'anions' while positive ions are called 'cations'.
• Like charges repel each other and unlike charges attract each other.
• Solutions of lemon juice and vinegar are good conductors of electricity. This means that, they get ionised when dissolved in water i.e., they give positive and negative ions in water.
• Unlike pure (or distilled) water, ordinary water/tap water/mineral water are good conductors of electricity because they contain dissolved salts which are in ionised form i.e., they contain positive and negative ions.
• The water that we get from sources such as taps, hand pumps, wells and ponds is not pure. It may contain several salts dissolved in it. Small amount of mineral salts are naturally present in it. This water is thus a good conductor of electricity.

Q. Your elders might have cautioned you against touching an electrical appliance with wet hands. But do you know why it is dangerous to touch an electrical appliance with wet hands?

• Explanation Our body or our skin is not insulating but it is slightly conducting. The conductivity of dry skin is quite low. When your hands are wet i.e., the skin is wet, it is more conducting than dry skin. As a result, it allows more current to pass through your body. The combination of water (ordinary water) and 220 volt electricity that comes at our home is highly dangerous because the high voltage and good conductivity of wet skin allow large (possibly fatal) currents to flow.
• Small amounts of mineral salts present naturally in water are beneficial for human health. However, these salts make water conducting. So, we should never handle electrical appliances with wet hands or while standing on a wet floor.

Q. In some situations, even though the liquid is conducting, the bulb may not glow when connected to an electric tester. What can be the reason?

• Explanation An electric bulb glows due to the heating effect of electric current. This means the filament of the bulb gets heated to a high temperature and it starts glowing. However, if the current through a circuit is too weak, the filament does not get heated sufficiently and it does not glow. The current in the circuit may be weak due to many reasons. For example, a material may be slightly conducting i.e., it may not conduct electricity as easily as a metal. As a result, the circuit of the tester may be complete and yet the current through it may be too weak to make the bulb glow. Also, the voltage of battery used in the electric tester may be quite low so that even a material is a good conductor but the current produced is very weak in the circuit. This current may not be sufficient to glow the bulb.

Q. Air is a poor conductor of electricity. But you may have also read that during lightning, an electric current pass-through air. How is it possible? What important result you can conclude regarding the insulators from the above case?

• Explanation Lightning is an electric discharge between clouds and Earth's surface that takes place due to the development of electric charges on the clouds. When the electric charge on the clouds is very large, the voltage between the clouds and Earth's surface (or ground) becomes so high that electric current passes through air. From the above case, we can conclude that 'under some special conditions materials classified as poor conductors also allow electricity to pass through them'.

4.0Chemical effects of electric current

When an electric current passes through a pure metallic conductor, whether solid or liquid (e.g. Mercury), there is no chemical effect. That is, no change in the chemical composition of the conductor takes place. There is only the heating effect or magnetic effect of current passing through them. But, there is a chemical change when electric current passes through ionic solutions.

• The chemical changes that take place when an electric current passes through ionic solutions is called 'chemical effect of electric current'.

Some basic terms

Electrolytes: The compounds which give ions either in molten state or in solution are called 'electrolytes'. Electrolytes are poor conductors of electricity in solid states, but they become good conductors in their molten state or in solution. This is due to the fact that ions can move freely in molten state or in solution.

• Electrolytes that conduct electricity are solutions of acids, bases and salts.

Acids: An acid is a substance that produces hydrogen ions in a water solution. It has the ability to produce these ions that gives acids their characteristic properties. When an acid dissolves in water, ions interact with water molecules to form ions, which are called hydronium ions. All acids taste sour. The familiar, sour taste of many foods is due to acids. However, taste never should be used to test for the presence of acids. Some acids can damage tissue by producing painful burns. Acids are corrosive. Some acids react strongly with certain metals, eating away the metals and forming metallic compounds and hydrogen gas.

• Examples of acids : Nitric acid , hydrochloric acid , acetic acid , sulphuric acid .

Bases: Any substance that forms hydroxide ions, , in a water solution is a base. In addition, a base is any substance that accepts from acids. The definitions are related, because the ions produced by some bases do accept ions. One way to think about bases is as the opposites of acids. In the pure, undissolved state, many bases are crystalline solids. Bases feel slippery and have a bitter taste. Like strong acids, strong bases are corrosive, and contact with skin can result in severe burns.

• Examples of bases : Sodium hydroxide or caustic soda ( NaOH ), potassium hydroxide , calcium hydroxide , ammonium hydroxide .
• "Taste and Touch" should never be used to test for the presence of a base or an acid.

Neutralisation: Neutralization is a chemical reaction between an acid and a base that takes place in a water solution. When an acid reacts with base, salt and water are produced. For example, when acid, HCl is neutralised by base, NaOH , a salt, NaCl and water ( ) are produced.

Salts: A salt is a compound formed when the negative ions from an acid combine with the positive ions from a base. For example,

• Examples of salts : Sodium chloride ( NaCl ), magnesium chloride , calcium chloride , copper sulphate , silver nitrate .

Solutes and solvents: In a solution, one substance is dissolved in another. In the sugar solution, sugar which is the substance being dissolved, is the solute. Water, which is the substance doing the dissolving, is the solvent. When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent. In salt water, salt is the solute and water is the solvent. In carbonated soft drinks, carbon dioxide gas is one of the solutes and water is the solvent.

• The electric conduction which takes place in the solutions of electrolytes due to the movement of negatives ions and positive ions of the electrolytes towards the oppositely charged electrodes is called electrolytic conduction.
• When a liquid dissolves in another liquid, the substance present in the larger amount is usually called the solvent. A solution in which water is the solvent is called an aqueous solution.

5.0Electrolysis

The production of a chemical reaction by passing an electric current through an electrolyte is called 'electrolysis'.

• The process by which free elements are liberated or deposited at electrodes in an electrolytic solution when an electric current is passed through it is called 'electrolysis'. Process : Electrolysis is carried out in an electrolytic cell (or voltameter). An electrolytic cell consists of a tank or vessel called 'electrolytic bath'. Two electrodes (conducting pieces) are immersed in a solution of an electrolyte. These electrodes are connected to the external circuit consisting of a battery. The positively charged electrode is called 'anode' and negative ions migrate to anode. The negatively charged electrode is called 'cathode' and positive ions migrate to cathode.
  
  An electrolytic cell

At cathode, reduction of positive ion (cation) takes place and at anode oxidation of negative ion (anion) takes place. Thus in this way, a chemical change takes place by passing electric current through the electrolytic cell.

Effects that can be observed during electrolysis

The passage of an electric current through a conducting solution causes chemical reactions. Some of the observable chemical effects of the electric current are : (1) Bubbles of a gas may form on the electrodes. (2) Deposits of metal may be seen on electrodes. (3) Colour change of solutions may occur.

• The reaction would depend on what solution and electrodes are used.
• The bubbles of gas are formed during the electrolysis of water. During the electrolysis chemical reactions take place inside the electrolyte. Hence colour of the electrolytic solution will change indicating the chemical change.

• In Electrolysis process- Cation (Positive ion) moves towards cathode (Negative electrode) Anion (Negative ion) moves towards anode (Positive electron)

Electrolysis of copper sulphate solution construction

(1) It consists of an electrolytic cell with two copper electrodes (plates) partly immersed in an aqueous solution of (electrolyte). (2) The Cu electrodes are connected to the two terminals of a battery. The positively charged electrode is called 'anode' and the negatively charged electrode is called 'cathode'. Working (1) The electrolyte is in the form of free ions (copper ion) and ions (sulphate ions). (2) A steady current is passed in the circuit with help of a battery. The electrons flow from the negative terminal of the battery to the cathode via connecting wire. (3) Since the cathode is negatively charged, the positively charged ions move towards it. At cathode, the ions get reduced by incoming electrons from the external circuit. (Reduction of Cu )

The Cu atoms so produced get deposited on the cathode. (4) Since the anode is positively charged, the negatively charged sulphate ions move towards it. At anode, these ions react with copper plate to give ions and release electrons. (Oxidation of Cu ) (5) The copper ions get into the solution, while release electrons flow back to the positive terminal of the battery via the connecting wire.

• The overall process results in deposition of copper at cathode and loss of equivalent amount of copper from anode. The concentration of in solution remains same. The process is also called 'electrodeposition' of copper.
  
  Electrolysis of copper sulphate solution
• Electrolysis is made up of two words, electro and lysis where electro means electricity and lysis is a Greek word meaning breakdown.

Electrolysis of water

This process is carried out in an electrolytic cell which consists of two platinum electrodes, these being chosen because platinum does not dissolve into electrolyte and does not react with products of electrolysis (see figure). The electrodes are immersed in water (electrolyte). A small amount of acid is added to increase the conductivity by increasing the number of free ions in the solution. The evolved gases are collected in inverted tubes surrounding the electrodes. The chemical reaction involved in electrolysis of water is given by,

Here, hydrogen is liberated at cathode while oxygen is liberated at anode.

• In electrolysis of water, by using electricity water as a compound split into hydrogen and oxygen.
• The salt and water in potatoes act as an electrolyte and generate electricity between two electrodes.

Why potato conducts electricity?

The potato does not participate directly in the reaction. It acts as an electrolyte due to presence of water, phosphoric acid , and some minerals containing calcium, iron, magnesium, potassium ions. This facilitates the transport of the ions through it, while keeping the copper and zinc electrodes apart. It is phosphoric acid , which facilitates the electro-chemical reaction of zinc with copper. Zinc is an active metal, which reacts readily with acid to liberate electrons. The acid's active ingredient is positively charged hydrogen , so the transfer of electrons takes place between the zinc and the acid. The zinc is oxidized and the acid is reduced to hydrogen gas , which you can see bubbling out around the electrodes. The reaction at the copper electrode removes the electrons from the copper and attaches them to the hydrogen ions in the phosphoric acid.

Oxidation at Zn electrode: Here, zinc looses 2 electrons, goes into the solution while 2 electrons remains there on the zinc electrode making Zn electrode negative.

Reduction at Cu electrode: Here, hydrogen ions gain electrons from copper electrode forming hydrogen gas. Since electrons are removed from the copper electrode, it becomes positively charged.

Net Reaction: When we connect LED to the circuit, electrons from the zinc electrode moves towards the copper electrode through the LED. Thus, an electric current flow through the LED making it glow.

• An electrolytic cell is a cell in which electrolysis takes place i.e., a current is passed through the electrolyte from an external source. It is also called 'Voltameter'. It utilises the electric energy to brought out chemical changes.
• Oxidation takes place at the anode (loss of electrons), reduction takes place at cathode, (gain of electrons).

6.0Electroplating

It is the most common application of electrolysis for coating of thin layer of metals like nickel, silver, gold, zinc, etc., on the surface of other metal or alloy. T. The process of depositing a thin and uniform layer of a metal on any conducting surface is called 'electroplating'. (i) In this process, the conducting material to be electroplated is made 'cathode' and the metal whose coating is required on the cathode material is used as anode. (ii) A soluble salt which contains ions of anode material is taken as electrolyte when the current is passed through the circuit, following reactions take place. (iii) At anode, oxidation of metal atom to be coated on cathode material takes place.

The ions moves towards the negatively charged cathode through the electrolyte. (iv) At cathode, reduction of these ions takes place and their atoms get deposited on cathode.

Thus, anode material decreases gradually and this material gets deposited on the cathode material [see figure].

• Electroplating is a very useful process. It is widely used in industry for coating metal objects with a thin layer of a different metal. The layer of metal deposited has some desired property, which the metal of the object lacks.

Applications of electroplating

(1) Chromium plating is done on many objects such as car parts, bath taps, kitchen gas burners, bicycle handlebars, wheel rims and many others. This is because chromium has a shiny appearance and it does not corrode. Also, it resists scratches. (2) Jewellery makers electroplate silver and gold on less expensive metals. These ornaments have the appearance of silver or gold and are less expensive. (3) Tin cans, used for storing food, are made by electroplating tin onto iron. Tin is less reactive than iron. Thus, food does not come into contact with iron and is protected from getting spoilt. (4) Iron is used in bridges and automobiles to provide strength. However, iron tends to corrode and rust. So, a coating of zinc is deposited on iron to protect it from corrosion and formation of rust.

Purification of metals (refining of metals)

In this process impure metal is used as anode and pure metallic strip is used as cathode. A soluble salt of pure metal is taken as electrolyte. On passing current, the impure metal anode dissolves but only the pure metal deposits on the cathode. e.g. , etc. are refined by using electrolysis.

Extraction of metals from ores

Many metals are extracted from their ores using electrolysis. e.g. (i) Aluminium is extracted by electrolysis of molten bauxite . (ii) Sodium is extracted from molten NaCl .

Electrolysis of silver nitrate solution

It takes place in an electrolytic cell with two silver electrodes (plates) partly immersed in an aqueous solution of silver nitrate (electrolyte). The Ag electrodes are connected to the two terminals of a battery (see figure). The positively charged electrode is called 'anode' and the negatively charged electrode is called 'cathode'. The electrolyte is in the form of free ions (silver ion) and ions (nitrate ions).

Electrolysis of silver nitrate solution

On passing current:

• At cathode : (reduction) The Ag atoms so produced get deposited on the cathode.
• At anode : (oxidation) The silver ions get into the solution, while released electrons flow back to the positive terminal of the battery via the connecting wire. The overall process results in deposition of silver at cathode and loss of equivalent amount of silver from anode. The concentration of in solution remains same.

7.0Some Basic Terms

1. Electric Shock :- A sudden painful feeling that you get if electricity goes through your body.
2. Bound :- To tie something or someone tightly or to fasten something.
3. Leads :- Metal strips for conduction of electricity.
4. Terminal :- Positive and negative polarity of any electric circuit.
5. Ionised :- The process by which ions are formed by gain or loss of an electron from an atom or molecule.
6. Compass :- A device used to determine geographical direction.
7. Corrosive :- A corrosive substance causes damage by chemical action.
8. Sour :- Having a sharp taste like that of a lemon.
9. Equivalent :- Equal or interchangeable in value, quantity, significance, etc.
10. Conductivity :- The ability to move heat or electricity from one place to another.
11. Soldering :- A process used for joining metal parts to form a mechanical or electrical bond.
12. Indicator :- Something that gives information or shows something; a sign.
13. Reduction :- Process of gaining electrons.
14. Oxidation :- A process that occurs when atoms or groups of atoms lose electrons.
15. Gradually :- Slowly over a period of time or a distance.