Electric cells, Electric circuits, Electrolysis

ELECTRIC CIRCUIT

Electric current is simply electric charge in motion. In conductors such as cables or wire, the current consist of swam of moving electron. Electric cells are chemical devices, which are capable of causing an electric current to flow. This produces electric force, which pushes the current along. If there is a complete circuit of conductors by which current can leave from one end to terminal of the cell and travel round to the other terminal, a current will flow. This current will be the at any point round the circuit and of the line is broken, the current is stopped or switched off. The electrons flow from the negative terminal or cathode of the cell to the procedure terminal or anode

TYPES OF ELECTRIC CELLS

Electric cells are divided into two namely: the primary cells and the secondary cells

PRIMARY CELLS

These are those cells in which current is produced as a result of an irreversible chemical charge.

SECONDARY CELLS

These cells are those which can be recharged when they run down by passing current backwards through them.

There are three component in a cell .Two of them are electrodes in the primary cell, the two electrodes are of different metals (graphite is often used). The third item is the container bearing the electrolyte. Examples of electrolyte are strips of aluminum, Carbons (graphite) copper, iron lead and zinc.

THE SIMPLE PRIMARY CELL (VOLTAIC CELL)

A simple cell can be made by placing two different electrodes (metals) in an electrolyte. Two wire are then used to connect these metals to a voltmeter, an instrument which measure the potential different between any two point in an electric circuit. If a deflection is noticed it means that the cell creates a voltage. If the deflection is done to the right it mean that the electrode, or anode, which is connected to the positive terminal of the voltmeter is the positive electrode, or anode, while the one is connected to the negative terminal is the negative electrode or cathode. If the deflection is however done to the left, a reconnection should be done.

The two major deflects of a simple cell are polarization and local action

POLARIZATION

The cell is characterized by the release of “hydrogen bubbles.” The bubbles collect at the positive electrode and insulate it. This shows down and eventually stops the working of the cell. This defect is called polarization.

This defect can be corrected either by occasionally brushing the plates, which is highly in convenient, or by using a depolarize e.g. manganese oxide. This oxides hydrogen to form and so removes the hydrogen bubble.

LOCAL ACTION

This occurs when pure zinc is not in use. The impurities in the zinc results in the gradual wearing away of the zinc plates. This can be prevented by cleaning the zinc with H₂SO₄ and then rubbed with mercury. The mercury amalgamates the zinc by covering the impurities thereby preventing it from coming into contact with electrolyte.

EVALUATION

1. What is a cell?

2.  Explain the defects of a named cell.

DEFINITION OF SIMPLE TERMS

ELECTRIC CIRCUIT

Electric current is simply electric charge in motion. In conductors such as cables or wire, the current consist of swam of moving electron. Electric cells are chemical devices, which are capable of causing an electric current to flow. This produces electric force, which pushes the current along. If there is a complete circuit of conductors by which current can leave from one end to terminal of the cell and travel round to the other terminal, a current will flow. This current will be the at any point round the circuit and of the line is broken, the current is stopped or switched off. The electrons flow from the negative terminal or cathode of the cell to the procedure terminal or anode

TYPES OF ELECTRIC CELLS

Electric cells are divided into two namely: the primary cells and the secondary cells

PRIMARY CELLS

These are those cells in which current is produced as a result of an irreversible chemical charge.

SECONDARY CELLS

These cells are those which can be recharged when they run down by passing current backwards through them.

There are three component in a cell .Two of them are electrodes in the primary cell, the two electrodes are of different metals (graphite is often used). The third item is the container bearing the electrolyte. Examples of electrolyte are strips of aluminum, Carbons (graphite) copper, iron lead and zinc.

THE SIMPLE PRIMARY CELL (VOLTAIC CELL)

A simple cell can be made by placing two different electrodes (metals) in an electrolyte. Two wire are then used to connect these metals to a voltmeter, an instrument which measure the potential different between any two point in an electric circuit. If a deflection is noticed it means that the cell creates a voltage. If the deflection is done to the right it mean that the electrode, or anode, which is connected to the positive terminal of the voltmeter is the positive electrode, or anode, while the one is connected to the negative terminal is the negative electrode or cathode. If the deflection is however done to the left, a reconnection should be done.

The two major deflects of a simple cell are polarization and local action

POLARIZATION

The cell is characterized by the release of “hydrogen bubbles.” The bubbles collect at the positive electrode and insulate it. This shows down and eventually stops the working of the cell. This defect is called polarization.

This defect can be corrected either by occasionally brushing the plates, which is highly in convenient, or by using a depolarize e.g. manganese oxide. This oxides hydrogen to form and so removes the hydrogen bubble.

LOCAL ACTION

This occurs when pure zinc is not in use. The impurities in the zinc results in the gradual wearing away of the zinc plates. This can be prevented by cleaning the zinc with H₂SO₄ and then rubbed with mercury. The mercury amalgamates the zinc by covering the impurities thereby preventing it from coming into contact with electrolyte.

EVALUATION

1. What is a cell?

2.  Explain the defects of a named cell.

Leclanche cell

Leclanche cells are of two types : the wet and the dried types. The wet leclanche cell consists of a zinc rod at the cathode in solution of ammonium chloride contained in a glass vessel. The anode is a carbon rod contained in a porous pot and is surrounded by manganese chloride as a depolarize

An e.m.f. is set up by the zinc, the carbon and the electrolyte, which drives a current from zinc to carbon through the cell. This carbon is at a higher potential than the zinc. When an external circuit is connected to the cell, current flows from carbon to zinc out side. The e.m.f is set up because zinc reacts with the ammonium chloride to form zinc chloride, ammonia and hydrogen, and electrons are released. These electrons flow from the zinc plate to the carbon plate out side the cell.

Hydrogen reacts with the manganese dioxide and oxidizes it to form water. The e.m.f of a leclanche cell is 1.5 voIts, defect include When the cell has worked for sometime, the rate of hydrogen production becomes greater than rate at which it is oxidized by the manganese dioxide, hence the formation of polarization. Therefore the cell must be allowed to rest from time. These primary cells are restricted to intermittent current supply because they do not give continuous service.They are too heavy to carry about without spilling the liquid. For the dry leclanche cell, the defect of heaviness is overcome.

The ammonium chloride electrolyte is a jelly-like material and not aligned solution. The positive electrode is a carbon rod surrounded by a packed mixture of manganese dioxide and powered carbon, inside a zinc container, which is the negative electrode.

The dry cell can be carried about easily E.g. torch batteries, and transistor radio batteries. Due to local action, they deteriorate after sometime.

THE DANIEL CELL

This is also a primary cell invented to counter the problem of polarization. The zinc rod is the negative electrode while the positive electrode is the container. The electrolyte is dilute tetrasulphate (vi) acid contained in a porous pot around the zinc rod, and the depolarize is copper tetraoxosulphate (vi) in the surrounding copper container. The diaphoreses is mush more efficient than the leclanche cell. The e.m.f. is of a constant  value of   l..08V.

THE LEAD-ACID ACCUMULATOR

This is the most common one. It consists of lead oxide as the positive electrode, lead as the negative electrode and tetra oxosulphate (vi) acid as the electrolyte. During the discharge, when the cell is given out current both plates gradually charge to lead tetraoxosulphate (vi) while the acid gradually becomes more dilute and the density decreases. When fully charged the relative density and e.m.f. of the cell are 1.25 and 2.2v respectively. But when discharged, they are reduced to 1.5 and less than 2.0v respectively. The rod density of the cell should not be allowed to drop 1.15 before it is recharged.

MAINTENANCE OF LEAD ACID ACCUMULATORS

1 The liquid level must be maintained by using distilled H₂^O

2. The cell should be charge if relative density of acid falls below 1.15. it is fully charged when relativedensity of acid is 1.25. It is tested with a special hydrometer.

3.If the cell is not in use for a long time, it should be discharge from time to time or the acid remove and thecell dried.

4. The battery should be kept clean so that current does not leak away across the casing between the terminals.

THE ALKALINE OR NIFE ACCUMULATORS

The name is gotten from the chemical symbol nickel (Ni) and iron (Fe). The positive electrode is made of nickel hydrogen while the negative plate is either of iron or calcium. The electrolyte is potassium hydroxide dissolve in water. This cellslast longer and lead acid cells keep their charge longer and they require less maintenance. They are used for emergencies in factories and hospitals. They are expensive and bulky with a small e.m.f value, about 1.25v.

EVALUATION

1. What is the advantage of dry leclanche. Cell over wet leclanchecell.?
2. How can polarization and local action beprevented.

Summary

READING ASSIGNMENT

New School Physics for Senior Secondary Schools (M.WAnyakohaPages 397 – 402).

GENERAL EVALUATION

1. A block of mass 2.0Kg resting on a smooth horizontal plane is acted upon simultaneously by two forces, 10N due north and 10N due east. The magnitude of the acceleration produced by the forces on the block is.
2. Two forces 3N and 4N act on a body due north and due east respectively. Calculate the equilibrant.

ELECTROLYSIS

Is the process whereby a liquid conducts electricity by the movement of positive and negative ions within the liquid while undergoing chemical changes.

ELECTROLYTES

Are liquid, which allow the electricity through them. Such electricity is salt solutions, alkalis and dilute acids (acidulated water).

NON-ELECTROLYTES

Are liquids, which do not allow electricity to pass through them. Such liquids include distilled water, alcohol, liquid paraffin and sugar solution.

NOTE

Metals and hydrogen are deposited at the cathode, while non-metals and oxygen are deposited on the anode. The anode may dissolve in solution. 

Electrolysis does not manufacture electric charges and it is the “splitting’ of compounds by electricity. E.g water decomposes into oxygen and hydrogen by electric current.  Electrolysis begins when the electric circuit is completed and ends abruptly when the electric circuit is broken. 

EVALUATON

1. Explain the term electrolysis?
2. What is an electrolyte                                     

FARADAY’S LAWS OF ELECTROLYSIS

FARADAY’S FIRST LAW OF ELECTROLYSIS

States that the mass of a substance liberated during the process of electrolysis is proportional to the quantity of electricity passed through the electrolyte

FARADAY’S SECOND LAW OF ELECTROLYSIS

States that the relative masses of substances liberated by the same quantity of electricity are proportional t their chemical equivalents.

SIMPLE CALCULATIONS

 If M is the mass of substance deposited when a current q flows for time t, then the quantity of electricity of electricity which flows is flows is It, and

         m = Z It.

         Where, Z = electrochemical equivalent  (e.c.e) the substance.

        …. Z = m/ It   

            I = current in              t = time in see                         m = mass of subs in grammes.

EVALUATION

1. What is meant by the statement ^“ the electrochemical equivalent of copper is 0.000 33 g /coulomb.

2. A current of 3A maintained for 50 minutes deposits 3.048g of zinc at the cathode. Determine the electrochemical equivalent of zinc.

APPLICATIONS OF ELECTROLYSIS

In industry, electrolysis is used in electroplating of metals, purification of metals and electrolytic production of metals from compounds.

ELECTROPLATING

This process is used in coating cutlery and other articles with copper, silver, chromium, nickel or gold. The article to be plated is used as the cathode and the coating metal is used as the anode. The electrolyte is a solution of a salt of a salt of the plating metal. For example, in the silver –plating of a spoon is made the cathode, pure silver is the anode, and silver nitrate solution is the electrolyte (see figure below). Two anodes would be placed, one on each side of the spoon so that back and front would be plated at once.

The silver nitrate dissociates in solution into silver ion and nitrate ions.

AgNo          →     Ag⁺+ NO^-3

When electricity is passed through the solution, the Ag ions move towards the cathode where they are discharged and the spoon becomes coated with metallic silver. The NO remains in solution combining with silver from the anodes to form more silver nitrate, thus, staying at its original concentration.

THE PURIFICATION OF METALS

In the electrolysis of copper sulphate using copper electrodes, copper is deposited at the cathode while at the same time the copper from the anode goes into solution.

In purification of copper metal, the impure copper is made the anode while the pure copper is made the cathode. When current is passed, copper ions are dissolved from the anode and deposited at the cathode leaving the impurities behind.  The pure copper is used in manufacture of electric cables because of its low resistance.    

THE ELECTROLYTIC PREPARATION OF METALS FROM COMPOUNDS

 Metals such as aluminum, sodium and potassium are prepared from their molten chlorides or hydroxide by the process of electrolysis.

EVALUATION

1. Mention at least two uses of electrolysis

2.  Explain  how electrolysis can be used to calibrate an ammeter?

READING ASSIGNMENT

New School Physics for Senior Secondary Schools (M.WAnyakohaPages 385- 395).

GENERAL EVALUATION

1. A wheel and axle of radii 80cm and 20cm respectively is used to raise a load of weight 800N by the application of 250N. Calculate the efficiency of the machine.
2. A machine of efficiency 80% is used to lift a box. if the effort applied by the machine is twice the weight of the box, calculate the velocity ratio of the machine

WEEKEND ASSIGNMENT

1. Which of a-d below is correct?(i)Ordinary torch battery is an example of primary cell.(ii) Accumulations have very high interne resistance.(a)(i) only (b) (ii) only  (c)(iii) only  (d) (i) and (ii) only
2. Which of the following statement is not true? (a) The chemical action in a primary cell is irreversible (b) Lead-acid  accumulation can be recharged (c) Lead-acid accumulator has large internal resistance (d) A secondary cell can be recharged
3. The defect in simple cell which result in a back e.m.fand increase in internal resistance is known as (a)local action (b) reduction (c)polarization (d) oxidation
4. Which of the following instrument is most accurate for comparing e.m.f of two cell? (a) Wheatstone bridge (b) galvanometer (c) potentiometer (d)meter bridge
5. Which of the following devices coverts mechanical energy to electric current? (a) Battery (b) Photocell (c) Thermopile (d) Dynamo

THEORY

1. Briefly differentiate between primary cell and secondary cells.
2. In electrolysis of copper tetraxosulphate (vi) using copper electrodes, 1.53g of copper wire deposited in 30 minutes. Determine the average current used.(Z=3.29 x 10^-4)