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Transcript of INVESTIGATORY PROJECT
CHANGE IN EMF OF A DANIEL CELL WITH CONCENTRATION AND TEMPERATURE
The Daniel cell is a type of electrochemical cell invented in 1836 by John Frederic Daniel, a British chemist and meteorologist, and consisted of a copper pot filled with a copper sulfate solution, in which was immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode. He was searching for a way to eliminate the hydrogen bubble problem found in the voltaic pile, and his solution was to use a second electrolyte to consume the hydrogen produced by the first. Zinc sulfate may be substituted for the sulfuric acid. The Daniel cell was a great improvement over the existing technology used in the early days of battery development. A later variant of the Daniel cell called the gravity cell or crowfoot cell was invented in the 1860s by a Frenchman named Callaud and became a popular choice for electrical telegraphy.
The Daniel cell is also the historical basis for the contemporary definition of the volt, which is the unit of electromotive force in the International System of Units. The definitions of electrical units that were proposed at the 1881 International Conference of Electricians were designed so that the electromotive force of the Daniel cell would be about 1.0 volts. With contemporary definitions, the standard potential of the Daniel cell at 25 °C is actually 1.10 V.
In the Daniel cell, copper and zinc electrodes are immersed in a solution of copper(II) sulfate and zinc sulfate respectively. At the anode, zinc is oxidized per the following half reaction:
Zn(s) → Zn2+(aq) + 2e- . . (Standard electrode potential -0.7618 V )
At the cathode, copper is reduced per the following reaction:
Cu2+(aq) + 2e- → Cu(s) . . (Standard electrode potential +0.340 V )
The total reaction being:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) . . ( Open-circuit voltage 1.1018 V )
Features of Daniel Cell:-
• Zinc rod at which oxidation occurs is called the anode while the copper rod at which the reduction takes place is called cathode.
• The half-cell reaction occurring at anode is called oxidation -half cell reaction while the occurring at cathode is called reduction.
• The two half-cell reactions always take place simultaneously i.e . . . . Half cell reaction cannot take place immediately.
• Since electrons are produced at zinc electrode, it is rich in electrons and pulls these electrons into the external circuit and hence acts as negative pole. The copper electrode on the other hand is deficient in electrons and thus pulls the electrons from the external circuit and act as positive pole.
• The electrons flow from negative pole to positive pole in the external circuit. However, conventionally the current is said to flow in opposite direction i.e. from positive pole to negative pole in the external circuit.
• The concentration of copper sulphate solution decreases with passage of time as the cell operates, consequently the current fall with passage of time.
An electrochemical cell is a device by which electric current energy is generated at the cost of chemical energy due to chemical action taking place in the cell. They are of two types:
1. Primary cells
2. Secondary cells
PRIMARY CELL: It is that cell in which electrical energy is produced due to chemical energy. The chemical reaction in the cell is irreversible.
E.g.: Daniel cell, Voltaic cell, Leclanche cell
SECONDARY CELL: It is that cell in which the electrical energy is first stored up as a chemical energy and when the outside circuit is closed to draw the current from the cell the stored chemical energy is reconverted into electrical energy. The chemical reactions are reversible in this cell. The secondary cells are also called storage cell or accumulators.
E.g.: Lead acid accumulator, Edison cell.
Here we are going to discuss about the change in emf of the Daniel cell which is a primary cell, with respect to concentration and temperature.
E.m.f [ELCTROMOTIVE FORCE]
Electromotive force, also called emf, is the voltage developed by any source of electrical energy such as a battery or dynamo.
EMF of cells means , when current flows through two points a potential difference generated by a cell draws no current is called EMF
Inside a source of emf that is open-circuited, the conservative electrostatic field created by separation of charge exactly cancels the forces producing the emf. Thus, the emf has the same value but opposite sign as the integral of the electric field aligned with an internal path between two terminals A and B of a source of emf in open-circuit condition (the path is taken from the negative terminal to the positive terminal to yield a positive emf, indicating work done on the electrons moving in the circuit
An electrical voltage difference is sometimes called an emf. The points below illustrate the more formal usage, in terms of the distinction between emf and the voltage it generates:
1. For a circuit as a whole, such as one containing a resistor in series with a voltaic cell, electrical voltage does not contribute to the overall emf, because the voltage difference on going around a circuit is zero. (The ohmic IR drop plus the applied electrical voltage is zero. See Kirchhoff's Law). The emf is due solely to the chemistry in the battery that causes charge separation, which in turn creates an electrical voltage that drives the current.
2. For a circuit consisting of an electrical generator that drives current through a resistor, the emf is due solely to a time-varying magnetic field that generates an electrical voltage that in turn drives the current. (The ohmic IR drop plus the applied electrical voltage again is zero. See Kirchhoff's Law)
3. A transformer coupling two circuits may be considered a source of emf for one of the circuits, just as if it were caused by an electrical generator; this example illustrates the origin of the term "transformer emf".
4. A photodiode or solar cell may be considered as a source of emf, similar to a battery, resulting in an electrical voltage generated by charge separation driven by light rather than chemical reaction.
5. Other devices that produce emf are fuel cells, thermocouples, and thermopiles.
In the case of an open circuit, the electric charge that has been separated by the mechanism generating the emf creates an electric field opposing the separation mechanism. For example, the chemical reaction in a voltaic cell stops when the opposing electric field at each electrode is strong enough to arrest the reactions. A larger opposing field can reverse the reactions in what are called reversible cells.
The electric charge that has been separated creates an electric potential difference that can be measured with a voltmeter between the terminals of the device. The magnitude of the emf for the battery (or other source) is the value of this 'open circuit' voltage. When the battery is charging or discharging, the emf itself cannot be measured directly using the external voltage because some voltage is lost inside the source. It can, however, be inferred from a measurement of the current I and voltage difference V, provided that the internal resistance r already has been measured: ℰ = V + Ir.
Zn(s)+Cu2+ (aq) ®Zn2+(aq)+Cu(s)
To check the change in E.m.f of Daniel cell with concentration and temperature
1. Two beakers
2. Zinc and copper plate
3. Filter paper
5. Connecting wires
6. Card board
7. KNO3 solution
8. 1 M,0.1 M,0.01 M Solution of
Daniel cell is an arrangement to convert the chemical energy of the redox reaction into electric energy.
It consists of a tube filled with semi-solid paste obtained by adding gelative or agar to the solution of strong electrolyte such as Nacl, NH4NO3.KNO3 etc, which does not change chemically during the process.
Function of salt bridge
• To complete the electrical circuit by allowing the solution to flow from one solution to another without mixing the two solutions.
• To maintain electrical neutrality of solution in two half-cells.
1. Take two beakers and pour the required chemicals in respective beaker and mark them for identification.
2. Take two square to slide in and connecting wire to their screw.
3. Connect negative of the voltmeter to the anode and its positive to the cathode
4 . Take filter paper long enough to dip into both the solution. Dip the filter paper in KNo3 solution and put it as a salt bridge.
5. Put on the electrode voltmeter set up. Note the reading quickly and then put of the electrode voltmeter set up.
6. For measuring variation with temperature with change in area of electrode use the different size of electrode and then do step 5 again.
7. For measuring variation with temperature heat the solution and then do step 5 again.
8. For measuring variations with change in concentration of electrolyte ,use the electrolytes of different molarity and then do step 5 again.
On concluding this project we came to know about the change in emf of Daniel cell with respect to concentration and temperature. Also this project deals with electrochemical cells and different type of electrochemical cells.
I am concluding this project with a hope that anyone can clarify his/her doubt against Daniel cell, emf and change in emf, through this project.