Electrochemical cells
Electrochemical cells
Electrochemical cells are devices having the following functions (i) conversion of chemical energy to electrical energy and (ii) conversion of electrical energy to chemical energy. electrochemical cells with former type of functions are called Galvanic cell e.g. Daniel cell, Laclanche cell, bichromate cell etc. and cells with latter type of functions are called electrolyte cells.
Galvanic cells
When a zinc rod is immersed directly in copper sulphate solution, zinc starts dissolving and copper will start precipitating along with liberation of heat energy.
When a copper rod is immersed in zinc sulphate solution, there is no change. This indicates that the zinc has more tendencies to lose electrons than copper. Therefore it is learnt that the zinc has more tendency to lose electrons than copper. Therefore it is learnt that the zinc can be oxidized by zinc ions by itself undergoing reduction and copper cannot be oxidized by zinc ions. Similarly, if a copper rod is immersed in silver nitrate, copper starts dissolving, this is noticed by the change of colour of the solution to blue colour and silver starts precipitating. If the case is reversed there will not be any change. This indicates that zinc can be oxidized by silver. Therefore the tendency to lose or gain electron is a relative property with respect to each other.
The above reaction of zinc and copper is a redox reaction i.e., a combination of both oxidation and reduction half reactions.
By conducting the redox reaction indirectly the decrease in the potential energy of the chemical reaction can be converted in to electrical energy.
The reduction reaction is always followed by reduction and vice versa. An electrochemical cell or galvanic cell or voltaic cell is a device which produces electrical energy by conducting a redox chemical reaction indirectly and the decrease in the potential energy of the chemical reaction appears in the form of electrical energy.
Construction
The electrochemical cell consists of two chambers. In the first compartment a zinc rod is immersed in zinc sulphate solution (zinc electrode) and copper rod is immersed in copper sulphate solution (copper electrode) in the second apartment. The two compartments are connected to each other by means of a salt bridge and connecting wires. Salt bridges are a bent glass tube containing jelly like potassium sulphate and both ends are plugged with glass wool. This will provide the contact between the electrolytes. The zinc rod and copper rod are connected to an ammeter using connecting wires to check the production of electrical signal. When both electrodes are connected, a deflection is noted in the ammeter which shows the production of electrical current in the circuit.
Mechanism
Reaction at anode
When connections are made, zinc rod gets oxidized and starts dissolving I to the solution, leaving behind the electrons. The accumulated electrons at the zinc rod start flowing through the outer circuit through the conducting wires and reach the right hand compartment.
Reactions at cathode
The copper ions present in the solution at the second compartment move towards the copper rod and the electrons coming from right hand compartment are consumed or taken up by these copper ions and the copper gets deposited on the copper rod.
The overall reaction is redox reaction. By conducting the reaction indirectly, a flow of electrons in the outer circuit takes place. Here there is no heat energy produced, as in the case of the same reaction conducted directly by immersing a zinc rod in copper sulphate solution. In this case the decrease in potential energy of the chemical reaction appears in the form of electrical energy.
Direction of flow of electrons
The direction of flow of electrons is indicated by the direction in which the ammeter needle deflects. The electrons are produced in the left hand electrode means copper sulphate, where reduction occurs. The direction of flow of electrons i.e., negative electricity is from the oxidation occurs. The direction of flow of electrons i.e., negative electricity is from the oxidation half cell to reduction half cell. by convention, the direction of flow of electrons. Therefore the direction of flow of current is from right electrode to the left electrode.
Charge of electrode
By IUPAC convention a cell has to be constructed in such a way that the electrode where oxidation occurs has to be written on the left hand side and the electrodes where reduction occurs has to be written on the left hand side. The electrons are produced at the left hand electrode and due to this accumulation of electrons it acquires negative charge. Then electrons will flow to the right hand electrode where it is consumed by copper 2+ ions. Hence it acquires the positive charge. Therefore according to this convention, the electrode at which oxidation occurs is called anode while electrode at which reduction occurs is called cathode. When an electric current is drawn from a Daniel cell, zinc electrode is called the negative terminal as it gives out electrons and copper electrode is called positive terminal.
Electrode and electrode potential
When an element metal or gas, is in contact with its own ions in solution it will constitute an electrode. The tendency of the electrode to lose or gain electrons when it is in contact with its own ionic solution is called electrode potential. The tendency to lose electrons is the tendency to get oxidized; hence it is called oxidation potential. The tendency to gain electrons is the tendency to get reduced; hence it is called reduction potential.
Generally a metal M consists of metal ion with its valence electrons that binds them together. When a metal is in contact with its own ionic solution, the flow of metal cations from the metal to the aqueous phase takes place leaving behind electrons in the metal lattice or metal cations in the aqueous solution will gains electrons and gets deposited on the metal. This will create an equilibrium between positive and negative developed on the metal and the positively or negatively charged free ions in the solution.
This equilibrium of positive and negative ions in metal and its surrounding solution is called Helmholtz electrical double layer. This develops a difference in potential between the metal and surrounding ions. This equilibrium potential difference between the metal and the surrounding solution is called the single electrode potential.
Standard electrode potential
The electrode potential depends on the concentration of the electrolyte and its temperature. Standard electrode potential of an electrode is the tendency of the electrode to lose or gain electrons when it is in contact with its own ionic solution of unit concentration and at 25°C. In other words it is the equilibrium potential difference between the metal electrode and its surrounding ionic solution of unit concentration at 25°C.
It is not possible to determine the absolute value of single electrode potential; since it is not possible to conduct an oxidation and reduction reactions i.e. redox reactions. Therefore only the difference in potential can be measured.
Electromotive force (EMF)
The difference in potential which causes the flow of current from one electrode of higher potential to another electrode of lower potential is called the electro motive force (emf) of cell. Unit of emf is volt.
By convention we compare the reduction potential of all electrodes with respect to each other and the reduction potential of the right hand electrode should always be higher than the reduction potential of the left hand electrode.
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