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1. InterfaceElectrochemistry is the study of reactions in which charged particles (ions and/or electrons) cross the interface between two phases of matter, such as the interface between a solid and a liquid (=electrode /electrolyte).

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2.1. Overview(1) Electrochemistry is a study of redox reaction:

- Reduction [a reactant gains electron(s)]- Oxidation [a reactant loses electron(s)]

Zn Cu

wire

Salt bridge

Cu(II) sulfate Zn(II) sulfate

electrons

Reduction:Cu2+(aq) + 2e- → Cu(s)

Oxidation:Zn(s) → Zn2+(aq) + 2e-

Total Cell Reaction:Zn(s)+Cu2+(aq) → Zn2+(aq)+Cu(s)

The metallic rods in the beaker are connected to the ammeter by means of an insulated wire through a key. Ammeter is used to know the passage of current which moves in opposite direction to the flow of electrons. The solution in the two beakers are connected by an inverted U-tube containing saturated solution of some electrolyte such as KСl, KNO3, NH4OH which does not undergo a chemical change during the process.

The two openings of the U-tube are plugged with some porous material such as glass wood or cotton. The U-tube which connects the two glass beakers is called a salt-bridge.

When the circuit is completed by inserting the key in the circuit, it is observed that electric current flows through external circuit as indicated by the ammeter. The following observations are made:

Therefore, the current flows from copper to zinc

N.B. The flow of electric current is taken opposite to the flows of electrons

There observation can be explained as:During the reaction, zinc is oxidized to Zn2+ ions which go into the solution. Therefore,

the zinc rod gradually loses its weight. The electrons released at the zinc electrode move towards the other electrode through outer circuit. Here, these are accepted by Cu2+ ions of CuSO4 solution which are reduced to copper.

The zinc electrode where electrons are released or oxidation occurs s called anode while the copper electrode where electrons are accepted or reduction occurs is called cathode.

The two containers involving oxidation and reduction half reactions are called half cells. The zinc rod dipping into a ZnSO4 solution is oxidation half cell and the copper electrode dipping into a CuSO4 solution is reduction half cell

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2.1. Overview(2) Reduction reactions take place heterogeneously at Interfaces between electrodes and electrolyte(s):

- Anode at which oxidation reaction(s) take(s) place.- Cathode at which reduction reaction(s) take(s) place.

Zn2+(aq)

e-

Zn(s)

e-

Cu(s)Cu2+(aq)

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1. Interface

N.B. The galvanic cells which consists of the zinc rod dipping into a ZnSO4 solution and the copper electrode dipping into a CuSO4 solution is Daniell cell.

Its formula is:

Salt bridge and its function. It’s usually an inverted U-tube filled with concentrated solution of inert electrolyte. The essential requirements of electrolyte are:

a) The mobility of the anion and cation of the electrolyte should be almost same.

b) The ions of the electrolyte are not involved in electrochemical change.

c) The ions do not react chemically with the species of the cell.

Generally, salts like KCl, KNO3, etc. are used. The seturated solutions of these electrolytes are prepared in agar agar jelly or gelatin. The jelly keeps the electrolyte in semi-solid phase and thus prevents mixing.

The important functions of the salt bridge are:a) Salt bridge completes the electrical circuit.b) Salt bridge maintains electrical neutrality of two half cell

solution.The accumulation of charges in the two half cells (accumulation

of extra positive charge in the solution around the anode according to the realizing of Zn2+ in excess and accumulation of extra negative charge in the solution around the catode due to excess of SO4

2- ) is prevented by using salt bridge, which provides a passage for the flow of the charge in the internal circuit.

REPRESENTATION OF AN ELECTROCHEMICAL CELLAn electrochemical cells or galvanic cell consists of two

electrodes: anode and cathode. The electrolyte solution containing these electrodes are called half cells.

The following conventions are used in representing an electrochemical cell:

1. A galvanic cell is represented by writing the anode (where oxidation occurs) on the left hand side and cathode (where reduction occurs) on the right hand side.

2. The anode of the cell is represented by writing metal or solid phase first and then the electrolyte (or the cation of the electrolyte) while the cathode is represented by writing the electrolyte first and then metal or solid phase.

3. The salt bridge which separates the two half cells is indicated by two vertical lines.

Electrode Potential and E.M.F. of a galvanic cellElectrode PotentialThe flow of electric current in an electrochemical cell

indicates that a potential difference exists between two electrodes.

If the metal has relatively high tendency to get oxidised, its atom will lose electrons readily and form Cu2+ ions, which go into the solution. The electrons lost on the electrode would be accumulated on the metal electrode and the electrode acquires a slight negative charge with respect to the solution. Some of the Cu2+ ions from the solution will take up electrons and become Cu atoms. After some time, an equilibrium will be established as:

When such an equilibrium is attained, it results in separation of charges (negative on the electrode with respect to the solution).

Forming the double layer

1- metal2 - solutionCu

Similarly, if the metal ions have relatively greater tendency to get reduced, they will take electrons from the electrode. As a result, a net positive charge will be developed on the electrode with respect to the solution. This will also result into separation of charges (positive on the electrode with respect to the solution).

Due to separation of charges between the electrode and the solution, an electrical potential is set up between metal electrode and its solution.

The electrical potential difference set up between the metal and its solution is known as electrode potential.

2. Reduction potential. The tendency of an electrode to gain electrons or to get reduced.

The electrode potential may be of two types:

1. Oxidation potential:The tendency of an electrolyte to lose electrons or to

get oxidised

E.M.F. or Cell Potential of a CellThe difference between the electrode potentials of the two

electrodes constituting an electrochemical cell is known as electromotive force (e.m.f.) or cell potential of a cell. This acts as a driving force for the cell reaction. The potential difference is expressed in volts.

20Cu2+(aq) + 2e- → Cu(s) +0.34 V

Zn2+(aq) + 2e- → Zn(s) -0.76 V

StandardReductionPotential

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2.2. Thermodynamics

In electrochemistry, the standard hydrogen electrode (SHE) potential is taken as a reference point.