Electrochemistry

 
Bhavsar Manali N.
Smt.S.M.Shah Pharmacy College 

Note: This Material is a study Purpose Only. Do not use in commercialization.


Electrochemistry:

• The changes in which electrical energy is produced as a result of chemical change. The devices used to produce electrical energy from chemical reactions are called electrical cells, galvanic or voltic cells.
• In these cells, oxidation and reduction reaction reactions occur in separate containers called half cells and the red-ox reaction is spontaneous.

• The arrangement consists of two beakers, one of with contains 1,0 M solution of zinc sulphate and the other 1,0 M solution of copper sulphate. A zinc rod is dipped into ZnSO4 solution while a copper rod is dipped into CuSO4 solution. These metallic rods are known as electrodes.
• 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:

• 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:

• The mobility of the anion and cation of the electrolyte should be almost same.
• The ions of the electrolyte are not involved in electrochemical change.
• The ions do not react chemically with the species of the cell.

• The important functions of the salt bridge are:

• Salt bridge completes the electrical circuit.
• Salt bridge maintains electrical neutrality of two half cell solution.

REPRESENTATION OF AN ELECTROCHEMICAL CELL

• An electrochemical cells or galvanic cell consists of two electrodes: anode and cathode. The electrolyte solution containing these electrodes is 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 cell

• The flow of electric current in an electrochemical cell indicates that a potential difference exists between two electrodes.
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• If the metal has relatively high tendency to get oxidized, 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 equilibrium is attained, it results in separation of charges (negative on the electrode with respect to the solution).
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Application of the electrochemical series

• Relative strengths of oxidizing and reducing agents

The substances which have lower reduction potentials are stronger reducing agents while have higher reduction potentials are stronger oxidations agent.

• Calculation of the E.M.F. of the cell.

E0cell = E0(cathode) - E0(anode)

• Predicting feasibility of the reaction.

In general, a red-ox reaction is feasible only if the species to release electrons must have lower reduction potential as compared to the species which is to accept electrolytes.

• To predict whether a metal can liberate hydrogen from acid or not

In general, only those metals can liberate hydrogen from the acid which have negative values of reduction potentials , - E0 values.