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Electrochemistry and buffer

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Electrochemistry

Electrochemistry is the branch of science that deals with the relationship between chemical reactions and electricityas well asconversions of chemical energy into electrical energy and vice-versa.

Conductors

Metallic conductors

Electrolytic conductors

Passage of electricity is due to the flow of electrons.

Passage of electricity is due to the flow of cations and anions.

It doesn’t involve any chemical change( but physical may e.g. heating effect)

Chemical changes occur during passage of electricity.

Electrochemical cell

A setup designedfor carrying out electrochemical reaction i.e. chemical reaction accompanied by consumption or generation of electrical energy is called electrochemical cell. It is of two types:

  1. Electrolytic cell: In this cell, consumption of electrical energy brings about a chemical change. E.g. electrolytic setup for acidulated water, electrolysis of NaCl.
  2. Galvanic cell(or voltaic cell): In this cell, electrical energy is produced by the chemical energy of the reacting substances. Eg. Daniel cell.

Salt bridge

Salt bridge is used to establish electrical contact between two electrolytes in galvanic cells by eliminating direct liquid-liquid junction. Typical salt bridge is a U-shaped glass tube filled with KCl-containing agar-agar gel. Substitutes for KCl can be KNO3, NH4NO3, etc. Electrolyte used in salt bridge is chosen such that the mobility of its cations and anions is almost same. Its functions are:

  • It maintains electrical contact between two half cells and completes the circuit.
  • It eliminates liquid junction potential (which occurs in liquid-liquid junction due to unequal rates of migration of anions and cations at the junction).

Electrode potential and Single electrode potential

The potential difference between electrode and the solution is termed as electrode potential. When a metallic rod is dipped into its soluble salt solution, some electrons outrush into the solution leaving the rod. The diffused electrons and the metallic ions formed are bound by electric forces between them of certain potential which is called single electrode potential. The electric bilayer for during this process is called Helmholtz layer.

Standard electrode potential is the electrode potential measured with respect to SHE at standard conditions mentioned below:

  1. 1M concentration of ions in the solution
  2. 1atm pressure for the gaseous substances involved in the reaction
  3. 0oC temperature

Standard hydrogen electrode

Standard hydrogen electrode(SHE) consists of a hydrogen electrode in which hydrogen gas is maintained at 1 atm pressure in equilibrium with H+ ions in its 1M solution. Electric contact is provided by inert metal like Platinum. The emf of SHE is arbitrarily taken 0.0V and is used as reference electrode to measure electrode potential of other electrodes.

Nernst equation

Let’s consider an electrochemical reaction:

aA + bB cC + dD

The emf of the cell reaction at non-standard conditions is calculated by using Nernst equation mentioned below:

E = Eo - RTlnQ (i)

Where E : cell emf at standard conditions

Eo: cell emf at non-standard condition

R: universal gas constant



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