Galvanic cells

Galvanic cells

Galvanic Cells -It is a type of electrochemical cell where electrons are generated spontaneously through a redox reaction; these electrons pass through an external circuit. Another name for a galvanic cell is a voltaic cell.

Daniel Cell

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Salt Bridge-

  • ·        Salt bridge is an inverted U-tube like structure.
  • ·        The tube is filled with concentrated solution of an inert electrolyte.
  • ·        Most of the time salts like KCl, KNO3, NH4NO3 are used as electrolyte.

Significance of salt bridge:

  • ·        When a metal is placed in a solution of its ions, the metal acquires either a positive or negative charge with respect to the solution on account of this a definite potential difference is developed between the metal and the solution. This potential difference is called electrode potential.
  • ·        Oxidation potential - When electrode is negatively charged with respect to solution i.e. it acts as anode, oxidation occurs.
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  • ·        Reduction potential - When electrode is positively charged with respect to 'Solution i.e. it acts as cathode reduction occurs.
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  • ·        Oxidation potential is the reverse of reduction potential.
  • ·        Higher is the reduction electrode potential value of a metal, greater is the tendency of the metal to gain electrons and get reduced.
  • ·        It connects the solutions of two half-cells and completes the cell circuit.
  • ·        It prevents transference or diffusion of the solutions from one half-cell to the other.
  • ·        It keeps the solutions in two half-cells electrically neutral.
  • ·        It prevents liquid-liquid junction-potential, i.e., the potential difference which arises between two solutions when in contact with each other.

  • ·        A broken vertical line or two parallel vertical lines in a cell reaction indicates the salt bridge.

Electrodes

·        In order to pass the current through an electrolytic conductor, two rods or plates are always needed which are connected with the terminals of a battery. These rods/plates are called Electrodes.

·        The electrode where oxidation reaction takes place is the anode and electrode where reduction takes place is the cathode.

 Electrochemical Series
Characteristics of electrochemical series

  • ·        Elements having positive standard electrode oxidation potential, lose electrons readily and are good reducing agents. This tendency decreases with decrease in Eo values.
  • ·        Elements having negative values of oxidation potential, gain or accept electrons readily and are good oxidising agents. This tendency increases with decrease in Eo value.
  • ·        An element constituting an electrode having higher oxidation potential can spontaneously displace another element, which forms an electrode with lower oxidation potential, from the solution of its salt. For example, Zn metal can displace Fe2+ and Cu2+ ions from their respective solutions to give Fe and Cu metals respectively. Iron (Fe) can displace Cu2+ ions but not Zn2+ ions from their solution



Electrode potential

  • ·        When a metal is placed in a solution of its ions, the metal acquires either a positive or negative charge with respect to the solution on account of this a definite potential difference is developed between the metal and the solution. This potential difference is called electrode potential.
  • ·        Oxidation potential - When electrode is negatively charged with respect to solution i.e. it acts as anode, oxidation occurs.

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  • ·        Reduction potential - When electrode is positively charged with respect to 'Solution i.e. it acts as cathode reduction occurs.
  • ·        
  • ·        Oxidation potential is the reverse of reduction potential.
  • ·        Higher is the reduction electrode potential value of a metal, greater is the tendency of the metal to gain electrons and get reduced.

 Standard electrode potential - When unity is the concentrations of all the species involved in a semi-cell, the electrode potential is known as the standard electrode potential.

EMF of a cell-It is the difference in the potential across left and right electrodes due to which electrons flow from anode to cathode.

Standard cell potential- When the concentrations of all the species involved in a half-cell is unity then the cell potential is equal to standard electrode potential. It is  denoted by Ecello

Reference Electrode (Standard Hydrogen Electrode, SHE or NHE)

SHE is defined as the electrode in which pure and dry hydrogen gas is bubbled at 1 atmospheric pressure and 298 K on a platinized platinum foil through a solution containing H positive ions at unit activity.



Representation of electrode-

Advantages of standard hydrogen electrode

  • SHE is used as a reference electrode. When it is coupled with any other electrode whose potential is to be determined. The potential of the cell is then measured using a potentiometer.
  •  Since the potential of SHE is zero, the potential of the cell is equal to the potential of another electrode or e.m.f. of the cell itself. Thus when SHE is used, the correction for its own potential is not necessary.
  •  It can be used over the entire pH range.
  •  It gives no salt error.
  • It consists of a pH scale with voltage measurement.