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Electrochemistry: Lead Storage Battery and Protection Against Corrosion

Lead Storage Battery

  • A lead storage battery consists of alternate plates of anodes and cathodes.
  • The anode consists of lead plate and the cathode of a grid of lead packed with lead dioxide .
  • The electrolyte is an aqueous solution of sulphuric acid which is or . (See Fig) .
Illustration: Lead Storage Battery

Anodic process (oxidation) :

Cathodic process (reduction) :

Net reaction

  • The battery can be recharged by passing current through it .
  • The battery can be charged and discharged a large number of times.

Fuel Cell

  • The fuel cells are based on combustion reactions of fuels like hydrogen, methane, methanol etc which are fed continuously into fuel cells.
  • One of the most successful fuel cells uses the combustion reaction of hydrogen (Fig) .
Illustration: Fuel Cell
  • The electrodes used in it are made of phorus carbon impregnated with catalyst (finely divided or ) .
  • The electrolyte used is an aqueous solution of .
  • Hydrogen gas is bubbled around the anode and oxygen gas around the cathode.

The electrode process are:

Anodic process (oxidation) :

Cathodic process (reduction) :

Overall process:

It was used as a power source in Apollo space programme and water produced was used for drinking purpose by the crew of the aircraft.

Cell Emf and Gibbs Energy

The maximum amount of work that a cell in its standard state can perform is given by

The significance of sign is that the work is done by the cell.

The maximum amount of useful work which a system can perform is equal to decrease in Gibbs energy thus

If is negative, then the cell reaction is spontaneous otherwise not.

Corrosion

Illustration: Corrosion
  • Corrosion is the process of deterioration of metals due to their exposure to environment.
  • It slowly forms a layer of oxides, basic carbonates or other salts on the surface of metals.
  • Example of corrosion are Rusting of iron and tarnishing of silver objects.
  • Corrosion is essentially an electrochemical process involving anodic oxidation of metals resulting in the formation of their ions and a cathodic process that consumes the electrons.
  • One spot on the surface of iron becomes anode and undergoes oxidation.

Anodic process:

The electrons released here move through the metal to another place which acts like a cathode. The reduction of oxygen takes place in presence of ions

Cathodic process:

The overall process is

The positive sign of indicates that this reaction occurs spontaneously.

ions formed here are further oxidized to ions by atmospheric oxygen to form rust

Protection Against Corrosion

There are two general methods used for protection of metallic objects against corrosion.

Use of Protective Layer

Many substances are used for applying a protective layer to prevent or limit corrosion:

  • Machinery, tools and automobile engine parts are often protected by a thin film of oil or grease.
  • Metallic doors, windows, coolers are painted with a suitable paint to prevent corrosion.

Cathodic Protection

  • Cathodic protection of a metal can be done by coating it with a more reactive or more electropositive metal with smaller electrode potential.
  • Galvanization: Iron sheets or other objects are coated with a layer of zinc by dipping them in molten zinc or using electrolysis.
  • Cathodic protection can also be provided by attaching, rather than coating, a piece of more reactive metal to the object to be protected.