write the chemical name and formula of coloured layer formed on copper and silver due to corrosion
Answers
when silver react with hydrogen sulphide in presence of air (oxygen)
to form a silver sulphide in black colour
Equation is :-
2Ag + H2S ---------- Ag2S +H2
copper metal is naked to moist air(oxygen)
due to
formation of copper carbonate ( corrosion of copper) it develops a green layer
Equation is:-
2Cu + O2 + CO2 + H2O → CuCO3.Cu(OH)2
Given sufficient exposure to air and moisture the surface of Iron develops reddish-brown coat. It makes the metal surface porous or friable over the course of time. It is a redox reaction[1] which culminates in the formation of Iron (III) oxides ie Fe2O3.nH2O and Iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3). This process is called Corrosion and in particularly Iron’s case it is called rusting.
Iron also develops “Green rust”[2] when it is kept in oxygen deprived surroundings like say under water. In presence of acidic radicals like carbonate, Chloride or Sulphate it develops crystalline compounds having greenish colour.
The metal Copper corrodes in a very negligible manner . Copper lies ranks very low in electrochemical series and has an inertia towards corrosion. But given suitable conditions like presence of oxides, carbon-dioxides, sulphur bearing compounds , it slowly corrodes on the surface giving a dark colour which gradually changes in to green. It is primarily due to formation of chlorides, sulphates, sulphides of the metal. Unlike Iron its topmost layer acts as protective cover to the underlying metal. The coating on the metal is often patina. [3]
Noble metals like silver also corrode in presence of some chemical compounds like Hydrogen Sulphide. This process is called tarnishing[4] . It often appears as a dull, gray or black film or coating over metal. Tarnish is a surface phenomenon that is self-limiting, unlike rust. Only the top few layers of the metal react, and the layer of tarnish seals and protects the underlying layers from reacting. Silver tarnish is usually comprised of Ag2O or Silver(I) oxide which gives a darker color to the metal.