Question

Give reason for the following statement: Copper and brass utensils should be tinned.

Answer 1

Hint: The oxidation of metals in the presence of air and moisture damages and degrades the entire surface of metals by forming a layer of oxide on it. Several methods are incorporated to prevent such degradation of metals

Complete answer:
Metals are electropositive and due to this nature of metals they tend to lose electrons easily to an electronegative metal. So, when metals get exposed to air or water, they lose electrons to an oxygen atom, and oxygen gets reduced. As a result, the metal oxide is formed. This oxidation of metals in the presence of air and moisture damages the entire surface of metals by forming a layer of oxide on it. This degradation of metal is known as corrosion.
Similarly, the utensils made of copper and brass also undergo this process of corrosion when exposed to air and water. The copper metal reacts with oxygen to form a cuprous oxide that is red and this cuprous oxide further oxidizes and forms black-colored cupric oxide.
\[\begin{align}
  & 2\text{Cu(s)}+\frac{1}{2}{{\text{O}}_{\text{2}}}\text{(g)}\to \text{C}{{\text{u}}_{\text{2}}}\text{O(s) (red)} \\
 & \text{C}{{\text{u}}_{\text{2}}}\text{O(s)}+\frac{1}{2}{{\text{O}}_{\text{2}}}\text{(g)}\to 2\text{CuO(s) (black)} \\
\end{align}\]
This cupric oxide when comes in contact with atmospheric gases forms a blue-green colored patina.
\[\begin{align}
  & 2\text{CuO(s)}+\text{C}{{\text{O}}_{\text{2}}}\text{(g)}+{{\text{H}}_{\text{2}}}\text{O(l)}\to \text{C}{{\text{u}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}{{\left( \text{OH} \right)}_{\text{2}}}\text{(s) (green)} \\
 & \text{4CuO(s)}+\text{S}{{\text{O}}_{3}}\text{(g)}+3{{\text{H}}_{\text{2}}}\text{O(l)}\to \text{C}{{\text{u}}_{\text{4}}}\text{S}{{\text{O}}_{\text{4}}}{{\left( \text{OH} \right)}_{\text{6}}}\text{(s) (green)} \\
\end{align}\]
Tinning is also known as tin plating. It is a process of coating a thin sheet or strip of steel with tin which is known as tinplate on a metallic surface of copper or brass. It is used to prevent rust and corrosion while maintaining the electrical and thermal conductivity of metals.
Brass is an alloy of copper and zinc. It also undergoes corrosion when exposed to air and moisture. Brass can become oxidized due to atmospheric exposure to form a layer known as patina similar to the corrosion of copper.
Hence, to prevent the formation of an oxide layer, brass and copper utensils should be tinned.

Additional information:
 If the brass contains larger quantities of zinc, a destructive corrosion process takes place known as dezincification. The statue of liberty is made of copper and its greenish color is due to the formation of the oxide layer of copper.

Note:
When iron rusts it forms a flaky layer and weakens the metal. While the blue-green patina formed due to copper’s oxidation blocks the metal’s tendency to undergo further corrosion and thus protects the metal surface from further damage.