Question

Two reactions involving water are shown.
 $ X\cdot anhydrous\left[ cobalt\left( II \right)chloride \right]~+water\to hydrated\left[ cobalt\left( II \right)\text{ }chloride \right] $
 $ Y\cdot iron+oxygen+water~\to ~rust $ Which reactions are reversible by heating?
(A) X-yes; Y-yes
(B) X-yes; Y-no
(C) X-no; Y-yes
(D) X-no; Y-no

Answer 1

Hint: The rusting is an electrochemical reaction in which iron transfers its electrons to the oxygen. The iron gets reduced and the oxygen gets oxidized. The rate of this reaction is affected by water and electrolytes. The presence of water increases the rusting.

Complete step by step solution:
Let us begin our answer by knowing about the rusting of iron only. First, we will see what rust is. The Rust can be defined as the iron oxide formed on anything made of iron having reddish brown color. The iron oxide is produced by the action of oxygen in water on the iron. The rust is hydrated $ iron\left( III \right)oxide(F{{e}_{2}}{{O}_{3}}\cdot n{{H}_{2}}O) $ and $ iron\left( III \right)hydroxide\left[ Fe\left( OH \right),Fe{{\left( OH \right)}_{3}} \right]O) $
So, the rusting of iron is a redox reaction between iron and water in the atmosphere. Now, our question is whether both air and moisture are required for the rusting. Yes, this is true. Both the air and moisture are needed for the rusting of iron.
The oxygen is a good oxidizing agent. The air contains oxygen which reacts with iron thereby changing its oxidation state. The iron losses electrons i.e. reduction of iron takes place. The reaction can be written as; $ Fe\to F{{e}^{2+}}+2{{e}^{-}} $
So, both the air and moisture are required for the rusting of iron. The oxygen comes from air and water comes from moisture. Rusting of iron is an irreversible reaction because chemical change takes place in this process. Heating of hydrated $ cobalt\left( II \right)~ $ chloride dehydrates it. Hence only $ X $ is reversible on heating.
Therefore, correct answer is option B i.e. X-yes; Y-no.

Note:
It must be noted that in the presence of salt, say if it is sea water, then the rusting takes place more quickly as it forms an electrochemical reaction. Several processes are devised to prevent rusting of iron. Galvanization is one technique in which the iron is covered with a layer of metallic Zinc on the upper side. The cathodic protection is also used to stop corrosion.