Question

Hydrated cobalt (II) chloride decomposes on heating. The equation for the reaction is
$CoC{l_2}.6{H_2}O \to CoC{l_2} + 6{H_2}O$
The reaction is reversed by adding water.
Which row describes the color change and the type of reaction for the reverse reaction?
A. Colour change- blue to pink, type of reaction- endothermic
B. Colour change – blue to pink, type of reaction – exothermic
C. Colour change – pink to blue, type of reaction – endothermic
D. Colour change – pink to blue, type of reaction - exothermic

Answer 1

Hint: The cobalt chloride is a chemical compound with formula $CoC{l_2}$ which is blue in color. The cobalt chloride changes its color in presence of water. The cobalt chloride hexahydrate is pink in color.

Complete step by step answer: Cobalt chloride is a chemical compound which changes its colour in presence of humidity. As the humidity increases in the cobalt chloride, the color changes from blue to pink.
Due to the increase in humidity, the water is absorbed by cobalt chloride. The crystal structure of cobalt chloride rearrange itself so that the water molecule can enter.
When the water molecule enters, the two water molecules surround the cobalt atom which forms a dihydrate. The cobalt chloride dihydrate is purple in color.
The hydration reaction is shown below.
$CoC{l_2} + 2{H_2}O \to CoC{l_2}.2{H_2}O$
The cobalt chloride is blue in color and cobalt chloride dihydrate is purple in color.
On further increase in the humidity, the structure of crystal again changes and four water molecules are added which form cobalt chloride hexahydrate.
$CoC{l_2}.2{H_2}O + 4{H_2}O \to CoC{l_2}.6{H_2}O$
The cobalt chloride dihydrate is purple in color and cobalt chloride hexahydrate is pink in color.
The cobalt chloride hexahydrate decomposes on heating to form cobalt chloride and water.
The reaction is shown below.
$CoC{l_2}.6{H_2}O \to CoC{l_2} + 6{H_2}O$
Cobalt chloride hexahydrate is pink in color and cobalt chloride is blue in color.
When the reaction is reversed by adding water, the blue solution of cobalt chloride changes to pink color and heat is evolved, therefore the reaction is exothermic.
So, the correct answer is “Option B”.

Note:
The property of cobalt chloride to change its color in presence of humidity helps to make cobalt chloride a humidity indicator which is applied in weather instruments.