Question

Anhydrous ferric chloride is prepared by:
A. dissolving $Fe{(OH)_3}$ in concentrated $HCl$ .
B. dissolving $Fe{(OH)_3}$ in dilute $HCl$ .
C. passing dry $HCl$ over heated iron scrap.
D. passing dry $Cl_2$ gas over heated iron scrap.

Answer 1

Anhydrous ferric chloride is prepared by passing dry $Cl_2$ gas over heated iron scrap. The heated iron scrap has a large surface area and thus, the chlorine gas reacts rapidly because of this surface area and due to high temperature, anhydrous $Fe{(Cl)_3}$ is formed.

Complete answer:
Now, in order to explain the answer, we need to understand each option provided in the question.
A. When $Fe{(OH)_3}$ is dissolved in concentrated $HCl$, it produces ferric chloride ($FeC{l_3}$ ) and water. Thus, anhydrous ferric chloride is not formed. The reaction is as follows:
$Fe{(OH)_3} + 3HCl \to FeC{l_3} + 3{H_2}O$
B. When $Fe{(OH)_3}$ is dissolved in dilute $HCl$, it produces hydrated ferric chloride ($FeC{l_3}$ ).
C. Whenever dry $HCl$ is passed over heated iron scrap, hydrogen gas is released. Thus, there is no formation of any anhydrous ferric chloride.
D. Anhydrous ferric chloride is prepared by passing dry $C{l_2}$ gas over heated iron scrap. The reaction involved is as follows:
$2Fe(s) + 2C{l_2}(g)\xrightarrow{\Delta }2FeC{l_3}$
Thus, from the reaction it is clearly visible that the ferric chloride formed in the reaction does not have any traces of moisture in it.

So, the correct answer is Option D.

Note:
Iron(III) chloride is used in sewage treatment and drinking water production as a coagulant and flocculant. In this application, $FeC{l_3}$ in slightly basic water reacts with the hydroxide ion to form a flock of iron(III) hydroxide. It is also used as a leaching agent in chloride hydrometallurgy.