Question

Where is $FeC{{l}_{3}}$ used? Is it a Lewis acid or base?

Answer 1

$FeC{{l}_{3}}$ is an inorganic compound which is chemically known as ferric chloride or iron (III) oxide. It is a crystalline anhydrous solid in which iron exists in its $+3$ oxidation state. It is non-combustible and is widely used in purification of water and sewage treatment.

Complete answer:
Although there are many uses of $FeC{{l}_{3}}$ like it is used in organic synthesis as a catalyst, as drying reagent in some reactions due to its anhydrous form, widely used in energy storage system but it is majorly used in chemistry labs for ferric chloride test which is conducted to test the presence of phenol in an unknown sample compound. The ferric chloride test involves following steps:
Step-1: The unknown sample is first dissolved in aqueous solution of ethanol.
Step-2: Few drops of dilute solution of ferric chloride are added.
Step-3: The presence of phenol in the given sample is indicated when it turns to red, green, purple or blue colour after adding ferric chloride.
Now, in $FeC{{l}_{3}}$ , iron exists in its $+3$ oxidation state which is the highest possible oxidation state for iron which means it has no further tendency to donate electrons to form compounds with higher oxidation state. But it has a tendency to accept electrons in order to change from its $+3$ oxidation state to its $+2$ oxidation state.
Thus, we can conclude that $FeC{{l}_{3}}$ is used in ferric chloride tests for phenols and is a Lewis acid.

Note:
Remember that a Lewis acid is a species which has a tendency to accept electrons and act as electrophiles whereas a Lewis base is a species which has a tendency to donate a pair of electrons and acts as a nucleophile. Generally, molecules with positive charge are considered as the Lewis acids and the molecules with negative charge or lone pair of electrons are considered as the Lewis bases.