Iron(II) chloride, also known as ferrous chloride (\(\text{FeCl}_2\)), is a light green to off-white crystalline solid in its anhydrous form. \(\text{FeCl}_2\) is highly soluble, meaning it readily dissolves in water. At \(20^\circ \text{C}\), approximately \(68.5\) grams of \(\text{FeCl}_2\) can dissolve in \(100\) milliliters of water, and this solubility increases as the water temperature rises.
The Chemistry of Dissolution
Ferrous chloride is an ionic compound composed of the positively charged iron(II) cation (\(\text{Fe}^{2+}\)) and two negatively charged chloride anions (\(\text{Cl}^{-}\)). To dissolve, the attractive forces holding the ions in the solid crystal lattice must be overcome.
Water molecules are highly polar, with partial negative charges near oxygen and partial positive charges near hydrogen. When the salt is introduced, these polar water molecules surround the ions on the crystal surface. The negative ends of the water molecules are attracted to the \(\text{Fe}^{2+}\) cations, and the positive ends are attracted to the \(\text{Cl}^{-}\) anions.
This attraction releases hydration energy. For dissolution to occur, the hydration energy must be greater than the lattice energy holding the solid structure together. Since ferrous chloride is highly soluble, the solid dissociates completely into its aqueous ions: \(\text{FeCl}_{2}(\text{s}) \rightarrow \text{Fe}^{2+}(\text{aq}) + 2\text{Cl}^{-}(\text{aq})\). The \(\text{Fe}^{2+}\) ions become surrounded by a hydration shell of water molecules in solution.
Properties of the Aqueous Solution
A freshly prepared, dilute solution of ferrous chloride typically displays a pale green color. This coloration is characteristic of the hydrated iron(II) ion (\(\text{Fe}^{2+}\)) in water.
The \(\text{pH}\) of the solution is generally slightly acidic, usually around \(3.5\) to \(4.0\). This acidity arises because the \(\text{Fe}^{2+}\) ion undergoes a minor reaction with water, known as hydrolysis. The \(\text{Fe}^{2+}\) ion acts as a weak acid, donating a proton (\(\text{H}^{+}\)) to the solvent, which lowers the solution’s \(\text{pH}\).
The aqueous solution lacks stability when exposed to air. The dissolved iron(II) is highly susceptible to oxidation by dissolved oxygen. This reaction converts the iron(II) cation (\(\text{Fe}^{2+}\)) to the iron(III) cation (\(\text{Fe}^{3+}\)), forming iron(III) chloride (\(\text{FeCl}_3\)). This oxidation causes the solution’s color to change from pale green to a yellowish-brown. To maintain integrity, the solution must be stored away from air or under an inert atmosphere.
Practical Applications and Uses
Ferrous chloride is widely used in industrial and environmental applications. One of its main roles is as a coagulant and flocculant in municipal and industrial wastewater treatment. When added to wastewater, the dissolved \(\text{Fe}^{2+}\) ions react with and clump together suspended solids and fine particles, a process called flocculation.
This process is effective for removing pollutants such as phosphates and sulfides from water. Ferrous chloride is also utilized for odor control in wastewater systems by chemically reacting with and reducing hydrogen sulfide gas.
Beyond water treatment, ferrous chloride serves as a precursor in the synthesis of other iron compounds, including iron oxide pigments used in paints and coatings. Its reducing properties also make it useful in organic synthesis and in the dyeing industry, where it acts as a mordant to help fix dyes to fabrics.