Animated Solution for Chemistry - d and f-Block Elements: Iron exhibits +2 and +3 oxidation states. Which of the following statements about iron is incorrect?

Ferrous vs. Ferric

When iron enters the chemical arena, it primarily does so in two distinct avatars: the $+2$ oxidation state (Ferrous, $F{e}^{2+}$) and the $+3$ oxidation state (Ferric, $F{e}^{3+}$). While they are born from the same parent atom, the loss of that one extra electron in $F{e}^{3+}$ drastically alters its chemical personality. To understand why, we need to unlock the secrets of Fajans' Rule.

The Master Key

Fajans' Rule is a beautiful concept that bridges the gap between purely ionic and purely covalent bonds. It states that no bond is $100\%$ ionic. Every cation has a tendency to distort the electron cloud of a neighboring anion, pulling it towards itself. This phenomenon is called polarization.

The polarizing power of a cation depends on two critical factors: its charge and its size. A higher positive charge acts like a stronger magnet, while a smaller size allows the cation to get closer to the anion's electron cloud. Therefore, Polarizing Power $\propto \frac{\text{Charge}}{\text{Size}}$.

When we compare $F{e}^{2+}$ and $F{e}^{3+}$, the Ferric ion ($F{e}^{3+}$) has a higher charge ($+3>+2$) and a smaller ionic radius. Consequently, $F{e}^{3+}$ possesses a significantly higher polarizing power. It distorts the anion's electron cloud much more effectively, leading to a greater sharing of electrons. Thus, $F{e}^{3+}$ compounds are predominantly covalent, while $F{e}^{2+}$ compounds are predominantly ionic.

Basicity, Volatility, and Hydrolysis

Now that we have established the ionic/covalent nature of these compounds, let's evaluate the given statements one by one.

1. Basic Nature of Oxides: The basicity of a metal oxide is directly linked to its ionic character. An ionic oxide easily dissociates in water to release oxide ions ($O^{2-}$), which then form hydroxide ions ($O{H}^{-}$), making the solution basic. Since $F{e}^{2+}$ forms more ionic compounds, Ferrous oxide ($FeO$) is more basic than Ferric oxide ($F{e}_2{O}_3$). Statement (a) is correct.

2. Ionic Character: As derived directly from Fajans' Rule, the lower oxidation state ($+2$) results in less polarization. Therefore, Ferrous compounds are relatively more ionic than Ferric compounds. Statement (b) is correct.

3. Volatility: Volatility refers to how easily a substance vaporizes. Ionic compounds are held together by strong, extensive electrostatic forces in a rigid crystal lattice, making them highly non-volatile. Covalent compounds, however, exist as discrete molecules held together by weaker intermolecular forces (like Van der Waals forces), making them relatively more volatile. Since Ferrous compounds are more ionic, they are less volatile than the corresponding Ferric compounds. Statement (c) is correct.

4. Hydrolysis: Hydrolysis is the chemical breakdown of a compound due to reaction with water. Covalent metal halides (like $FeC{l}_3$) undergo hydrolysis much more readily than ionic halides (like $FeC{l}_2$). The high charge density on the $F{e}^{3+}$ ion strongly attracts the oxygen atom of water molecules, facilitating the breaking of $O-H$ bonds and the formation of $Fe-OH$ bonds. Therefore, Ferric compounds are more easily hydrolysed.

Statement (d) claims that "Ferrous compounds are more easily hydrolysed than the corresponding ferric compounds." Based on our analysis of Fajans' Rule and the nature of hydrolysis, this is the exact opposite of the truth. Ferric compounds are the ones that hydrolyse more easily.

Thus, statement (d) is the incorrect statement, making it the right answer to our question.