Problem 176 - A Complete Resource Book in Chemistry for JEE Main

Question

Iron exhibits \(+2\) and \(+3\) oxidation states. Which of the following statements about iron is incorrect? [2012] (a) Ferrous compounds are less volatile than the corresponding ferric compounds (b) Ferrous compounds are relatively more ionic than the corresponding ferric compounds. (c) Ferrous oxide is more basic in nature than the ferric oxide (d) Ferrous compounds are more easily hydrolysed than the corresponding ferric compounds

Step-by-Step Solution

Answer

Statement (a) is incorrect.

Step 1: Understanding Oxidation States

Iron (Fe) can have two common oxidation states: +2 (ferrous) and +3 (ferric). These different oxidation states affect the properties of the compounds formed by iron.

Step 2: Analyzing the First Statement

Statement (a) claims that ferrous compounds are less volatile than ferric compounds. Typically, higher oxidation state compounds (ferric) are more covalent and more volatile compared to lower oxidation state compounds (ferrous), which are more ionic and less volatile. Thus, this statement is generally considered incorrect.

Step 3: Evaluating the Second Statement

Statement (b) states that ferrous compounds are more ionic compared to ferric compounds. As lower oxidation states usually form more ionic compounds, this statement is true as ferrous (+2) state is more ionic than ferric (+3) state.

Step 4: Examining the Third Statement

Statement (c) suggests that ferrous oxide is more basic than ferric oxide. This is accurate because ionic oxides (lower oxidation states) tend to be more basic, while covalent oxides (higher oxidation states) are more acidic.

Step 5: Considering the Fourth Statement

Statement (d) asserts that ferrous compounds are more easily hydrolyzed than ferric compounds. However, ferric compounds are generally more covalent and prone to hydrolysis due to the higher positive charge attracting water molecules, making this statement incorrect.

Step 6: Conclusion: Identify the Incorrect Statement

Based on the analysis, statements (a) and (d) seem inconsistent with typical chemical behavior. Between these, (a) is less likely correct due to how oxidation states affect volatility. Hence statement (a) is incorrect.

Key Concepts

Ferrous vs Ferric Compounds

When discussing ferrous and ferric compounds, it is crucial to understand their chemical differences arising from the oxidation states of iron. Ferrous compounds derive from iron in the +2 oxidation state, while ferric compounds contain iron in the +3 oxidation state. These states significantly affect their properties.

• **Ferrous Compounds (+2):** - Generally more ionic. - Exhibit less covalency due to the lower oxidation state. - Higher electron availability leads to reduced molecular stability.
• **Ferric Compounds (+3):** - Tends to be more covalent. - This higher covalency results from the increased positive charge, drawing electrons more tightly. - Exhibits greater molecular stability.

Understanding these characteristics helps in determining the behavior of compounds in various chemical reactions and processes.

Volatility in Chemistry

Volatility in chemistry refers to how readily a substance vaporizes. This property is closely linked to the molecular structure and the nature of chemical bonds. In the context of ferrous vs ferric compounds, their volatility is influenced by their oxidation states. Compounds with higher oxidation states, like ferric compounds, generally display greater volatility. This is because:

• More covalency means weaker lattice structures making them easier to vaporize.
• The +3 oxidation state leads to enhanced electron sharing, weakening inter-molecular forces compared to +2.

This is why ferric compounds are typically more volatile than their ferrous counterparts, which tend to have stronger ionic bonds, making them less volatile.

Ionic and Covalent Bonds

Chemical bonds form due to the interaction of electrons between atoms. These bonds are primarily ionic or covalent.

Ionic Bonds

Ionic bonds occur when there is a complete transfer of electrons from one atom to another, creating ions. They are usually seen in compounds involving metals and nonmetals, with one providing electrons and the other accepting.

• Characterized by high melting and boiling points.
• Typically soluble in water due to the polar nature of water aiding dissolution.
• Example: Sodium chloride (NaCl).

Covalent Bonds

Covalent bonds form when two atoms share electrons. This type of bond is more prevalent in compounds made entirely of nonmetals.

• Lower melting and boiling points compared to ionic compounds.
• Solubility mainly in organic solvents rather than water.
• Example: Water (H₂O).

Understanding the nature of these bonds assists in predicting compound properties like solubility, volatility, and reactivity.

Acidic and Basic Nature of Oxides

Oxides, compounds formed by the reaction of oxygen with another element, can be acidic or basic based on their chemical composition and the oxidation state of the central element.

Basic Oxides

Basic oxides occur when metals react with oxygen. These oxides react with acids to form salt and water and display basic properties.

• Common in metals with low oxidation states (like ferrous oxides).
• Examples include Na₂O and CaO, which neutralize acids to form salts.

Acidic Oxides

Acidic oxides arise chiefly from nonmetals or metals in high oxidation states. These react with bases to form salts and water.

• Typically associated with high oxidation states (like ferric oxides).
• Examples include CO₂ and SO₃, capable of forming acids upon reaction with water.

The taxonomy of oxides is vital for predicting how they might react in different chemical environments, playing a key role in industrial processes and environmental chemistry.