Question

Which one of the following carbides is most likely to be non-stoichiometric? (a) \(\mathrm{SiC}\) (b) \(\mathrm{CaC}_{2}\) (c) \(\mathrm{Fe}_{3} \mathrm{C}\) (d) None of these

Short answer

The most likely non-stoichiometric carbide is (Fe_3C).

Step-by-step solution

Understanding Carbides

Carbides are compounds composed of carbon and a less electronegative element. There are different types including ionic, covalent, and interstitial carbides. Non-stoichiometric compounds often occur due to interstitial structures, where atoms can take variable occupancy in a lattice.

Identifying Carbide Types

(a) (SiC) is a covalent carbide and typically forms in a 1:1 stoichiometry as silicon and carbon share electrons to form a strong covalent bond. (b) (CaC_2) is an ionic carbide where calcium donates electrons to carbon. It is known to form a stoichiometric compound. (c) (Fe_3C) is an interstitial carbide, also known as cementite, where iron can form variable stoichiometry with carbon due to its peculiarity in the metallic lattice.

Determining Non-Stoichiometry Potential

Stoichiometry typically varies in interstitial carbides, given the flexibility of metal atoms allowing variations in the number of interstitial carbon atoms. (Fe_3C) can exhibit such behavior due to potential excess or deficiency of carbon atoms within the metallic iron lattice.

Final Answer Selection

Among the choices, (Fe_3C) is an interstitial carbide with potential for non-stoichiometry because of its metallic structure, which can accommodate defects or variable carbon content without altering the overall iron lattice.

Key concepts

Non-stoichiometric Compounds

Non-stoichiometric compounds are intriguing and slightly elusive when it comes to chemistry. These are compounds where the proportions of the elements cannot be expressed as simple ratios of small whole numbers. This happens because of variations in the number of atoms within the crystal lattice. A classic example is interstitial carbides, where metal atoms form a lattice, and carbon atoms occupy spaces or 'interstices' between the metals. In such cases, the number of carbon atoms can vary, leading to non-stoichiometry. The ability of a compound to accommodate different numbers of atoms without changing its structure is what makes non-stoichiometric compounds unique and fascinating.

Covalent Carbide

Covalent carbides are formed when carbon bonds with elements that are close to it on the periodic table, usually non-metals or metalloids. The prime example of a covalent carbide is silicon carbide (2SiC2). In these carbides, carbon forms strong covalent bonds, sharing its electrons with the other element to form a robust structure. These bonds are quite stable and result in hard, wear-resistant materials. Because of the precise electron pairing in covalent bonds, covalent carbides such as 2SiC2 usually maintain a stoichiometric 1:1 ratio and are not prone to non-stoichiometry.

Interstitial Carbide

Interstitial carbides are formed when small carbon atoms fit into the spaces or interstices within a metallic lattice. In these structures, metals are the dominant component, often forming a robust metallic lattice that provides a 'host' for carbon atoms. This type of carbide is most prone to non-stoichiometry because the metallic lattice can accommodate a variable number of carbon atoms without significant structural change. For example, 2Fe_3C2 is known as cementite, an interstitial carbide where the presence of carbon can vary slightly. This flexibility in the iron lattice allows the compound to maintain stability even when the exact carbon content fluctuates.

Ionic Carbide

Ionic carbides occur when carbon pairs with metals that are strongly electropositive, typically forming a relationship where electrons are transferred from the metal atoms to the carbon. In this type of carbide, like 2CaC_22, the carbon acts as an electron acceptor, resulting in strong ionic bonds. These bonded structures are more rigid regarding their stoichiometry. Ionic carbides tend to be stoichiometric because the transfer of electrons occurs in a fixed ratio, usually dictated by the charge balance between the participating atoms. This means that ionic carbides do not exhibit the flexibility for variability seen in interstitial carbides, thereby resisting non-stoichiometric behavior.