Problem 36 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Statement-1 : Between \(\mathrm{SiCl}_{4}\) and \(\mathrm{CCl}_{4}\), only \(\mathrm{SiCl}_{4}\) reacts with water. Statement- \(\mathbf{2}: \mathrm{SiCl}_{4}\) is ionic and \(\mathrm{CCl}_{4}\) is covalent.

Step-by-Step Solution

Answer

Statement 1 is true, but Statement 2 is not a correct explanation for it.

Step 1: Understand the Reactivity with Water

The exercise mentions that SiCl₄ reacts with water while CCl₄ does not. This difference in reactivity with water stems from the molecular structure and bonding nature of these compounds. SiCl₄ can undergo hydrolysis because silicon can expand its coordination number beyond 4, forming Si(OH)₄ and releasing HCl, while carbon in CCl₄ cannot.

Step 2: Analyze Statement 2

Statement 2 suggests that the difference in behavior is due to ionic character in SiCl₄ and covalent character in CCl₄. However, both SiCl₄ and CCl₄ are molecular covalent compounds, not ionic. The reactivity difference is not due to ionic character but due to differences in molecular geometry and bonding.

Step 3: Evaluate the Validity of the Reason

The true reason for SiCl₄'s reactivity with water is the ability of Si to expand its octet and form a stable product through hydrolysis, rather than any ionic character. CCl₄ does not react with water due to the lack of vacant d orbitals and the strong covalent nature of the C-Cl bond.

Key Concepts

Hydrolysis of SiCl4

The process of hydrolysis involves a chemical reaction with water. In the case of silicon tetrachloride, SiCl₄, when it comes into contact with water, it reacts to form silanols, specifically Si(OH)₄, along with the release of hydrochloric acid (HCl) as a byproduct. This reaction occurs because silicon can expand its coordination number. Unlike carbon in carbon tetrachloride (CCl₄), which holds onto its tetrahedral shape, silicon in SiCl₄ can extend beyond the typical four bonds, allowing it to form bonds with additional oxygen atoms from the water molecules. This expanded coordination ability promotes the hydrolysis of SiCl₄ whereas CCl₄ does not undergo this process. The end-product of SiCl₄ hydrolysis demonstrates why silicon compounds often behave differently from their carbon analogs, highlighting the unique behavior of semimetals in chemistry.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule, which can significantly influence a compound's reactivity and properties. In the case of SiCl₄, it has a tetrahedral geometry similar to CCl₄. However, the presence of d orbitals in silicon allows for an expansion beyond the stable geometry in special reactions. In the geometry of SiCl₄, silicon is the central atom bonded to four chlorine atoms, arranged symmetrically around it forming a tetrahedral structure. This geometric arrangement is typical of sp³ hybridization, where the central atom forms four single covalent bonds that extend symmetrically in space. While both SiCl₄ and CCl₄ share this tetrahedral geometry, the molecular details differ. Silicon's ability to form additional bonds due to empty d orbitals allows it to undergo reactions like hydrolysis, something which CCl₄'s strict geometry without accessible d orbitals does not allow.

Covalent Bonding vs Ionic Character

Covalent and ionic bonds represent the two major types of chemical bonding between atoms, with differences in electron sharing and transfer. Silicon tetrachloride (SiCl₄) is primarily a covalent compound. It consists of strong covalent bonds formed by sharing electrons between the silicon and each of the four chlorine atoms. Covalent bonds typically occur between nonmetal atoms, involving different degrees of sharing and no significant transfer of electrons. When we consider ionic character, we're discussing how much a bond behaves like a true ion transfer rather than equal sharing. In this scenario, neither SiCl₄ nor CCl₄ exhibits significant ionic character; rather, both are firmly covalent. Therefore, the reactivity seen in SiCl₄ in reactions like hydrolysis is not attributed to ionic behavior but rather to the unique ability Si has to bond with oxygen in water, leveraging its covalent characteristics and structural flexibility, unlike the more rigid covalent bonds in CCl₄.