Question

An oxide \(X\) in its normal form is almost non-reactive due to very high \(X\) - O bond enthalpy. It resists the attack by halogens, hydrogen and most of acids and metals even at elevated temperatures. It is only attacked by HF and \(\mathrm{NaOH}\). The oxide \(X\) is (a) \(\mathrm{SiO}_{2}\) (b) \(\mathrm{CO}_{2}\) (d) \(\mathrm{PbO}_{2}\) (c) \(\mathrm{SnO}_{2}\)

Short answer

The oxide X is (a) \(\mathrm{SiO}_{2}\).

Step-by-step solution

- Evaluate Reactivity and Chemical Inertness

Consider the given properties of the oxide X: high bond enthalpy between X and oxygen (O), resistance to reaction with halogens, hydrogen, acids, and metals at high temperatures, and reactivity with only hydrofluoric acid (HF) and sodium hydroxide (NaOH).

- Analyze the Given Options

Analyze the chemical behavior of the given oxides with respect to reactivity and resistance to various chemicals. While CO2 is a gas and quite reactive especially in aqueous solutions (forming carbonic acid), PbO2 and SnO2 are stable but not as resistant to chemical attacks as the properties described. SiO2, on the other hand, is known for its high bond enthalpy and inertness, only reacting significantly with HF and strong bases like NaOH.

- Determine the Correct Option

Given the description of X's properties, SiO2 matches perfectly because it is well known for its exceptional chemical inertness and high bond enthalpy with oxygen, making it highly resistant to reactions with various substances except for HF and NaOH.

Key concepts

SiO2 Properties

Silicon dioxide, or SiO2, is a chemical compound commonly found in nature as quartz, and it is renowned for its remarkable durability. SiO2 exhibits high thermal stability and can withstand temperatures up to approximately 1600°C. This impressive thermal threshold highlights its utility in high-temperature applications, such as furnace construction.

Furthermore, SiO2 is insoluble in water and doesn't conduct electricity, making it an excellent electrical insulator useful in electronics. A prominent trait of SiO2 is its exceptional hardness, which is crucial in manufacturing abrasive and cutting tools.

Regarding its susceptibility to chemical reactions, SiO2's chemical inertness is exceptional. It is generally unreactive with many acids, such as sulfuric acid (H2SO4) and hydrochloric acid (HCl), under normal conditions. The strong silicon-oxygen (Si-O) bonds contribute to its resistance against most reagents, except for a few like hydrofluoric acid (HF) and strong bases like sodium hydroxide (NaOH).

Bond Enthalpy

Bond enthalpy, often referred to as bond dissociation energy, is a fundamental concept in chemistry that quantifies the strength of a chemical bond. It represents the energy required to break one mole of bonds in a gaseous substance and is usually expressed in kilojoules per mole (kJ/mol).

High bond enthalpy indicates strong chemical bonds within a compound. The Si-O bond in SiO2, for instance, is particularly strong, with a high bond enthalpy due to the semi-covalent and partially ionic character, which requires a significant amount of energy to be disrupted. This high bond enthalpy is a direct contributor to the chemical inertness of SiO2 since reactions typically only occur if the added energy overcomes these strong bonds.

Understanding bond enthalpy is essential when predicting the chemical reactivity and stability of molecules. Strong bonds with high enthalpy values suggest that the molecule will be less reactive and more resistant to breaking apart under standard conditions.

Chemical Reactivity of Oxides

Oxides are compounds composed of an element or a compound bonded with oxygen atoms. The chemical reactivity of oxides can vary significantly based on their nature and the types of bonds present. Typically, metallic oxides tend to be more basic and reactive, while non-metallic oxides are more acidic and less reactive.

Oxides can be categorized based on their acid-base characteristics. Basic oxides, usually from metals, react with acids to form salts and water, while acidic oxides, typically derived from non-metals, react with bases to also form salts and water. On the other hand, amphoteric oxides can react with both acids and bases. Examples include aluminum oxide (Al2O3) and zinc oxide (ZnO).

The reactivity of oxides is also influenced by their bond enthalpy; higher bond enthalpy usually denotes less reactivity. Thus, SiO2's low chemical reactivity is directly attributed to its high bond enthalpy. In contrast, oxides with lower bond enthalpies can be more susceptible to chemical reactions because their bonds can be broken more readily, leading to different behavioral patterns when interacting with other substances.