Question

Explain the following observations: (a) For a given oxidation state, the acid strength of the oxyacid in aqueous solution decreases in the order chlorine \(>\) bromine \(>\) iodine.(b) Hydrofluoric acid cannot be stored in glass bottles. (c) HI cannot be prepared by treating Nal with sulfuric acid. ( \(\mathbf{d}\) ) The interhalogen ICl_ is known, but BrCl_ is not.

Short answer

In summary, the acid strength of oxyacids decreases in the order chlorine > bromine > iodine due to their electronegativity. Hydrofluoric acid cannot be stored in glass bottles because it reacts with and dissolves the glass. HI cannot be efficiently prepared by treating NaI with sulfuric acid due to undesirable side reactions. The interhalogen ICl is more stable and likely to form compared to BrCl, due to the greater electronegativity difference and size discrepancy between iodine and chlorine.

Step-by-step solution

Observation (a): Acid strength of oxyacids in aqueous solution

To explain the order of the acid strength of the oxyacids in aqueous solution, we need to consider the following factors: 1. Electronegativity of the central atom 2. The number of oxygen atoms bonded to the central atom 3. The oxidation state of the central atom The acid strength of an oxyacid relates to the strength of the O-H bond. A stronger O-H bond means that it is harder to release H+ ions, making the oxyacid a weaker acid. The O-H bond strength is affected by the electronegativity and size of the central atom. When the central atom is more electronegative, it attracts electrons from the O-H bond, weakening the O-H bond, and making it easier to release the H+ ions, making the oxyacid a stronger acid. In the case of chlorine, bromine, and iodine, chlorine is the most electronegative, followed by bromine and then iodine. Therefore, for a given oxidation state, the acid strength of the oxyacid in aqueous solution decreases in the order chlorine > bromine > iodine.

Observation (b): Hydrofluoric acid cannot be stored in glass bottles

Hydrofluoric acid (HF) is a highly corrosive acid and reacts with glass, which is primarily composed of silicon dioxide (SiO2). The reaction is as follows: \[ SiO_2 + 4 HF -> SiF_4 + 2 H2O \] In this reaction, HF reacts with SiO2, forming silicon tetrafluoride (SiF4) and water. This process results in the dissolution of the glass, making it unfit for storing hydrofluoric acid.

Observation (c): Preparation of HI by treating NaI with sulfuric acid

Treating NaI with sulfuric acid to prepare hydrogen iodide (HI) can result in an undesirable side reaction. The primary reaction should be: \[ 2 NaI + H_2SO_4 -> 2 HI + Na_2SO_4 \] However, HI can be oxidized by sulfuric acid, leading to the formation of iodine (I2) and water: \[ 8 HI + H_2SO_4 -> 4 I_2 + 4 H_2O + H_2S \] Because of this side reaction, HI cannot be prepared efficiently by treating NaI with sulfuric acid.

Observation (d): Presence of interhalogen ICl and absence of BrCl

Interhalogens are formed by the reaction between elements of different electronegativity and size. In general, interhalogens with a larger electronegativity difference and size discrepancy are more likely to form and be stable. The electronegativity of Iodine (I) is lower than that of Bromine (Br), while that of Chlorine (Cl) remains constant. The greater the electronegativity difference, the greater the polarity and strength of the bond, making it more likely to form. Additionally, the size of Iodine is larger than that of Bromine, allowing it to form a stronger bond with Chlorine. Considering these factors, ICl is more stable and thus more likely to form than BrCl, which explains the presence of interhalogen ICl and the absence of BrCl.

Key concepts

Electronegativity and Acid Strength

Understanding the relationship between electronegativity and acid strength is crucial in predicting the reactivity of oxyacids. Electronegativity is the ability of an atom to attract bonding electrons to itself. When the central atom in an oxyacid is highly electronegative, it effectively pulls electron density away from the O-H bond, weakening it and facilitating the release of the H+ ion. This process is what defines the strength of an acid: the easier it is to release a proton, the stronger the acid.

For example, in the case of halogen oxyacids (like those containing chlorine, bromine, and iodine), chlorine is the most electronegative. This makes oxyacids of chlorine stronger acids than those of bromine or iodine. This concept is key to understanding why, for a given oxidation state, the acid strength decreases from chlorine to iodine.

Hydrofluoric Acid Reactivity with Glass

Hydrofluoric acid (HF) is notoriously known for its ability to etch glass, an attribute that makes storing it quite a challenge. The primary component of glass is silicon dioxide (SiO2), and HF reacts with it to form silicon tetrafluoride (SiF4) and water. This reaction undermines the structural integrity of the glass container, leading it to dissolve.

Therefore, alternative container materials such as plastic or lead must be used to store HF safely. This reactivity with glass is not just a trivial fact, but an important safety consideration in handling HF in the laboratory.

Preparation of Hydrogen Iodide

The synthesis of hydrogen iodide (HI) is an interesting example of how certain expected reactions can be thwarted by side reactions. When sodium iodide (NaI) is treated with sulfuric acid, theoretically, one would expect hydrogen iodide and sodium sulfate to form. However, in practice, HI is oxidized by sulfuric acid, leading to the production of iodine and water, which makes this method inefficient.

This side reaction can be mitigated by using a different method of preparation, like reacting iodine with hydrazine, which effectively produces hydrogen iodide without the undesired side reaction.

Interhalogen Compounds Stability

Interhalogen compounds, like ICl, are formed by halogens of different sizes and electronegativities. The stability of these compounds is directly related to the difference in electronegativity and the size of the halogens involved. Larger differences in electronegativity and complementary sizes create a stronger, more polarized bond.

ICl, which involves iodine and chlorine, is stable due to the significant electronegativity difference and the large size of iodine compared to chlorine. However, BrCl does not form under normal conditions because the size and electronegativity differences between bromine and chlorine are not favorable for a stable interhalogen compound to exist.