Problem 36 - Chemistry The Central Science In Si Units

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. (d) The interhalogen \(\mathrm{ICl}_{3}\) is known, but \(\mathrm{BrCl}_{3}\) is not.

Step-by-Step Solution

Answer

(a) More electronegative halogens form stronger acids. (b) HF reacts with glass. (c) HI reduces sulfuric acid. (d) Iodine can stabilize ICl3 due to its size and ability to expand its valence shell.

Step 1: Electronegativity Explanation

For (a), the acid strength is influenced by the electronegativity of the halogen in the oxyacid. Chlorine, with the highest electronegativity, can stabilize the negative charge on the oxyanion better than bromine or iodine after deprotonation. Thus, chlorine oxyacids are the strongest, followed by bromine and iodine.

Step 2: Glass Reactivity Clarification

For (b), hydrofluoric acid (HF) cannot be stored in glass because it reacts with silicon dioxide (2encountered in glass materials) to form silicon tetrafluoride (SiF4), effectively dissolving the glass.

Step 3: Unfavorable Reaction Explanation

For (c), HI cannot be prepared by treating NaI with sulfuric acid because HI is a strong reducing agent and will reduce sulfuric acid (2forming hydrogen sulfide (H2S) or sulfur instead of the desired hydrogen iodide.

Step 4: Molecular Stability Factor

For (d), ICl3 exists because iodine can expand its valence shell and form stable trivalent compounds due to its larger atomic size and lower electronegativity. In contrast, bromine cannot do this effectively with chlorine because of less efficient electron sharing and smaller atomic sizes.

Key Concepts

Acid Strength

Acid strength refers to the ability of an acid to donate a proton (H\(^+\)) in an aqueous solution. In the context of oxyacids containing halogens, such as chlorine, bromine, and iodine, the strength of the acid is linked to how well the negative charge can be stabilized after the loss of a proton. Chlorine is more electronegative than bromine and iodine, allowing it to stabilize the resulting oxyanion more effectively. This greater stabilization translates to stronger oxyacids when chlorine is involved. Thus, chlorine-based oxyacids are more potent than those containing bromine or iodine due to this increased electronegativity, which helps with the attractive forces holding onto the electrons.

• Chlorine has the highest electronegativity, making it the strongest acid.
• Bromine follows, although it doesn't stabilize the charge as effectively as chlorine.
• Iodine is the least effective, leading to the weakest acid strength.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. In the halogen group, electronegativity decreases as we move down the group from fluorine to iodine. This property is crucial in understanding the behavior and reactions of elements like chlorine, bromine, and iodine in compounds. High electronegativity means an atom can better stabilize negative charges, influencing properties such as acid strength.
In oxyacids, like those of chlorine, bromine, and iodine, the electronegativity of the central halogen atom determines how well it can accept and stabilize extra electron density after a proton is released. A higher electronegativity signifies stronger bond formation with oxygen in these acids, leading to better stability and, consequently, stronger acidic behavior.

• Chlorine's higher electronegativity translates to stronger acids.
• Bromine has a moderate electronegativity.
• Iodine, with the lowest electronegativity among the three, forms weaker acids.

Interhalogen Compounds

Interhalogen compounds are formed from two different halogens. These compounds generally have higher reactivity than diatomic halogens due to the difference in electronegativity between the two elements. Iodine trichloride (ICl₃) is a classic example of an interhalogen compound. It exists because iodine's larger atomic size allows it more flexibility in forming multiple bonds, leading to the stability of trivalent compounds.
In contrast, bromine and chlorine do not form a stable BrCl₃ compound. This is mainly due to the smaller atomic sizes and lesser ability to expand their valence shells like iodine, which restricts their ability to form stable trivalent structures. Therefore, iodine's ability to accommodate more electron pairs results in it successfully forming compounds with multiple bonds compared to bromine.

• ICl₃ is a stable interhalogen compound.
• BrCl₃ is not stable due to structural limitations.

Halogens

Halogens are a group of elements in the periodic table that include fluorine, chlorine, bromine, and iodine. They are known for their high reactivity, especially with alkali metals and certain nonmetals, to form salts. This reactivity is primarily due to their desire to gain an electron to complete their outer electron shell.
Each halogen has distinctive properties related to its size, electronegativity, and ability to form different types of compounds. For instance, fluorine, the smallest and most electronegative halogen, is extremely reactive and can etch glass by forming bonds with silicon. Iodine, with a much larger atomic size, is less electronegative than fluorine or chlorine but can form a variety of compounds, including interhalogen compounds, due to its ability to expand its valence shell easily.

• Fluorine is the most reactive and electronegative.
• Iodine can form complex compounds because of its size.
• Chlorine and bromine have intermediate properties.