Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are correct? (a) HF is a stronger acid than \(\mathrm{HCl}\). (b) among halide ions, iodide is the most powerful reducing agent. (c) fluorine is the only halogen that does not show a variable oxidation state. (d) \(\mathrm{HOCl}\) is a stronger acid than HOBr. (a) 2 and 4 (b) 2 and 3 (c) 1,2 and 3 (d) 2,3 and 4

Step-by-Step Solution

Answer

(d) 2, 3 and 4

Step 1: Evaluating Statement (a)

Examine the acidity of HF and HCl. HF is a weaker acid compared to HCl because HF forms a strong bond due to its high electronegativity, making it less likely to dissociate in water. Therefore, statement (a) is incorrect.

Step 2: Evaluating Statement (b)

Among the halide ions, iodide (I⁻) is the largest and can lose an electron more easily than the others (fluoride, chloride, bromide), making it the strongest reducing agent. Therefore, statement (b) is correct.

Step 3: Evaluating Statement (c)

Fluorine is the most electronegative element and does not show variable oxidation states; it is always -1. Other halogens can show multiple oxidation states. Therefore, statement (c) is correct.

Step 4: Evaluating Statement (d)

Compare the acidity of HOCl and HOBr. Acidity is influenced by the electronegativity of the halogen atom, with more electronegative atoms leading to stronger acids. Chlorine is more electronegative than bromine, so HOCl is a stronger acid than HOBr. Therefore, statement (d) is correct.

Step 5: Determine Correct Combination

Based on the evaluation, the correct statements are 2, 3, and 4. This corresponds to option (d).

Key Concepts

Acid Strength Comparison

When comparing the strength of acids like HF and HCl, it's crucial to understand how the electronegativity and bond strength play a role. HF, or hydrofluoric acid, showcases a strong bond between hydrogen and fluorine due to the high electronegativity of fluorine. This strong bond results in lesser dissociation in water, making HF a relatively weak acid. In contrast, hydrochloric acid (HCl) dissociates more readily in water, as the bond between hydrogen and chlorine is not as strong due to chlorine's lower electronegativity compared to fluorine. This characteristic means HCl is a stronger acid than HF. When comparing acid strengths, remember:

• Electronegativity of atoms affects bond strength.
• Stronger bonds lead to less dissociation, thus weaker acids.
• Acids that dissociate easily are considered stronger.

Understanding these principles helps explain why HF is less acidic than HCl.

Halogen Properties

Halogens are a group of elements found in Group 17 of the periodic table, and they exhibit unique properties due to their high reactivity and electronegativity. These elements include fluorine, chlorine, bromine, iodine, and astatine. The reactivity of halogens decreases down the group:

• Fluorine is the most reactive owing to its high electronegativity and small atomic size.
• Chlorine and bromine follow, exhibiting moderate reactivity.
• Iodine is less reactive compared to the aforementioned elements.

Another interesting property of halogens is their ability to form diatomic molecules, such as F₂, Cl₂, Br₂, and I₂. They tend to form negative ions, known as halides (e.g., F⁻, Cl⁻), by gaining an electron to achieve a stable electron configuration. Each halogen's unique properties influence its chemical behavior and interactions with other substances.

Oxidation States of Halogens

Halogens can exhibit various oxidation states in their compounds, except for fluorine, which always exhibits an oxidation state of -1 due to its high electronegativity. However, other halogens, such as chlorine, bromine, and iodine, display variable oxidation states. For example:

• Chlorine can exhibit oxidation states ranging from -1 to +7 in different compounds, such as in \(\text{Cl}^-\) for chloride or \(+7\) in perchlorates like \(\text{ClO}_4^-\).
• Bromine also shows multiple oxidation states, such as -1 in bromides and +5 in bromates like \(\text{BrO}_3^-\).
• Iodine can range from -1 in iodides to +7 in periodates like \(\text{IO}_4^-\).

These variable oxidation states are due to the availability of d-orbitals in heavier halogens, allowing them to participate in various types of bonding and chemical reactions.

Reducing Agents in Halides

In the world of chemistry, halides like iodide (I⁻) act as reducing agents, playing an important role in chemical reactions. A reducing agent donates electrons to other substances, which means it itself gets oxidized. Among the halides, iodide is considered the most powerful reducing agent. This is because its larger atomic size allows it to lose electrons more easily than smaller halides like fluoride or chloride. As a result:

• Iodide can effectively reduce other substances by donating electrons.
• The tendency to lose electrons makes iodide a strong reducing agent.
• The efficiency improves due to the increase in atomic size down the group.

This ability of iodide to act as a robust reducing agent is leveraged in many chemical reactions, highlighting the importance of halide ion properties in inorganic chemistry.