Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is not true? (a) HOCl is a stronger acid than HOBr (b) among halide ions, iodide is the most powerful reducing agent (c) HF is stronger acid than \(\mathrm{Cl}\) (d) fluorine is the only halogen that does not show a variable oxidation state

Step-by-Step Solution

Answer

Statement (c) is not true.

Step 1: Identifying Acids and Bases

Analyze statement (a): HOCl (Hypochlorous acid) and HOBr (Hypobromous acid) are compared. Generally, acids become stronger due to an increase in electronegativity. Chlorine (Cl) is more electronegative than bromine (Br), making HOCl a stronger acid than HOBr. Thus, statement (a) is true.

Step 2: Evaluating Reducing Potential

For statement (b): Halide ions are compared. Iodide (I-) is larger than other halides and holds electrons less tightly, making it the best at giving electrons away, hence, a powerful reducing agent. Thus, statement (b) is true.

Step 3: Comparing Acid Strength

Investigate statement (c): HF (Hydrofluoric acid) is compared with hydrochloric acid (HCl). HF is actually weaker than HCl because HF forms a strong bond that is harder to dissociate, whereas HCl dissociates more readily. Therefore, statement (c) is not true.

Step 4: Understanding Oxidation States

For statement (d): Fluorine is compared with other halogens in terms of oxidation states. It is the most electronegative element and can only exhibit an oxidation state of -1, unlike other halogens that show variable states. Thus, statement (d) is true.

Key Concepts

Acid Strength

Acid strength refers to the tendency of an acid to lose its proton (H⁺) in an aqueous solution. It is influenced by two main factors: the bond strength between the hydrogen and its associated atom, and the stability of the resulting anion once the proton has been removed. Here, we can examine the strength of acids like HF and HCl. Although fluorine is very electronegative, which would typically increase the acid strength, the HF bond is very strong and difficult to break. This makes HF a weak acid compared to HCl, which has a weaker bond and dissociates more easily.
When comparing acids like HOCl and HOBr, the situation varies because chlorine is more electronegative than bromine. This results in HOCl having a stronger pull on the electron pair, which makes the release of the H⁺ ion more favorable, thus increasing its acid strength over HOBr. Remember:

• Higher electronegativity usually leads to stronger acids.
• Stronger bonds mean weaker acids, due to difficulty in releasing H⁺.
• The stability of the generated anion also influences acid strength.

Halides

Halides are ions of halogens, which include fluorine, chlorine, bromine, and iodine. These halide ions possess different abilities to act as reducing agents. A reducing agent is a substance that donates electrons during a chemical reaction.
Iodide is the largest of the halide ions and has its valence electrons further from the nucleus. This results in iodide ions holding onto their outermost electrons less tightly. Consequently, iodide is a more powerful reducing agent than its smaller halide counterparts, as it readily loses electrons to other species. In summary:

• Iodide is the most effective reducing agent among halides.
• Larger ion size equates to weaker electron hold, facilitating electron donation.

Oxidation States

Oxidation states are indicative of the number of electrons an atom gains or loses during a chemical reaction. Distinctively, fluorine is quite unique among halogens because it only displays one oxidation state of -1. This occurs because fluorine, being the most electronegative element, has a strong affinity for electrons.
Unlike other halogens which can show variable oxidation states due to their ability to form compounds with different elements, fluorine’s extreme electronegativity limits its oxidation state variability. Consequently, this characteristic influences its chemical behavior significantly.

• Fluorine's oxidation state is always -1.
• Extreme electronegativity limits variability in oxidation states.
• Other halogens can exhibit multiple oxidation states.

Electronegativity

Electronegativity measures an atom's tendency to attract and hold onto electrons in a chemical bond. It's a crucial concept for understanding different chemical behaviors and properties. Generally, electronegativity increases across a period (from left to right in the periodic table) and decreases down a group (top to bottom).
Fluorine stands out as the most electronegative element, making it a strong player in chemical reactions. This pronounced electronegativity explains why fluorine can’t have variable oxidation states and why it forms strong bonds, as seen in HF.

• Electronegativity increases across periods and decreases down groups.
• It impacts bond strength and chemical reactivity.
• Fluorine is the most electronegative element and profoundly influences chemical reactions.