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 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

The correct answer is (d) 2, 3, and 4.

Step 1: Analyze Statement (a)

Compare the strength of HF and HCl. Hydrochloric acid (HCl) is a strong acid, and it ionizes completely in water. Hydrofluoric acid (HF) is a weak acid and does not ionize completely. Hence, HCl is a stronger acid than HF. So, statement (a) is incorrect.

Step 2: Analyze Statement (b)

Determine the reducing power of halide ions. Among halide ions, iodide (3I^-3) is the most powerful reducing agent due to its large size, lower electronegativity, and ability to easily donate electrons. Therefore, statement (b) is correct.

Step 3: Analyze Statement (c)

Check the oxidation states of halogens. Fluorine (3F3) has only one oxidation state of 3-13, as it is the most electronegative element. Other halogens like chlorine, bromine, and iodine can exhibit variable oxidation states. Thus, statement (c) is correct.

Step 4: Analyze Statement (d)

Determine the acid strength between 3HOCl3 and 3HOBr3. The strength of oxyacids increases with the electronegativity of the halogen. Since chlorine is more electronegative than bromine, 3HOCl3 is a stronger acid than 3HOBr3. Therefore, statement (d) is correct.

Step 5: Compare Answer Choices

Based on the analysis, the correct statements are 2, 3, and 4. Therefore, option (d) is the correct answer.

Key Concepts

Acid Strength

Acid strength is a measure of an acid's ability to donate a proton to a base. In simple terms, it tells us how likely an acid is to give away its hydrogen ion. It's important to remember:

• Strong acids, like Hydrochloric acid (HCl), completely ionize (break apart to form ions) in water.
• Weak acids, like Hydrofluoric acid (HF), do not fully ionize in water.

For a practical understanding, think of HCl as being very confident and always completing the action of ionizing, whereas HF is quite hesitant and doesn't fully go through with it.

This difference comes down to the bond strength of the H-X bond (where X is the halogen). In the case of HCl, the bond is weaker, allowing it to ionize completely, making it a stronger acid than HF. Thus, chemically speaking, a stronger acid makes a more significant contribution to conducting electricity in solution.

Oxidation States

An oxidation state is a number assigned to an element in a chemical compound, reflecting the number of electrons lost or gained by an atom of that element. It helps to understand how electrons are distributed in an atom when forming a compound.

Here's what to remember about halogens:

• Fluorine always has an oxidation state of \(-1\).
• Other halogens like chlorine, bromine, and iodine can have multiple oxidation states, going positive or negative depending on the compound they're in (such as \(+1, +3, +5,\ ext{etc.}\).

Why only \(-1\) for fluorine? It's because fluorine is the most electronegative element on the periodic table and is only happy grabbing electrons, not sharing or losing them. Understanding oxidation states is crucial in predicting reactions and understanding electron transfer in compounds.

Fluorine plays it safe, never letting go of the cozy \(-1\) state, while the others enjoy a bit more flexibility in electronegativity adventures.

Reducing Agents

Reducing agents are substances that donate electrons to another—essentially acting as electron givers or electron philanthropists! Understanding reducing agents is all about electron donation: a key concept in many chemical reactions.

When looking at halide ions:

• Iodide (I⁻) ions are particularly interesting. They are great reducing agents due to their size and ability to donate electrons easily.
• Larger ions have electrons that are further from the nucleus, making them easier to give away.

Think of the iodide ion as having a big bag of donations it's eager to hand out, simply because it's easier to manage than a tightly packed smaller ion like fluoride. The larger the ion, the weaker its hold on outer electrons, enhancing its ability to act as a reducing agent.

This characteristic makes iodide a substantial player in redox reactions, eagerly offering its electrons and thus, is considered the most potent reducing agent among halide ions.