Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following orders is not in accordance with the property stated against it? (a) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\); electronegativity (b) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2} ;\) bond dissociation energy (c) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\); oxidizing power (d) \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF} ;\) acidic property in water

Step-by-Step Solution

Answer

Option (b) is incorrect for bond dissociation energy.

Step 1: Understanding the Problem

We are given four options, each listing an order of substances according to a specified chemical property. Our task is to identify which order does not correctly represent the stated property: electronegativity, bond dissociation energy, oxidizing power, and acidic property in water.

Step 2: Evaluating Option (a): Electronegativity

Electronegativity is the tendency of an atom to attract electrons. For halogens, the order of electronegativity is: \( \mathrm{F}_{2} > \mathrm{Cl}_{2} > \mathrm{Br}_{2} > \mathrm{I}_{2} \). Thus, option (a) is correct concerning electronegativity.

Step 3: Evaluating Option (b): Bond Dissociation Energy

Bond dissociation energy is the energy required to break a bond. For halogens, \( \mathrm{Cl}_{2} > \mathrm{Br}_{2} > \mathrm{F}_{2} > \mathrm{I}_{2} \), indicating that \( \mathrm{F}_{2} \) has lower bond dissociation energy due to its weak bond compared to \( \mathrm{Cl}_{2} \). Therefore, the order given in (b) is incorrect.

Step 4: Evaluating Option (c): Oxidizing Power

Oxidizing power is the ability to oxidize other substances. The order is \( \mathrm{F}_{2} > \mathrm{Cl}_{2} > \mathrm{Br}_{2} > \mathrm{I}_{2} \), which matches the order given in (c). Hence, it is correct for oxidizing power.

Step 5: Evaluating Option (d): Acidic Property in Water

Acidic property in water for hydrogen halides generally increases down the group: \( \mathrm{HI} > \mathrm{HBr} > \mathrm{HCl} > \mathrm{HF} \). The order given in (d) is accurate for acidity in water.

Step 6: Conclusion

By assessing each option, we determine that the order in option (b) does not correctly represent the property in question, as \( \mathrm{F}_{2} \) should not be greater than \( \mathrm{Cl}_{2} \) for bond dissociation energy.

Key Concepts

Electronegativity

Electronegativity is a key concept in chemistry, describing how strongly an atom can attract electrons towards itself. In the periodic table, electronegativity tends to increase across a period and decrease down a group. Among the halogens, fluorine (\( \mathrm{F}_{2} \)) is the most electronegative element, which means it has a strong ability to attract electrons. This makes fluorine incredibly reactive since it can easily pull electrons from other atoms.

Here's the general order of electronegativity for the halogens:

• Fluorine (\( \mathrm{F}_{2} \)): highest electronegativity
• Chlorine (\( \mathrm{Cl}_{2} \))
• Bromine (\( \mathrm{Br}_{2} \))
• Iodine (\( \mathrm{I}_{2} \)): lowest electronegativity

The strong electron-attracting power of fluorine is unmatched, which is why its position at the top of this order is so secure.

Bond Dissociation Energy

Bond dissociation energy refers to the energy needed to break a bond into two atoms. For diatomic molecules like the halogens, this can provide insight into the stability of the molecules. Interestingly, in the case of halogens, bond strength isn't intuitively reflected by their atomic sizes alone.

The order for bond dissociation energy amongst the halogens isn't as straightforward:

• Chlorine (\( \mathrm{Cl}_{2} \)): strongest bond
• Bromine (\( \mathrm{Br}_{2} \))
• Fluorine (\( \mathrm{F}_{2} \)): unusually weak bond
• Iodine (\( \mathrm{I}_{2} \)): weakest bond

Though smaller atoms like fluorine might be expected to form strong bonds due to their proximity, the repulsion between the non-bonding electrons in fluorine causes it to have a weaker bond than chlorine. This makes fluorine's bond dissociation lower than \( \mathrm{Cl}_{2} \), highlighting that size is just one part of the equation.

Oxidizing Power

Oxidizing power refers to a substance's ability to gain or accept electrons from a different substance, essentially causing that second substance to lose electrons. In the context of halogens, fluorine (\( \mathrm{F}_{2} \)) stands out for its exceptionally high oxidizing power, often attributed to its high electronegativity and ability to stabilize new electron configurations.

Here’s the typical order of oxidizing power:

• Fluorine (\( \mathrm{F}_{2} \)): most powerful oxidizing agent
• Chlorine (\( \mathrm{Cl}_{2} \))
• Bromine (\( \mathrm{Br}_{2} \))
• Iodine (\( \mathrm{I}_{2} \)): least powerful oxidizing agent

This hierarchy explains why fluorine is used in many industrial processes as an oxidizing agent, capable of reacting even with substances that are usually inert.

Acidic Property in Water

The acidic property in water refers to the substance's ability to donate protons (\( \mathrm{H}^+ \)) when dissolved in water. For hydrogen halides (compounds of hydrogen with halogens), this property is influenced by bond strength and stability of the resulting ions.

Typically, the strength of an acid is inversely related to the strength of the bond between hydrogen and the halogen. For the hydrogen halides:

• Hydroiodic acid (\( \mathrm{HI} \)): strongest acid
• Hydrobromic acid (\( \mathrm{HBr} \))
• Hydrochloric acid (\( \mathrm{HCl} \))
• Hydrofluoric acid (\( \mathrm{HF} \)): weakest acid

As we move down the group, bond strength decreases, making it easier for the hydrogen halide to ionize and release hydrogen ions into the water. This creates stronger acids in the sequence from \( \mathrm{HF} \) to \( \mathrm{HI} \).