AIPMT PRELIMS 2006 Chemistry Trends in the Periodic Table MCQ Question

Let's analyze the question step-by-step.

Question: Which one of the following orders is not in accordance with the property stated against it?

Options: A) HI > HBr > HCl > HF : Acidic property in water
B) F₂ > Cl₂ > Br₂ > I₂ : Electronegativity
C) F₂ > Cl₂ > Br₂ > I₂ : Bond dissociation energy

Correct Answer: D (This seems to be a typographical error, as the options only go up to C. Let's analyze options A, B, and C.)

Explanation of Each Option

Option A: HI > HBr > HCl > HF : Acidic property in water

This order is correct. The acidic strength of hydrogen halides in water increases from HF to HI. This can be attributed to two main factors:

1. Bond Strength: The bond strength decreases down the group, which means that HI has a weaker H-I bond compared to HCl or HF. Weaker bonds are easier to break, resulting in stronger acids.
2. Stability of Anions: The larger iodide ion (I⁻) is more stable than fluoride (F⁻) due to its larger size and ability to distribute charge over a greater volume. Thus, as we move from HF to HI, the acidic strength increases.

Option B: F₂ > Cl₂ > Br₂ > I₂ : Electronegativity

This order is correct. Electronegativity is the ability of an atom to attract electrons in a bond. The electronegativity decreases down the group in the periodic table:

• Fluorine has the highest electronegativity (3.98 on the Pauling scale), followed by chlorine (3.16), bromine (2.96), and iodine (2.66).
  Thus, the correct order of electronegativity is indeed F₂ > Cl₂ > Br₂ > I₂.

Option C: F₂ > Cl₂ > Br₂ > I₂ : Bond dissociation energy

This order is incorrect. The bond dissociation energy is the energy required to break a bond in a molecule. For diatomic halogens, the bond dissociation energy decreases as we move down the group from fluorine to iodine:

• Reason: The bond length increases as the size of the halogens increases, leading to a weaker bond. Fluorine has a high bond dissociation energy due to its smaller size and strong overlap of its orbitals, while iodine has a much lower bond dissociation energy because of its larger atomic size and weaker bond strength.

Conclusion

• Correct Answer: C is the option that does not conform to the stated property. The bond dissociation energy order should actually be $F_2 < Cl_2 < Br_2 < I_2$ instead of the given order.

Summary

• Option A correctly states the order of acidic strength due to bond strengths and anion stability.
• Option B correctly reflects the trend in electronegativity, which decreases down the group.
• Option C incorrectly states the order of bond dissociation energies; thus, it is the option that does not align with the described property.

Key Concepts

• Acid Strength: Increases as bond strength decreases.
• Electronegativity: Decreases down the group.
• Bond Dissociation Energy: Decreases down the group due to increased atomic size.

This breakdown should help clarify why each option is correct or incorrect, focusing particularly on the principles of periodic trends.