Question

In which of the following arrangements, the sequence is not strictly according to the property written against it? \(\quad\) [2009] (a) \(\mathrm{HF}<\mathrm{HCl}<\mathrm{HBr}<\mathrm{HI}\) : increasing acid strength (b) \(\mathrm{NH}_{3}<\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{SbH}_{3}:\) increasing basic strength (c) \(\mathrm{B}<\mathrm{C}<\mathrm{O}<\mathrm{N}:\) increasing first ionization enthalpy (d) \(\mathrm{CO}_{2}<\mathrm{SiO}_{2}<\mathrm{SnO}_{2}<\mathrm{PbO}_{2}:\) increasing oxidising power

Short answer

(b) NH3<PH3<AsH3<SbH3: increasing basic strength is incorrect.

Step-by-step solution

Analyze Option (a)

The sequence given is \( \mathrm{HF} < \mathrm{HCl} < \mathrm{HBr} < \mathrm{HI} \) with increasing acid strength. This sequence is correct since, as we move down the halogen group from HF to HI, the bond strength weakens, leading to a greater tendency to donate \( \mathrm{H^+} \), thus increasing acid strength.

Analyze Option (b)

The sequence \( \mathrm{NH_3} < \mathrm{PH_3} < \mathrm{AsH_3} < \mathrm{SbH_3} \) is said to show increasing basic strength. This is incorrect because ammonia (\(\mathrm{NH_3}\)) is a stronger base compared to the other hydrides in the group due to its smaller size and higher ability to donate the lone pair of electrons. Thus, basic strength actually decreases down the group.

Analyze Option (c)

The sequence \( \mathrm{B} < \mathrm{C} < \mathrm{O} < \mathrm{N} \) is for increasing first ionization enthalpy. This is incorrect because nitrogen has a higher first ionization enthalpy than oxygen due to its half-filled \(2p\) subshell, which is more stable.

Analyze Option (d)

The sequence \( \mathrm{CO_2} < \mathrm{SiO_2} < \mathrm{SnO_2} < \mathrm{PbO_2} \) is for increasing oxidizing power. This sequence is generally correct as oxidizing power increases down the group in the carbon family as the oxidation state \( +4 \) becomes more stable in heavier elements.

Key concepts

Acid Strength Trend

Understanding acid strength is crucial to explaining the behavior of acids like HF, HCl, HBr, and HI. Acid strength indicates how easily an acid can donate a proton (H⁺). As we move down the halogen group from fluorine to iodine, acids generally become stronger. This is primarily because:

• The bond strength between hydrogen and the halogen decreases.
• In HF, the bond is exceptionally strong due to fluorine's high electronegativity and small size.
• As we go from HF to HI, bond strength weakens, resulting in a greater tendency to donate H⁺.

Thus, HF is the weakest acid, and HI is the strongest among the given acids. This trend helps in predicting the reactivity and strength of different halogen acids.

Basic Strength Trend

The concept of basic strength is about an atom or molecule's ability to accept protons or donate electron pairs. In the context of hydrides like NH₃, PH₃, AsH₃, and SbH₃, the basic strength trend is not straightforward. Ammonia (NH₃) stands out as the strongest base among them because:

• NH₃ has a smaller atomic size, making the lone pair on nitrogen more available for donation.
• Larger hydrides like PH₃, AsH₃, and SbH₃ have more diffuse electron clouds, reducing their ability to donate electron pairs.

Contrary to the given sequence, basic strength decreases as we move down the group.

Ionization Enthalpy

Ionization enthalpy is the energy required to remove an electron from an atom in its gaseous state. It is a key indicator of an element's chemical reactivity. The correct trend does not always follow atomic number order. For instance:

• While you might expect a linear increase from B to N, nitrogen's half-filled p-subshell provides extra stability.
• Hence, nitrogen has a higher ionization enthalpy than oxygen, which disrupts the expected trend.
• The expected increase in ionization enthalpy from B to N is disrupted by this subshell stability.

Understanding this concept is crucial in predicting the behavior of elements, especially in chemical bonding and reactions.

Oxidizing Power Trend

Oxidizing power refers to a substance's ability to accept electrons during a chemical reaction. For oxides like CO₂, SiO₂, SnO₂, and PbO₂, the trend in oxidizing power is influenced by oxidation states and the element's position in the group:

• Heavier elements like PbO₂ show greater oxidizing power as the oxidation state +4 becomes more stable.
• In contrast, lighter elements like CO₂ exhibit less oxidizing power due to their stable double bonds.
• The increase in oxidizing power down the group is due to the stabilization of higher oxidation states in heavier elements.

This trend helps explain the reactivity differences among oxides in a group, impacting both industrial processes and natural phenomena.