Answer the question on basis of passage given below:

All the members of Group 15 elements form covalent hydrides with hydrogen as $NH_3, PH_3, AsH_3, SbH_3, BiH_3$. Group 16 elements form covalent hydrides with hydrogen as $H_2O, H_2S, H_2Se, H_2Te, H_2PO$. Group 17 elements form covalent hydrides with hydrogen as $HF, HCI, HBr, HI$. Based on these facts answer the following question.

In aqueous solution, the acidic strength of hydrides of Group 17 elements i.e. $HF, HCl, HBr, HI$ increases as:

$HF <HCl <HBr < HI$

The correct answer is Option (1) → $HF <HCI <HBr < HI$.

The acidic strength of the hydrides of Group 17 elements \((HF, HCl, HBr, HI)\) depends on how easily they can donate protons \((H^+)\) when dissolved in water. The key factors that influence this are the bond strength between hydrogen and the halogen, the electronegativity of the halogen, and the stability of the conjugate base after the proton is released.

Factors Affecting Acidic Strength:

Bond Dissociation Energy (Bond Strength):

The bond strength between hydrogen and the halogen \((F, Cl, Br, I)\) decreases as we move down the group. This is because the size of the halogen atoms increases down the group, leading to longer and weaker bonds with hydrogen.

A weaker bond means it is easier to break and release a proton \((H^+)\), making the acid stronger. Conversely, a stronger bond makes it harder to release \(H^+\), resulting in a weaker acid.

The bond dissociation energies for the halogen-hydrogen bonds are as follows:

\(H–F: 565\, \ kJ/mol\)

\(H–Cl: 431\, \ kJ/mol\)

\(H–Br: 366\, \ kJ/mol\)

\(H–I: 298\, \  kJ/mol\)

As you can see, the \(H–F\) bond is the strongest, while the \(H–I\) bond is the weakest. Therefore, \(HI\) easily dissociates to release \(H^+\) and is the strongest acid, whereas \(HF\) holds onto its proton more tightly and is the weakest acid.

Electronegativity of the Halogen:

Electronegativity refers to the tendency of an atom to attract electrons. While fluorine is the most electronegative element in Group 17, its high electronegativity does not result in strong acidity for \(HF\). This is because, although fluorine strongly pulls electrons towards itself, the bond between \(H\) and \(F\) is very strong, making it difficult to release a proton.

Down the group, the electronegativity decreases as follows:

\(F\) (Fluorine): 3.98

\(Cl\) (Chlorine): 3.16

\(Br\) (Bromine): 2.96

\(I\) (Iodine): 2.66

However, electronegativity alone is not enough to determine acidic strength. The bond strength between hydrogen and the halogen plays a more dominant role in determining the acidic strength, especially in the case of \(HF\).

Stability of the Conjugate Base \((X^-)\):

When an acid donates a proton, the remaining species is called its conjugate base. For the hydrides of Group 17, the conjugate bases are \(F^-\), \(Cl^-\), \(Br^-\), and \(I^-\).

A more stable conjugate base means that the acid is more likely to dissociate and release a proton, making the acid stronger.

The stability of the conjugate base increases as we go down the group because the size of the halide ions increases. Larger ions like \(I^-\) can spread out the negative charge more effectively, making them more stable.

In contrast, \(F^-\) is a small ion and holds a high negative charge density, making it less stable than larger halide ions like \(Cl^-\), \(Br^-\), and \(I^-\). This is another reason why \(HF\) is a weaker acid, while \(HI\) is the strongest acid.

Analysis of the Hydrides of Group 17:

\(HF\) (Hydrogen Fluoride):

The \(H–F\) bond is very strong due to the small size and high electronegativity of fluorine. This makes it difficult for HF to release a proton.

Additionally, the conjugate base \((F^-)\) is less stable because of its small size and high negative charge density.

Therefore, \(HF\) is a weak acid.

\(HCl\) (Hydrogen Chloride):

The \(H–Cl\) bond is weaker than the \(H–F\) bond, so it is easier for \(HCl\) to release a proton compared to \(HF\).

The conjugate base \((Cl^-)\) is larger and more stable than \(F^-\), making \(HCl\) a stronger acid than \(HF\).

\(HCl\) is classified as a strong acid, and it dissociates almost completely in water.

\(HBr\) (Hydrogen Bromide):

The \(H–Br\) bond is weaker than the \(H–Cl\) bond, making it even easier for \(HBr\) to dissociate and release a proton.

The conjugate base \((Br^-)\) is larger and more stable than \(Cl^-\), making \(HBr\) a stronger acid*than \(HCl\).

\(HI\) (Hydrogen Iodide):

The \(H–I\) bond is the weakest among the Group 17 hydrides, meaning it is the easiest to break and release \(H^+\).

The conjugate base \((I^-)\) is the largest and most stable, making \(HI\) the strongest acid of the hydrides.

\(HI\) dissociates completely in water, making it a very strong acid.

Conclusion:

The acidic strength of the Group 17 hydrides increases as we move down the group because:

The bond strength between hydrogen and the halogen decreases, making it easier to release a proton.

The stability of the conjugate base increases as the size of the halide ion increases.

Correct Order of Acidic Strength:

The order of increasing acidic strength is: \(\mathbf{HF < HCl < HBr < HI}\)