Q.Explain why the $\text{F-F}$ bond dissociation enthalpy is anomalously lower than that of $\text{Cl-Cl}$, even though fluorine is the smaller atom.

Fluorine is the most electronegative element and lacks d-orbitals, so it shows only -1; the other halogens also show positive states (+1, +3, +5, +7).

Being the most electronegative element, fluorine can never be oxidised by any other element, so it exhibits only the -1 oxidation state (apart from 0 in F2). It also has no d-orbitals in its valence shell, so it cannot expand its octet to reach positive states. In contrast, Cl, Br and I have va …

Fluorine is the most electronegative element and lacks vacant d-orbitals, so it cannot expand its octet or lose electrons to a more electronegative partner; it never shows a positive oxidation state.

The other halogens (Cl, Br, I) are less electronegative and have accessible d-orbitals, so they can form bonds to more electronegative atoms (like O and F) and exhibit positive oxidation states (+1, +3, +5, +7), e.g. in oxoacids and interhalogens.

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A halogen can displace a halide ion lower in the group because oxidising power decreases down Group 17 (F$_2$ > Cl$_2$ > Br$_2$ > I$_2$).

The reaction is a halogen-displacement (single displacement redox) reaction:

$2KBr + F_2 \rightarrow 2KF + Br_2$

Only a halogen with greater oxidising power than Br$_2$ can oxidise Br$^-$ to Br$_2$. Fluorine, being the strongest oxidising agent among the halogens (highest electrode reduction potential, smallest atomic size, highest electronegativity), readily oxidises Br$^-$ ions.

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HF etches glass because it reacts with the SiO2 that glass is made of.

Glass is largely silicon dioxide (SiO2). Hydrofluoric acid attacks and dissolves silica:

$SiO_2 + 4HF \rightarrow SiF_4 + 2H_2O$

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Chlorine is a stronger oxidising agent than bromine, so it displaces bromine from bromide ion in solution.

Reasoning: Down Group 17, oxidising power (and standard reduction potential) decreases: F2 > Cl2 > Br2 > I2. A halogen higher in the group can oxidise (displace) the halide ion of a halogen lower in the group.

Since Cl2 is a stronger oxidising agent than Br2, passing Cl2 gas into an aqueous KBr solution oxidises Br^- to Br2 while Cl2 itself is reduced to Cl^-:

$Cl_2 + 2KBr \longrightarrow 2KCl + Br_2$ …

The Cl–Cl bond is the strongest of the halogen X–X bonds; F–F is unusually weak because of interelectronic repulsion between the small fluorine atoms.

Bond strength (bond dissociation enthalpy) among halogen molecules does not simply decrease down the group. Approximate values (kJ/mol):

• F–F ≈ 155 (anomalously low)
• Cl–Cl ≈ 242 (highest)
• Br–Br ≈ 193
• I–I ≈ 151 …