Ch 9 - Group 7: The Halogens

Comprehensive flashcards covering the physical properties, chemical reactivity, and laboratory tests for Group 7 halogens and their compounds.

Halogen

Highly reactive non-metals in Group 17 (VIIA) that act as salt-formers with metals and exist as stable, non-polar diatomic molecules ($X_2$).

Valence Electron Configuration of Halogens

$ns^2 np^5$, possessing 7 valence electrons and requiring 1 more to complete an octet.

Fluorine ($F_2$)

A pale yellow gas at room temperature.

Chlorine ($Cl_2$)

A pale green-yellow gas at room temperature used in municipal water treatment and PVC production.

Bromine ($Br_2$)

A reddish-brown volatile liquid at room temperature.

Iodine ($I_2$)

A dark grey/purple-black solid at room temperature that undergoes sublimation.

Volatility Trend of Halogens

Decreases down the group ($F_2 > Cl_2 > Br_2 > I_2$) because increasing numbers of electrons create larger, more polarizable electron clouds, leading to stronger London dispersion forces.

Halogen Bond Strength Trend

Generally decreases down the group ($Cl_2 > Br_2 > I_2$) as atomic radius increases and orbital overlap becomes less effective; however, the $F-F$ bond is anomalously weak due to lone pair repulsion.

Oxidizing Power of Halogens

Decreases down the group ($F_2 > Cl_2 > Br_2 > I_2$) as increasing atomic radius and electron shielding make it harder to attract/capture incoming electrons.

Displacement Reaction

A reaction where a more reactive halogen (higher in the group) forces a lower halide ion out of solution by acting as a stronger oxidizing agent.

Reactivity with Hydrogen ($H_2$)

Decreases down the group; $F_2$ reacts explosively in any condition, while $I_2$ forms a slow, reversible equilibrium at temperatures over $400^{\circ}C$.

Thermal Stability of Hydrogen Halides ($HX$)

Decreases down the group ($HF > HCl > HBr > HI$) because bond energies decrease as the orbital overlap between the H $1s$ orbital and the larger halogen valence orbitals becomes progressively poorer.

Reducing Power of Halide Ions ($X^-$)

Increases down the group ($F^- < Cl^- < Br^- < I^-$); $I^-$ is a potent reducing agent because its large ionic radius and electron shielding allow it to easily donate its valence electron.

Silver Nitrate Test for Halide Ions

The addition of aqueous silver nitrate ($AgNO_3$) to form insoluble precipitates: $Cl^-$ (white), $Br^-$ (cream), and $I^-$ (yellow).

Solubility of Silver Halides in Ammonia

Decreases down the group; $AgCl$ dissolves in dilute $NH_3$, $AgBr$ dissolves only in concentrated $NH_3$, and $AgI$ is insoluble in both.

Disproportionation

A type of redox reaction where the same element is simultaneously oxidized and reduced, such as chlorine reacting with water to form $HCl$ (oxidation state $-1$) and $HOCl$ (oxidation state $+1$).

Hypochlorous Acid ($HOCl$)

A small, neutral biocide formed during water chlorination that diffuses through microbial cell walls to destroy pathogens via oxidation.

Reaction of Sodium Chloride with Conc. $H_2SO_4$

A simple acid-base displacement forming hydrogen chloride gas: $NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl(g)$, as $Cl^-$ is too weak a reducing agent to reduce Sulfur.

Reaction of Sodium Iodide with Conc. $H_2SO_4$

A redox reaction where $I^-$ reduces $S^{+6}$ to $S^{-2}$ ($H_2S$ gas, rotten egg smell) and is itself oxidized to $I_2$ (purple vapor).