Halogen Redox Reactions

Oxidising power refers to the ability of a halogen ($X_2$) to attract and gain electrons to form a halide ion ($X^-$), effectively acting as an oxidising agent that gets reduced in the process.

The oxidising ability decreases down Group 7 ($F_2 > Cl_2 > Br_2 > I_2$) because as the atomic radius increases, the outer shell is further from the nucleus and experiences more shielding from inner electrons.

Consequently, the electrostatic attraction between the positive nucleus and an incoming electron becomes weaker, making it harder for larger halogens to gain an electron compared to smaller ones like fluorine or chlorine.

Disproportionation is a specific type of redox reaction where the same element is simultaneously oxidised and reduced from a single oxidation state to two different ones.

In cold dilute alkali ($15^\circ C$), chlorine reacts to form chloride and chlorate(I) ions: $Cl_2 + 2OH^- \rightarrow Cl^- + ClO^- + H_2O$ (Oxidation states: $0 \rightarrow -1$ and $+1$).

In hot concentrated alkali ($70^\circ C$), the chlorate(I) ion is thermally unstable and undergoes further disproportionation to the more stable chlorate(V) ion: $3Cl_2 + 6OH^- \rightarrow 5Cl^- + ClO_3^- + 3H_2O$ (Oxidation states: $0 \rightarrow -1$ and $+5$).

The reducing power of halide ions increases down the group ($Cl^- < Br^- < I^-$) because larger ions have outer electrons that are further from the nucleus and more shielded, making them easier to lose.

This trend is demonstrated by reacting solid halides with concentrated sulfuric acid ($H_2SO_4$), which acts as an oxidising agent.

While $Cl^-$ only performs an acid-base reaction to produce $HCl$ gas, $I^-$ is strong enough to reduce the sulfur in $H_2SO_4$ from $+6$ all the way to $-2$ in $H_2S$ (hydrogen sulfide).

Check Oxidation States: Always assign oxidation numbers to the halogen atoms before and after the reaction to verify if disproportionation has occurred.

Temperature Matters: In questions involving chlorine and $NaOH$, look for the temperature; $15^\circ C$ produces $ClO^-$ while $70^\circ C$ produces $ClO_3^-$.

Identify the Gas: When describing the reaction of $I^-$ with $H_2SO_4$, ensure you mention the specific observations for each sulfur product ($SO_2$ is choking, $S$ is a yellow solid, $H_2S$ smells of rotten eggs).

Solvent Layers: Remember that halogens are more soluble in organic solvents than in water; use this to explain why colors become more intense in the top organic layer during displacement tests.