What is the difference between the oxidising power of halogens and the reducing power of halide ions down Group 7?

Oxidising power of halogens decreases down the group as atoms get larger and attract electrons less strongly. Conversely, the reducing power of halide ions increases down the group because larger ions lose their outermost electron more easily due to increased shielding.

How does the reaction of $Cl^-$ with concentrated $H_2SO_4$ differ from the reaction of $I^-$ with the same acid?

Chloride only undergoes an acid-base reaction to produce $HCl$ because it is a weak reducing agent. Iodide is a strong reducing agent and undergoes multiple redox steps, reducing sulfur from +6 to +4 ($SO_2$), 0 ($S$), and -2 ($H_2S$).

When testing for halides with silver nitrate, why is ammonia used as a follow-up step?

The colors of silver chloride (white), bromide (cream), and iodide (yellow) can be very similar and difficult to distinguish by eye. Ammonia provides a definitive check because $AgCl$ dissolves in dilute $NH_3$, $AgBr$ requires concentrated $NH_3$, and $AgI$ will not dissolve in either.

What error occurs if a student uses hydrochloric acid instead of nitric acid to acidify silver nitrate for a halide test?

Hydrochloric acid contains chloride ions ($Cl^-$), which will immediately react with the silver nitrate to form a white precipitate of silver chloride. This would give a false positive result regardless of which halide was actually in the unknown sample.

What is a common mistake when writing the disproportionation reaction of chlorine in water?

Students often forget that the reaction is reversible ($Cl_2 + H_2O \rightleftharpoons HCl + HClO$) and may fail to identify that the oxidation state of chlorine changes from 0 to both -1 (in $HCl$) and +1 (in $HClO$).

Why is it incorrect to say that 'halogens get more reactive' down the group?

Reactivity in Group 7 refers to the ability to gain electrons (oxidising power), which actually decreases down the group. While the halide ions become more reactive as reducers, the elemental halogens become less reactive as oxidisers.

Define a disproportionation reaction using chlorine as an example.

A disproportionation reaction is a redox reaction where the same species is simultaneously oxidised and reduced. In the reaction of $Cl_2$ with water, chlorine (0) is reduced to $Cl^-$ (-1) and oxidised to $ClO^-$ (+1).

What is the chemical formula and appearance of the product formed when $I^-$ reduces $H_2SO_4$ to its lowest oxidation state?

The product is hydrogen sulfide ($H_2S$), which is a colorless gas. It is most easily identified by its distinct and unpleasant smell of rotten eggs.

State the trend in electronegativity down Group 7 and explain why it occurs.

Electronegativity decreases down the group. This is because the atomic radius increases and there is more shielding, which weakens the attraction between the nucleus and the bonding pair of electrons.

Why does the reducing power of halide ions increase as the ionic radius increases?

As the ion gets larger, the valence electrons are further from the nucleus and experience more shielding from inner shells. This reduces the electrostatic attraction, making it easier for the ion to donate an electron to another species.

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Chemical Properties of Group 7

Summary

The chemical behavior of Group 7 elements (halogens) is defined by their high electronegativity and their tendency to act as powerful oxidising agents. As the group is descended, the oxidising power of the elemental halogens decreases, while the reducing power of their corresponding halide ions increases, leading to distinct patterns in displacement reactions and interactions with concentrated sulfuric acid.

1. Oxidising Power of Halogens

Oxidising agents are species that accept electrons, and halogens ( $X_2$ ) do this to form 1- ions: $X_2 + 2e^- \rightarrow 2X^-$ .
The oxidising power decreases down the group from fluorine to iodine because the atomic radius increases and electron shielding becomes more significant.
As the outer shell moves further from the nucleus, the electrostatic attraction for an incoming electron weakens, making it harder for the atom to gain an electron.
Electronegativity, the ability of an atom to attract a bonding pair of electrons, follows the same downward trend for these reasons.

Diagram showing the decrease in oxidising power of halogens and the increase in reducing power of halide ions as you move down Group 7.

2. Displacement Reactions

3. Reducing Power of Halide Ions

4. Reaction with Concentrated Sulfuric Acid

5. Testing for Halide Ions

6. Disproportionation of Chlorine

7. Exam Strategy & Tips