3.1.3 Halogens

State physical states and colours of the halogens at room temperature.

Fluorine: very pale yellow gas; Chlorine: greenish gas; Bromine: red-brown liquid with orange fumes; Iodine: shiny grey solid that sublimes to purple gas.

What is the trend in melting and boiling points down Group 7?

Melting and boiling points increase down the group.

Why do melting and boiling points increase down Group 7?

Larger molecules have more electrons → stronger London (induced dipole) forces → more energy required to separate molecules.

What is the general molecular formula for halogens?

Halogens exist as diatomic molecules (X2).

What is the outer-shell electronic configuration of Group 7 elements?

s²p⁵.

How do halogens typically react in redox reactions?

They commonly gain one electron to form 1− halide ions.

What is the trend in reactivity of halogens down the group?

Reactivity decreases down the group because larger atoms with more shielding attract electrons less easily.

Explain halogen displacement reactions.

A more reactive halogen displaces a less reactive halogen from its aqueous halide ion (e.g., Cl2 displaces Br− and I−).

What colour indicates chlorine in aqueous solution?

Very pale green (often appears colourless).

What colour indicates bromine in aqueous solution?

Yellow/orange.

What colour indicates iodine in aqueous solution?

Brown (sometimes forms black solid).

What observation shows chlorine has displaced bromide in potassium bromide solution?

The solution turns yellow (Br2 formed).

What observation shows bromine has displaced iodide in potassium iodide solution?

The solution turns brown/purple (I2 formed).

What is observed when an organic solvent is added to an aqueous halogen test?

The organic layer shows the halogen’s colour: chlorine colourless, bromine yellow, iodine purple.

Write the displacement equation for chlorine displacing bromide.

Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq).

Write the displacement equation for bromine displacing iodide.

Br2(aq) + 2I−(aq) → 2Br−(aq) + I2(aq).

What is disproportionation?

A reaction in which the same element is simultaneously oxidised and reduced.

Give the reaction of chlorine with water (disproportionation).

Cl2(g) + H2O(l) → HClO(aq) + HCl(aq).

What are the products when chlorine disproportionates in cold NaOH?

Cl2 + 2NaOH → NaCl + NaClO + H2O (chloride and chlorate(I), bleach formation).

Why is chlorine used in water treatment?

Chlorine disinfects by killing bacteria; benefits of disinfection outweigh risks of toxic effects when controlled.

What happens to universal indicator if chlorine is added to water then bleaches?

It turns red (acidic HCl/HClO), then becomes colourless as HClO bleaches the indicator.

Write the reaction of chlorine with hot, concentrated NaOH.

3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O (formation of chlorate(V)).

What is the purpose of adding nitric acid before adding AgNO3 in halide tests?

Nitric acid removes carbonates (prevents Ag2CO3 precipitation) so silver halide precipitates can be observed clearly.

What is the silver nitrate test result for fluoride?

No precipitate with AgNO3 (fluoride gives no visible silver halide).

What precipitate forms with chloride and AgNO3?

White precipitate of AgCl.

What precipitate forms with bromide and AgNO3?

Cream precipitate of AgBr.

What precipitate forms with iodide and AgNO3?

Pale yellow precipitate of AgI.

How can you distinguish AgCl, AgBr and AgI using ammonia?

AgCl dissolves in dilute NH3; AgBr dissolves in concentrated NH3; AgI does not dissolve in NH3.

Why does BaSO4 formation slow reaction of Ba with H2SO4?

Insoluble BaSO4 coats the metal surface and prevents further reaction (analogous example from Group 2 reasoning).

Why is reactivity of halogens linked to atomic size and shielding?

Larger atoms (down group) have outer electrons further from nucleus and greater shielding, so they attract incoming electrons less strongly and are less oxidising.