Chlorine, halides and sulfuric acid

Chlorine and water or alkali

• Water: Cl2 + H2O ⇌ HCl + HClO. Chlorine goes from 0 to -1 and +1. Chloric(I) acid (HClO) kills bacteria, so chlorine treats drinking water.
• Cold dilute NaOH: Cl2 + 2NaOH -> NaCl + NaClO + H2O. Sodium chlorate(I) is bleach.
• Hot concentrated NaOH: 3Cl2 + 6NaOH -> 5NaCl + NaClO3 + 3H2O (0 to -1 and +5).

All three are disproportionation.

Halides with concentrated sulfuric acid

• NaCl: NaCl + H2SO4 -> NaHSO4 + HCl. Steamy fumes of HCl only; Cl- is too weak a reducing agent to reduce H2SO4.
• NaBr: HBr forms, then Br- reduces some H2SO4 to SO2 (S: +6 to +4). Steamy fumes and orange-brown Br2 vapour.
• NaI: HI forms, then I- reduces H2SO4 to SO2, S (yellow solid, 0) and H2S (smell of bad eggs, -2). Purple vapour and black solid iodine.

The trend

Halide ions get stronger as reducing agents down the group: the outer electrons of a larger ion are further from the nucleus and more shielded, so they are lost more easily.

Hydrogen halides

HX gases are very soluble in water and form acidic solutions: HCl + H2O -> H3O+ + Cl-. With ammonia gas they form dense white smoke of the ammonium salt: NH3 + HCl -> NH4Cl.

Worked example

Explain why sodium iodide gives hydrogen sulfide with concentrated sulfuric acid, but sodium chloride does not.

1. I- is a much stronger reducing agent than Cl-, because its outer electrons are further from the nucleus and more shielded.
2. I- can reduce sulfur from +6 all the way to -2 in H2S.
3. Cl- cannot reduce H2SO4 at all.

Answer: Reducing power of halides increases down Group 7.

Check yourself

1. Which products form when chlorine reacts with cold dilute sodium hydroxide?
   • NaCl and NaClO3 and H2O
   • NaCl and H2
   • NaClO and HCl
   • NaCl, NaClO and H2O
   Show answer

   NaCl, NaClO and H2O. Cold dilute alkali gives chloride and chlorate(I), which is bleach.

2. What is the oxidation number of chlorine in NaClO3, formed with hot alkali?
   Show answer

   +5. Na +1, O 3(-2) = -6, so Cl = +5.

3. What is seen when concentrated sulfuric acid is added to solid sodium chloride?
   • Purple vapour and a yellow solid
   • Orange-brown vapour
   • A smell of bad eggs
   • Steamy fumes only
   Show answer

   Steamy fumes only. Only HCl forms; Cl- cannot reduce the acid.

4. Which product shows sulfur reduced to oxidation state -2?
   • H2S
   • SO2
   • S
   • NaHSO4
   Show answer

   H2S. In H2S, sulfur is -2.

5. Which halide ion is the strongest reducing agent?
   • Br-
   • Cl-
   • I-
   • F-
   Show answer

   I-. Its outer electrons are furthest from the nucleus and most easily lost.

6. What forms when hydrogen chloride gas meets ammonia gas?
   • White smoke of ammonium chloride
   • Brown fumes of NO2
   • A yellow precipitate
   • A green gas
   Show answer

   White smoke of ammonium chloride. NH3 + HCl -> NH4Cl(s).

7. Why is chlorine added to drinking water?
   • It makes the water alkaline
   • It removes calcium ions
   • The HClO formed kills bacteria
   • It adds chloride for health
   Show answer

   The HClO formed kills bacteria. Chloric(I) acid is a disinfectant. The benefit outweighs the small risk from chlorine compounds.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.