Unit 12: Chemical Properties of Halogens & Halides III
Exploring key reactions with metals and water, and the important uses of these versatile elements.
12.9 Reactions of Halogens with Metals
As strong oxidising agents, halogens react directly with most metals to form metal halides (salts). In these redox reactions, the metal is oxidised (loses electrons) and the halogen is reduced (gains electrons).
The reactivity of the halogen decreases down the group. Therefore, reactions with fluorine are extremely violent, while reactions with iodine are much slower and often require heating.
Examples:
- Sodium and Chlorine: Hot sodium metal burns vigorously in chlorine gas
with a bright yellow flame to produce white, solid sodium chloride.
$$ 2Na(s) + Cl_2(g) \rightarrow 2NaCl(s) $$ - Iron and Chlorine: When hot iron wool is placed in chlorine gas, it
glows brightly and reacts to form a brown solid, iron(III) chloride. Chlorine is a
strong enough oxidising agent to oxidize iron to its higher +3 oxidation state.
$$ 2Fe(s) + 3Cl_2(g) \rightarrow 2FeCl_3(s) $$ - Iron and Iodine: In contrast, when iron is heated with iodine (a weaker
oxidising agent), it only forms grey-black iron(II) iodide. Iodine is not strong enough
to oxidize iron to the +3 state.
$$ Fe(s) + I_2(g) \rightarrow FeI_2(s) $$
Solved Examples:
-
Write a balanced equation for the reaction of magnesium with
bromine.
Solution: Magnesium is in Group 2 and forms a $Mg^{2+}$ ion. Bromine forms a $Br^-$ ion. The product is magnesium bromide, $MgBr_2$.
$Mg(s) + Br_2(l) \rightarrow MgBr_2(s)$ -
Explain why chlorine reacts more vigorously with magnesium than bromine
does.
Solution: Chlorine is higher up Group 17 than bromine, making it a smaller, more electronegative atom and a stronger oxidising agent. It accepts electrons from the magnesium more readily, leading to a more vigorous reaction.
12.10 Reactions of Chlorine with Water & Dilute Alkali
Chlorine undergoes a special type of redox reaction called disproportionation when it reacts with water and cold, dilute alkali. A disproportionation reaction is one where an element is simultaneously oxidised and reduced.
Reaction with Water
Chlorine reacts reversibly with water to a small extent, producing a mixture of hydrochloric
acid (HCl) and chloric(I) acid (HOCl).
$$ Cl_2(aq) + H_2O(l) \rightleftharpoons HCl(aq) + HOCl(aq) $$
In this reaction, one chlorine atom is reduced (oxidation state 0 to -1 in HCl), and the
other is oxidised (oxidation state 0 to +1 in HOCl). The chloric(I) acid is a powerful
oxidising agent and is responsible for chlorine's bleaching and disinfecting properties.
Reaction with Cold, Dilute Alkali (e.g., NaOH)
The reaction with cold, dilute sodium hydroxide is similar but goes to completion because
the alkali neutralizes the acidic products. The products are sodium chloride, sodium
chlorate(I), and water.
$$ Cl_2(aq) + 2NaOH(aq) \rightarrow NaCl(aq) + NaClO(aq) + H_2O(l) $$
Again, chlorine is simultaneously reduced (to -1 in NaCl) and oxidised (to +1 in NaClO). The
resulting solution, containing sodium chlorate(I) (NaClO), is sold as household bleach.
Solved Examples:
-
What is a disproportionation reaction?
Solution: A redox reaction in which a single element is both oxidized and reduced. -
What are the oxidation states of chlorine in the three species $Cl_2$, HCl,
and HOCl?
Solution: In $Cl_2$, the oxidation state is 0. In HCl, it is -1. In HOCl, oxygen is -2 and hydrogen is +1, so chlorine must be +1. -
Why is chlorine added to swimming pools?
Solution: It is added as a disinfectant. It reacts with water to form chloric(I) acid (HOCl), which kills bacteria and other harmful microorganisms.
12.11 Uses of Halogens & Halides
The reactivity and unique properties of halogens and their compounds lead to many important applications.
- Chlorine ($Cl_2$): Its most significant use is in water treatment and as a disinfectant for swimming pools. It is also used to manufacture household bleach (sodium chlorate(I)) and a huge range of organic compounds, including solvents and the polymer PVC (polyvinyl chloride).
- Iodine ($I_2$): Used as an antiseptic for sterilizing wounds (e.g., tincture of iodine).
- Sodium Chloride (NaCl): Used as a food preservative and flavoring (table salt), and for de-icing roads in winter.
- Sodium Chlorate(I) (NaClO): The active ingredient in household bleach, used for its powerful disinfecting and bleaching properties.
- Silver Bromide (AgBr): A light-sensitive compound that is the basis of traditional photographic film. When light strikes the film, the AgBr decomposes to form tiny, dark particles of silver, creating the negative image.
- Fluorides: Added to toothpaste and public water supplies to help prevent tooth decay by strengthening tooth enamel.
Solved Examples:
-
What property of chloric(I) acid makes it useful for water
purification?
Solution: It is a strong oxidising agent that is effective at killing harmful bacteria. -
Name the active ingredient in household bleach.
Solution: Sodium chlorate(I) (NaClO). - Why is sodium chloride spread on roads in
winter?
Solution: It lowers the freezing point of water, preventing ice from forming and melting existing ice. - How does silver bromide work in
photography?
Solution: It is a light-sensitive compound. When exposed to light, it decomposes to form small, dark particles of silver metal, which creates the image on the film. - What is the main use of chlorine gas?
Solution: Water treatment and disinfection.