Uses of chlorine and chlorate (I) (3.2.3.2)

How does Cl₂ react with water and what are the reactions that follow?

When Cl₂ is added to H₂O, HCl and HOCl (chloric (I) acid) are produced

Cl_2(g) + H₂O_(g) ⇌ HOCl_(aq) + HCl_(aq)

HCl then dissociates completely in water:

HCl_(aq) → H⁺_(aq) + Cl^-_(aq)

HOCl partially dissociates so is a weak acid:

HOCl_(aq) ⇌ H⁺_(aq) + ClO^-_(aq)

Chlorine also reacts with water to produce oxygen as the HOCl produced in the initial reaction is unstable in the presence of UV light:

HOCl_(aq) → 2HCl_(aq) + O_2(g)

How does Cl₂ react with NaOH?

Cl₂ reacts with cold, dilute NaOH_(aq) in a disproportionation reaction:

Cl_2(g) + 2NaOH_(aq) → NaClO_(aq) + NaCl_(aq) + H₂O_(l)

What are the uses of the solution formed from the reaction between Cl₂ and NaOH?

Uses:

• NaClO is used as a domestic bleach

  • Usually at a concentration of 5-8%

  • It’s essential for disinfection, making pools, hospitals and homes safe

How is chlorine used in water treatment?

Cl_2(g) + H₂O_(g) ⇌ HOCl_(aq) + HCl_(aq)

• The Cl₂ is in low concentrations

• HOCl sterilises the water, disrupting the cell membrane of bacteria

• Cl₂ remains active in the water supply, preventing recontamination

• Cl₂ can react with organic material, creating chloroalkanes which are carcinogenic but the concentration is too low to cause harm

Overall the benefits of water treatment by Cl₂ outweigh its toxic effects