The Halogens

Halogens: Group 17. Reactive. All halogens exist as diatomic molecules.
• Characteristics: Fluorine is yellow gas. Chlorine is green gas. Bromine is red/ brown liquid. Iodine
is grey solid.
• Boiling Point Increases Down:
– As you go down the group, greater number of electrons.
– So stronger London forces.
– So more energy required to break the intermolecular forces.
– So boiling point increases.
• Redox Reactions: Most common type of reactions of halogen. Each halogen atom is reduced,
gaining one electron to form 1- halide ion. Cl2 + 2e-
-> 2Cl-
. Another species has lost electrons to
halogen- it is oxidised.
• Oxidising Agent: A reagent that oxidises (takes electrons from) another species. Halogens are
oxidising agents.
• Halogen- Halide Displacement Reactions: A solution of chlorine (Cl2) added to two aqueous
solutions of bromide (Br-
) and iodide (I-
• E.g. potassium iodide produces iodide ions so a reaction of potassium iodide and chlorine.
Products- chloride ions (potassium chloride) and iodine.
• Displacement: If halogen added is more reactive than halide present, halogen displaces the halide
from solution compound. The solution changes colour. No reaction means no change.
• Cyclohexane: Solutions of iodine and bromine in water appear a similar orange- brown colour.
Organic non- polar solvent such as cyclohexane added and mixed. The non- polar halogens
dissolve more readily in cyclohexane than in water. Colours are easier to tell apart with
• Colours: Chlorine is pale green in water and cyclohexane. Bromine is orange in water and
cyclohexane. Iodine is brown in water and turns violet in cyclohexane layer. Whatever is not the
ion, produces the colour.
• Results and Reactivity: Chlorine has reacted both with Br- and I-
. Iodine has not reacted at all least
reactive. Shows that chlorine is more reactive. Fluorine most reactive.
• Full equation- Cl2 (aq) + 2NaBr (aq) -> 2NaCl (aq) + Br2 (aq).
Ionic equation- Cl2 (aq) + 2Br-
(aq) -> 2Cl-
(aq) + Br2 (aq).
By assigning oxidation numbers, chlorine is being reduced and bromine oxidised.
• Reactivity Trend Decreases Down:
– As you go down the group, atomic radius increases.
– More inner shells so shielding increases.
– Less nuclear attraction to capture an electron from another species.

– So reactivity decreases and become weaker oxidising agent.
• State and explain the trend in reactivity shown by these observations- don’t put one is more
reactive than other, write full explanation above in relation to element used. Chlorine gains
electron easiest because smallest atom, least shielded, greatest etc.
• One further displacement reaction to confirm reactivity- whichever one is missing.
• Disproportionation: It the simultaneous oxidation and reduction of the same element in the same
redox reaction. Reaction with chlorine + water and chlorine + cold dilute sodium hydroxide are
• It has been both oxidised and reduced. It has been oxidised from 0 in Cl2 to +1 in HClO. It has been
reduced from 0 in Cl2 to –1 in HCl.
• Chlorine and Water Reaction: Chloric (I) acid HClO. Bacteria are killed by chloric (I) acid and
chlorate (I) ions.
Cl2 (aq) + H2O (l) -> HClO (aq) + HCl (aq). Cl is +1 in HClO and -1 in HCl.
• Chloric (I) Acid: It acts as a weak bleach. Can add some indicator solution to solution of chlorine.
Chorine gas and damp litmus paper results in reaction between chlorine and water. Indicators first
turns red from presence HCl and HClO as both acidic. Then colour disappears as bleaching action
of chloric (I) acid (chlorate ions) takes effect.
• Chlorine and Cold Dilute Sodium Hydroxide: Reaction of chlorine and water is limited by low
solubility of chlorine in water. If water contains dissolved sodium hydroxide, much more chlorine
dissolves so another reaction takes place. Cold and dilute NaOH would be conditions.
Cl2 (aq) + 2NaOH (aq) -> NaClO (aq) + NaCl (aq) + H2O (l). Cl is +1 in NaClO and -1 in NaCl. State
3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O- hot concentrated sodium hydroxide.
• Sodium Chlorate (I): Resulting solution contains chlorate (I) ions ClOfrom the sodium chlorate (I)
NaClO formed. NaClO used in household bleach.
• Risks of Chlorine Use: Chlorine is toxic. Chlorine in drinking water can react with hydrocarbons to
form chlorinated hydrocarbons which are carcinogenic.
• Benefits of Chlorine Use: Bacteria are killed.
• Well ventilated area as bromine is toxic- as bromine is produced.
• Chlorine and water- 2F2 + 2H2O -> 4HF + O2