The Halogens (OCR A Level Chemistry A): Flashcards
Exam code: H432
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- FrontThe Halogens
Define halogen displacement reaction
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Define halogen displacement reaction
A halogen displacement reaction is a reaction in which a more reactive halogen displaces a less reactive halide ion from an aqueous solution, acting as an oxidising agent.
True or False?
Iodine can displace bromide ions from an aqueous solution of potassium bromide.
False.
Iodine is less reactive than bromine, so it cannot displace bromide ions. Only a more reactive halogen can displace a less reactive halide.
Why does the boiling point of halogens increase going down the group?
Larger molecules have more electrons, which strengthens the London (instantaneous dipole-induced dipole) forces between molecules, requiring more energy to overcome.
2Br- (aq) + .......... → 2Cl- (aq) + .......... (aq)
2Br- (aq) + Cl2 (aq) → 2Cl- (aq) + Br2 (aq)
Br- is oxidised (−1 to 0) and Cl2 is reduced (0 to −1).
What colour does iodine appear in an organic solvent layer?
Iodine appears purple in an organic solvent layer, which distinguishes it from bromine (yellow) and chlorine (colourless).
True or False?
Halogens act as oxidising agents in redox reactions.
True.
Halogens gain an electron from another species, decreasing in oxidation number (e.g. 0 to −1), so they are reduced and act as oxidising agents.
Why does the oxidising power of halogens decrease going down Group 7?
Atomic radius increases and shielding increases down the group, reducing the nuclear attraction for an incoming electron and making the halogen less effective as an oxidising agent.
Define disproportionation reaction
A disproportionation reaction is a reaction in which the same species is simultaneously both oxidised and reduced. The reaction of chlorine with water is a key example.
True or False?
The reaction of chlorine with water is a disproportionation reaction.
True.
Chlorine (oxidation state 0) is simultaneously oxidised to +1 in HClO and reduced to −1 in HCl, so the same species undergoes both changes.
What is the equation for chlorine reacting with water?
Cl2 (aq) + H2O (l) ⇌ HCl (aq) + HClO (aq)
The product HClO (chloric(I) acid) is the active sterilising agent that kills bacteria.
Cl2 (aq) + H2O (l) ⇌ .......... (aq) + HClO (aq)
Cl2 (aq) + H2O (l) ⇌ HCl (aq) + HClO (aq)
This is a disproportionation reaction — chlorine is both oxidised and reduced.
Why are the benefits of chlorinating water considered to outweigh the risks?
Chlorination kills bacteria that cause serious disease, and this public health benefit is considered greater than the low risks of toxic effects or chlorinated hydrocarbon formation.
True or False?
The mixture of NaCl and NaClO formed when chlorine reacts with sodium hydroxide is used as bleach.
True.
The NaCl/NaClO mixture is the active ingredient in bleach and is used to disinfect surfaces and kill bacteria.
What is the silver nitrate test (for halides)?
A silver nitrate test (for halides) is a qualitative test in which acidified AgNO3 solution is added to an unknown solution to identify halide ions by the colour of the precipitate formed.
True or False?
Silver iodide produces a white precipitate when AgNO3 solution is added to a solution containing iodide ions.
False.
Silver iodide (AgI) produces a yellow precipitate. The precipitates are: AgCl = white, AgBr = cream, AgI = yellow.
What are the colours of the precipitates formed when AgNO3 is added to solutions of Cl−, Br− and I−?
AgCl = white, AgBr = cream, AgI = yellow. Dilute then concentrated ammonia is used as a follow-up test to confirm which halide is present.
AgCl dissolves in .......... ammonia. AgBr dissolves only in .......... ammonia. AgI does not dissolve in either.
AgCl dissolves in dilute ammonia. AgBr dissolves only in concentrated ammonia. AgI does not dissolve in either.
This allows the three halides to be distinguished.
How do you test for sulfate ions in an unknown solution?
Acidify the sample with dilute HCl then add aqueous barium chloride solution. A white precipitate of BaSO4 (s) confirms sulfate ions are present.
True or False?
Dilute HCl is added before AgNO3 in a halide test to prevent false positives from carbonate ions.
True.
Carbonate ions would form a precipitate with Ag+ ions. Adding dilute HCl first removes CO32− ions by reacting them to form CO2 gas, eliminating the false positive.
What gas is produced when ammonium ions are heated with aqueous sodium hydroxide?
Ammonia gas (NH3) is produced. It can be identified by its pungent smell or by turning damp red litmus paper blue.
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