3.1.3

Halogen’s physical properties

(a)→halogens: at room temp exist as diatomic molecules [molecules made up of 2 atoms e.g. F2]; are simple molecular structures with weak London forces between diatomic molecules caused by induced dipoles [image]

→boiling point increases down group: more electrons in molecules so greater induced dipoles; stronger london forces; more difficult to separate larger molecules

Redox reactions and reactivity of halogens and compounds

(b)→halogens outer shell: 7 valence e- in form _s2 _p5; they react to form 1- ion in redox reactions [reduced as gain 1 extra e-] [are oxidising agents]

(c)→reactivity decreases down the group: so more reactive ones displace less reactive ones

→colour change of halogens in aq solutions: chlorine [pale green solution] displaces both bromine [yellow] and iodine [brown] {image}

→colour change of halogens in organic solutions: chlorine [colourless] displaces both bromine [yellow] and iodine [purple] [image]

(d)→trend in reactivity explained: oxidising power decreased down group so less reactive as;

  • atom radi increased

  • more shiedling

  • less nuclear attraction to attract electron from another species

(e)(i)→disproportionation reaction: where same species is both oxidised and reduced in a reaction e.g. chlorine with h20 and dilute aq alkali

→e.g. chlorine w water: used to clean water and make it drinkable; chlorine is oxidised [lose 1e-] in HClO and reduced in HCl [gain 1e-] [image]

  • HClO sterilises water by killing bacteria

  • HClO dissociates in water to form H+ and ClO- which further cleans water

  • universal indicator: red as acid then colourless as HClO bleaches colour

(ii)→e.g. chlorline w cold,dilute,aq NaOH: [image] chlorine oxidised [lost 1e-] in NaClO and reduced in NaCl [gain 1e-]

  • mixture of NaCl and NaClO used as bleach to kill bacteria

(f)→pros vs cons of adding chlorine to water:

  • pros: killing bacteria with sterilising agents formed by Cl2

  • cons: possible toxic by products e.g. cl2 gas from formation of chlorinated hydrocarbons

  • benefits outweighs risks

Characteristic reactions of halide ions

(g)→idenitifying halide ions: dissolve solution in nitric acid [to prevent false positives]; then adding silver nitrate solution forming precipitate of silver halide Ag+ (aq) + X- (aq) → AgX (s)

→what halides form what colour precipitates:

  • Silver chloride (AgCl) is a white precipitate

  • Silver bromide (AgBr) is a cream precipitate

  • Silver iodide (AgI) is a yellow precipitate

→adding dilute ammomia solution to precipitates:

  • AgCl dissolves

  • AgBr remains insoluble

  • AgI remains insoluble

→adding concentrated ammonia solution to precipitates:

  • AgCl dissolves

  • AgBr dissolves

  • AgI remains insoluble