Lecture 16. The Halogens. PhD Halina Falfushynska. Group 17 elements properties. The Group 17 elements are known as the halogens. All are non-metals. • The elements exist as diatomic molecules, X2.
PhD Halina Falfushynska
• Astatine is radioactive, with the longest lived isotope having a half life of only several hours.
4NaCl + MnO2 + 4H2SO4 → MnCl2 + 4NaHSO4 + 2H2O + Cl2
Electrolytic process: Chlorine is obtained by the electrolysis of brine (concentrated NaCl solution).
Cathode: 2 H+ (aq) + 2 e− → H2 (g)
Anode: 2 Cl− (aq) → Cl2 (g) + 2 e−
2NaBr + H2SO4 + H2O2 = Br2 + Na2SO4 + 2H2O
They react with metals to form halides. The reactivity of the halogens decreases down the group.
Mg (s) + Br2 (l) → MgBr2 (s)
2Al + 3Cl2 → 2AlCl3
The ionic character of the halides decreases in the order MF > MCl > MBr > MI where M is a monovalent metal.
2Na + Cl2 2NaCl
Halogensformmanyoxideswithoxygenbutmostofthemareunstable. Fluorineformstwooxides OF2and O2F2. Botharestrongfluorinatingagents. O2F2oxidisesplutoniumto PuF6andthereactionisusedinremovingplutoniumas PuF6fromspentnuclearfuel.
Chlorine, bromineandiodineformoxidesinwhichtheoxidationstatesofthesehalogensrangefrom +1 to +7. Orderofstabilityofoxidesformedbyhalogens, I > Cl > Br. Thehigheroxidesofhalogenstendtobemorestablethanthelowerones.
Halogens combine amongst themselves to form a number of compounds known as interhalogens of the types XX ′ , XX3′, XX5′ and XX7′ where X is a larger size halogen and X’ is smaller size halogen
Halogensreactwith a numberof non-metalstoformhalides
P4 + 6Cl2 → 4PCl3
S8 + 4Cl2 → 4S2Cl2
H2 + Cl2 → 2HCl
Reaction of read Phosphorus with Bromine. The oxidation of red phosphorus with bromine proceeds under combustion.
2NaOH + Cl2 → NaCl + NaOCl + H2O(cold and dilute)6 NaOH + 3Cl2 → 5NaCl + NaClO3 + 3H2O(hot and conc.)With dry slaked lime it gives bleaching powder.2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O
Ca(OH)2 + Cl2 → CaOCl2 + H2O
When chlorine is inhaled at concentrations above 30 ppm, it begins to react with water and cells, which change it into hydrochloric acid (HCl) and hypochlorous acid (HClO) - Chlorine water.
Cl2 + H2O → HCl + HClO
Oxidation of Iodide with Halogenates. The formation of iodine by oxidation of iodide with halogenates is dependent on the pH value.
A halogen oxidises halide ions of higher atomic number.F2 + 2X– → 2F– + X2 (X = Cl, Br or I)Cl2 + 2X– → 2Cl– + X2 (X = Br or I)Br2 + 2I– → 2Br– + I2
2F2 (g) + 2H2O (l) → 4H+ (aq) + 4F− (aq) + O2 (g)X2 (g) + H2O (l) → HX (aq) + HOX (aq)( where X = Cl or Br )4I− (aq) + 4H+ (aq) + O2 (g) → 2I2 (s) + 2H2O (l)
NaCl + H2SO4 → NaHSO4 + HCl (420K)
PCl5 + H2O → POCl3 + 2 HCl
2PBr3 + 6H2O → 2H3PO3 + 6HBr
Hydrochloric acid decomposes salts of weaker acids, e.g., carbonates, hydrogencarbonates, sulphites, etc.Na2CO3 + 2HCl → 2NaCl + H2O + CO2NaHCO3 + HCl → NaCl + H2O + CO2Na2SO3 + 2HCl → 2NaCl + H2O + SO2
Uses of Bromine
The main use of fluorine is toothpaste even though it isn’t as fluorine itself but instead as fluoride, a compound of fluorine.
Bromine is one of the main ingredients in camera films (as silver bromide).
---for bleaching woodpulp (required for the manufacture of paper and rayon), cotton and textiles
--in the extraction of gold and platinum
--in the manufacture of dyes, drugs and organic compounds such as CCl4, CHCl3, DDT, refrigerants, etc.
--in sterilising drinking water
---preparation of poisonous gases such as phosgene (COCl2), tear gas (CCl3NO2), mustard gas (ClCH2CH2SCH2CH2Cl).
Chlorine gas has also been used by insurgents against the local population and coalition forces in the Iraq War in the form of chlorine bombs. On March 17, 2007, for example, three chlorine filled trucks were detonated in the Anbar province killing two and sickening over 350.
When dissolved in water, iodine can be used as a strong antiseptic or as a test for starch.