C10 group 7 elements

• group 7 elements exist as diatomic molecules
• group 7 elements are called the halogens
• atomic radius increases down group 7 due to increasing numbers of electron shells
• reactivity decreases down group 7 because shielding increases so nuclear attraction decreases, making it harder to attract an electron
• melting and boiling point increase down group 7 because atoms get larger so have more electrons, giving them stronger van der Waals forces
• electronegativity decreases going down group 7 because atoms get larger so outer electrons are further from the nucleus, so shielding is increased
• halogens react by gaining electrons to become negative ions with a charge of -1
• halogens take part in redox reactions where they are reduced, so act as oxidising agents
• the oxidising ability of the halogens increases going up the group
• fluorine is the most powerful oxidising agent

  iodine is the least powerful oxidising agent
• halogens react with metal halides in aqueous solution, these are displacement reactions, the halide in the compound will be displaced by a more reactive halogen, but not by a less reactive halogen
• in the displacement reaction between halogens and metal halides, the metal ions are spectator ions, the halogen is an oxidising agent, and the halide ions are oxidised as they lose electrons to become stable halogens
• a halogen will always displace the ion of a halogen below it in the periodic table
• the reactions of fluorine in aqueous solution cannot be tested because it reacts with water
• halide ions can act as reducing agents, as they themselves are oxidised
• the reducing ability of halide ions increases down the group
• halide ion reducing ability increases down the group because ions are larger so there is more shielding and outer electrons are further from the nucleus, so it is easier to lose electrons
• equation for sodium fluoride reacting with conc sulfuric acid:

  NaF + H2SO4 -> NaHSO4 + HF
• sodium fluoride reacting with conc sulfuric acid is not a redox reaction because fluoride ions are weak reducing agents so cannot reduce the sulfur, instead it is an acid-base reaction
• equation for sodium chloride reacting with conc sulfuric acid:

  NaCl + H2SO4 -> NaHSO4 + HCl
• sodium chloride reacting with conc sulfuric acid is not a redox reaction because chloride ions are weak reducing agents so cannot reduce the sulfur, instead it is an acid-base reaction
• equation for sodium bromide reacting with conc sulfuric acid:

  NaBr + H2SO4 -> NaHSO4 + HBr

  2H+ + 2Br- + H2SO4 -> SO2 + 2H2O + Br2
• sodium bromide reacting with conc sulfuric acid is first an acid-base reaction, then the bromide ions are strong enough reducing agents to reduce the sulfur to sulfur dioxide, this is redox because sulfur oxidation state goes from +6 to +4 and bromine oxidation state goes from -1 to 0
• sodium iodide reacting with conc sulfuric acid is first an acid-base reaction, then the iodide ions are strong enough reducing agents to reduce the sulfur to hydrogen sulfide, this is redox because sulfur oxidation state goes from +6 to -2 and iodine oxidation state goes from -1 to 0
• acidified silver nitrate is used to test for halide ions in aqueous solution, as it forms a precipitate of insoluble silver halide which is coloured
• the test for halide ions is:

  • add dilute nitric acid HNO3 and silver nitrate AgNO3
  • results: Cl = white ppt, Br = cream ppt, I = yellow ppt
  • add dilute ammonia NH3
  • results: Cl ppt dissolves, Br and I ppts do not dissolve
  • add concentrated ammonia NH3
  • results: Cl and Br ppts dissolve, I ppt does not dissolve
• to test for halide ions, you must first add nitric acid because it removes any carbonate or hydroxide ions which would interfere with the reaction by forming silver carbonates or silver hydroxides
• chlorine is used to purify water for drinking and in swimming pools
• HClO is chloric acid which is an oxidising agent that kills bacteria by oxidation, it is also a bleach
• chlorine reacts with cold water to produce a mixture of chlorate (I) ions and chloride ions
• the equations for chlorine reacting with cold water are:

  Cl2 + H2O <-> HClO + HCl

  Cl2 + H2O <-> ClO(-) + Cl(-) + 2H(+)
• chlorine reacting with cold water is a disproportionation reaction as the chlorine is both oxidised and reduced, it goes from an oxidation state of 0 to -1 and +1
• in sunlight, chlorine is rapidly lost from swimming pools so it needs to be added frequently
• in the presence of sunlight/UV light, chlorine reacts with water by decomposing it into oxygen and hydrochloric acid
• the equation for chlorine reacting with water in the presence of sunlight/UV light is:

  2Cl2 + 2H2O -> 4HCl + O2
• an alternative to continuously chlorinating swimming pools is to add sodium or calcium chlorate (I)
• the equation for sodium chlorate reacting with water is:

  NaClO + H2O -> Na(+) + OH(-) + HClO
• if sodium chlorate (I) is added to a swimming pool, the pool then needs to be kept slightly acidic because this stops the equilibrium from shifting to the left and maintains production of HClO
• chlorine reacts with cold, dilute sodium hydroxide to form sodium chlorate (I) NaClO
• the equation for chlorine reacting with sodium hydroxide is:

  Cl2 + 2NaOH -> NaClO + NaCl + H2O