chapter 8 - reactivity trends

• reducing agents are substances that donate electrons to other species
• group 2 reaction with oxygen (Mg)

  2Mg + O2 -> 2MgO

  magnesium oxidised

  oxygen reduced
• group 2 reactions with water (Sr)

  Sr + 2H2O -> Sr(OH)2 + H2

  strontium is oxidised

  hydrogen is reduced
• group 2 reactions with dilute acids (Mg and HCl)

  Mg + 2HCl -> MgCl2 + H2

  magnesium is oxidised

  hydrogen is reduced
• metal + acid -> salt + hydrogen
• reactivity increases as you descend group 2, as ionisation energy decreases

  electrons are easier to remove
• group 2 oxides + water (ca)

  CaO + H2O -> Ca2+ + 2OH-
• solubility of group 2 hydroxides

  • increases down the group
• group 2 uses

  • calcium hydroxide neutralises acid soil
  • magnesium hydroxide (milk of magnesia) or calcium carbonate treats acid indigestion
• the halogens = group 7
• all halogen molecules exist as diatomic molecules
• halogen trends in boiling point

  • increases down the group
  • more electrons
  • stronger london forces
  • more energy required to break the intermolecular forces
• halogens are oxidising agent
• group 2 are reducing agents
• halogen-halide displacement reaction

  • Cl + I -> chlorine displaces iodine (goes violet - as iodine is released)
  • Cl + Br -> chlorine displaces bromine (goes orange - as bromine is released)
  • Br + I -> bromine displaces iodine ( goes violet - as iodine is released)
• in halogen-halide reaction, cyclohexane is added to see a layer of the colour at the top
• chlorine and bromine ionic equation

  Cl2 + 2Br- -> "Cl- + Br2
• fluorine is a pale yellow gas reacts with almost any substance
• astatine is the least reactive halogen
• halogen trend in reactivity down

  • atomic radius increases
  • more inner shells so shielding increases
  • less nuclear attraction to capture an electron form another species
  • reactivity decreases
• disproportionation is a redox reaction where the same element is reduced and oxidised
• reaction of chlorine and water

  Cl2 + H2O -> HClO + HCl
• reaction of chlorine with cold, dilute aqueous sodium hydroxide

  Cl2 + 2NaOH -> NaClO + NaCl + H2O
• chlorine use

  • benefits - bleach and kills bacteria
  • risks - irritant to lungs, can form chlorinated hydrocarbons which may cause cancer
• halogen, precipitation reactions with aqueous silver ions

  Ag+ + X- -> AgX
• test for carbonate

  Na2CO3 + 2HNO3 -> 2NaNO3 + CO2 + H2O

  1. in a test tube, add dilute nitric acid to the solid or solution to be tested
  2. if you see bubbles, the unknown compound could be a carbonate
  3. bubble the gas through limewater (calcium hydroxide). carbon dioxide will react to form a white precipitate
  4. equation -
  5. CO2 + Ca(OH)2 -> CaCO3 + H2O
• test for sulfate ions

  Ba2+ + SO42- -> BaSO4

  add barium nitrate, if sulfate is present, a white precipitate will form
• tests for halides

  Ag+ + X- -> AgX

  1. add aqueous silver nitrate, to an aqueous solution of a halide
  2. the silver halide precipitates are different colours - iodine - yellow, chlorine - white, bromine - cream
  3. add aqeuous ammonia to tets the solubility of the precipitate. chloride is soluble in dilute ammonia, bromide is soluble in concentrated ammonia and iodide is insoluble
• sequence of tests

  1. carbonate
  2. sulfate (will also get a white precipitate with a carbonate)
  3. halides (will also get a precipitate with carbonate and sulfate)
• mixture of ion testing

  1. carbonate test - continue until no more bubbles
  2. add excess barium nitrate, sulfate will precipitate out when filtered
  3. halide test
• test for ammonium ion

  NH4+ + OH- -> NH3 + H2O

  1. aqueous sodium hydroxide is added to a solution of ammonium ion
  2. ammonia gas is produced, you are unlikely to see bubbles as it is soluble
  3. mixture is warmed and ammonia gas released
  4. you may be able to smell ammonia, but test with a moist pH indicator paper, ammonia will turn the paper blue (alkaline)