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Complex Ions

Complex Ions. and stuff like that. Complex Ions. A complex ion is the exception to a rule... It is an ionic compound that has an overall charge. Complex ions are identified using square brackets. [ ]. Complex Ions. Are soluble. Usually formed from solutions containing precipitates.

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Complex Ions

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  1. Complex Ions and stuff like that.

  2. Complex Ions A complex ion is the exception to a rule... It is an ionic compound that has an overall charge. Complex ions are identified using square brackets. [ ]

  3. Complex Ions Are soluble. Usually formed from solutions containing precipitates. There are 7 you need to memorise.

  4. Complex Ions • Formed if a precipitate disappears when excess reagent is added. • Metal cations with several ligands attached. • Ligands have a pair of non-bonding electrons e.g. H20, NH3, OH-, SCN- • Usually* the number of ligands is twice the charge on the cation. E.g. Cu2+ forms [Cu(NH3)4]2+ * Except Al3+

  5. Complex ions

  6. Making "OH" Complexes [Al(OH)4]- [Zn(OH)4]2- [Pb(OH)4]2- These hydroxide complexes are made by adding excess hydroxide to a hydroxide precipitate.

  7. Making NH3 Complexes [Cu(NH3)4]2+ [Ag(NH3)2]+ [Zn(NH3)4]2+ These ammonia complexes are made by adding excess (ammonia) ammonium hydroxide to a hydroxide precipitate.

  8. [Al(OH)4]- Aluminium ions + Sodium hydroxide = Aluminium hydroxide ppt - Al(OH)3 Adding excess Sodium hydroxide = Al(OH)3 + OH- --> [Al(OH)4]-

  9. [Cu(NH3)4]2+ When used in small amounts OH- react, when used in large amounts (excess) NH3 reacts. Copper ions + Ammonia = Copper hydroxide ppt - Cu(OH)2 Adding excess Ammonia = Cu(OH)2 + 2OH- --> [Cu(NH3)4]2+

  10. add 2 drops of dilute NaOH solution Cloudy (precipitate); Fe(OH)3 is NOT soluble orange precipitate formsFe3+ New sample add 2 drops KSCN solution dark red solution confirms Fe3+ Clear solution: Product is soluble

  11. Precipitate is formed (solution becomes cloudy) Example: Mixing Iron(III)nitrate solution with sodium hydroxide solution A compound has been formed (from ions present) that is not soluble. Example: Iron3+ ions form a compound with OH- ions. The compound is not charged. Use swap and drop rule to find formula for compound. Example: Fe(OH)3 Solution changes colour and/or becomes clear. Example: Iron(III)hydroxide dissolves with potassium thiocyanate solution (red) A complex ion has been formed (from ions present) that is soluble. Example: Iron3+ ions form complex ions with SCN- ions (thiocyanate). The complex ion is charged. Learn formulae for 7 complex ions (square brackets). Example: [Fe(SCN)]2+

  12. Precipitate is formed (solution becomes cloudy) Example: Mixing Iron(III)nitrate solution with sodium hydroxide solution A compound has been formed (from ions present) that is not soluble. Example: Iron3+ ions form a compound with OH- ions. The compound is not charged. Use swap and drop rule to find formula for compound. Example: Fe(OH)3 Solution changes colour and/or becomes clear. Example: Iron(III)hydroxide dissolves with potassium thiocyanate solution (red) A complex ion has been formed (from ions present) that is soluble. Example: Iron3+ ions form complex ions with SCN- ions (thiocyanate). The complex ion is charged. Learn formulae for 7 complex ions (square brackets). Example: [Fe(SCN)]2+

  13. litmus stays red Na+ add NaOH solution, heat, test gas with red litmus. add 2 drops of dilute NaOH solution. no precipitateNH4+, Na+ litmus goes blue NH4+ white precipitate formsAl3+, Zn2+, Pb2+, Mg2+, Ba2+ add excessNaOH solution precipitate disappearsAl3+, Zn2+, Pb2+ add 2 drops, thenexcess NH3 solution precipitate remainsMg2+, Ba2+ add dilute H2SO4 solution white precipitate forms and disappears, Zn2+ white precipitate formsAl3+, Pb2+ colourless solutionMg2+ white precipitateBa2+ add dilute H2SO4 solution green precipitate formsFe2+ white precipitatePb2+ colourless solutionAl3+ add 2 drops KSCN solution orange precipitate formsFe3+ dark red solution confirms Fe3+ blue precipitate then deep blue solution Cu2+ add 2 drops, then excessNH3 solution blue precipitate formsCu2+ brown precipitate then colourless solution Ag+ add 2 drops, then excessNH3 solution brown precipitate formsAg+ Find the steps in the scheme where the 7 complex ions are formed. Hint: Look out for “excess” amounts added and the disappearance of a precipitate.

  14. precipitate disappearsAl3+, Zn2+, Pb2+ add excess NaOH solution Equations: Al3+ + 4 OH- → [ ] Zn2+ + 4 OH- → [ ] Pb2+ + 4 OH- → [ ] Complete the equations and work out the charge! Al(OH)4- Zn(OH)42- Pb(OH)42-

  15. White precipitate forms and disappearsZn2+ add 2 drops, then excess NH3 solution Complete the equation and work out the charge! Equation: Zn2+ + 4 NH3 → [ ] Zn(NH3)42+

  16. add 2 drops KSCN solution Dark red solution confirms Fe3+ Complete the equation and work out the charge! Equation: Fe3+ + SCN- → [ ] FeSCN 2+

  17. Blue precipitate, then deep blue solutionCu2+ add 2 drops, then excess NH3 solution Complete the equation and work out the charge! Equation: Cu2+ + 4 NH3 → [ ] Cu(NH3)42+

  18. Brown precipitate, then colourless solutionAg+ add 2 drops, then excess NH3 solution Complete the equation and work out the charge! Equation: Ag+ + 2 NH3 → [ ] Ag(NH3)2+

  19. Written questions for end of assessment • You will be given a list of observations • You need to methodically follow the flow charts and make conclusions and equations as if you were actually carrying it out in real life. • Eg: Question 1 • No ppt with silver nitrate means sulfate or nitrate ion • White precipitate with barium nitrate means it must be sulfate ion • Ba2+(aq) + SO42-(aq) BaSO4(s)

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