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. There are 7 you need to memorise.

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+ Al3+ forms tetra…. H/O and expt complex ions practical and theory EXPT 2 full unknowns writ and hand in steps, observations, name formula equations for ppts and balanced equations for complex ions formed – mark and return AS 2.1 takes 1.5 hours * Except Al3+

Complex ions Iron Thiocyanate FeSCN2+ Silver diamine [Ag(NH3)2]+ Aluminium tetrahydroxide [Al(OH)4]- Lead tetrahydroxide [Pb(OH)4]2- zinc tetrahydroxide [Zn(OH)4]2- Zinc tetraamine [Zn(NH3)4]2+ Copper tetraamine [Cu(NH3)4]2+ Learn all these, you are likely to be tested on those in red Fe3+ as SCN is 1- A precipitate of AgCl(s) (left) readily dissolves in an aqueous solution of NH3 because of the formation of the complex ion [Ag(NH3)2]+ (right). The moleuclar view of the final solution shows [Ag(NH3)2]+ and Cl- ions and NH3 molecules.

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.

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.

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

[Cu(NH3)4]2+ Copper ions + Ammonia = Copper hydroxide ppt - Cu(OH)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+

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

Observation What it means Species formed 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+

Observation What it means Species formed 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+

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. precipitate disappears Al3+, Zn2+, Pb2+ precipitate remains Mg2+, Ba2+ add 2 drops, then excess NH3 solution white precipitate forms Al3+, Zn2+, Pb2+, Mg2+, Ba2+ add excess NaOH solution add 2 drops of dilute NaOH solution. no precipitate NH4+, Na+ add NaOH solution, heat, test gas with red litmus. litmus stays red Na+ litmus goes blue NH4+ blue precipitate then deep blue solution Cu2+ blue precipitate forms Cu2+ brown precipitate forms Ag+ brown precipitate then colourless solution Ag+ add 2 drops, then excess NH3 solution orange precipitate forms Fe3+ green precipitate forms Fe2+ white precipitate forms Al3+, Pb2+ white precipitate forms and disappears, Zn2+ add dilute H2SO4 solution colourless solution Al3+ white precipitate Pb2+ add dilute H2SO4 solution colourless solution Mg2+ white precipitate Ba2+ add 2 drops KSCN solution dark red solution confirms Fe3+

Equations: Al3+ + 4 OH- → [ ] Zn2+ + 4 OH- → [ ] Al(OH)4 - precipitate disappears Al3+, 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)4 2- Pb(OH)4 2-

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

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

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

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

Testing for Ions Copy the grid.   CO32- OH- SO42- NO3- Mg2+ Na2+ Zn2+ Ba2+ 1. Add reagent to a sample of the solution 2. Observe carefully No precipitate Precipitate forms and remains if excess reagent is added Precipitate forms and dissolves with excess reagent (complex ion) 3. Test a fresh sample with a different reagent 4. By a process of elimination you can identify the ion

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)