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Qualitative Analysis- Carry out procedures to identify ions in solution

Qualitative Analysis- Carry out procedures to identify ions in solution. A.S. 2.2 (Chemistry 91162) 3 internal credits. Ions in Solution. An Ion is an atom which has gained or lost electrons Cations Have a positive charge Anions Have a negative charge. Cations. Anions.

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Qualitative Analysis- Carry out procedures to identify ions in solution

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  1. Qualitative Analysis-Carry out procedures to identify ions in solution A.S. 2.2 (Chemistry 91162) 3 internal credits

  2. Ions in Solution • An Ion is an atom which has gained or lost electrons • Cations • Have a positive charge • Anions • Have a negative charge

  3. Cations

  4. Anions

  5. Solubility of Ionic Compounds • Ionic compounds are soluble, insoluble or sparingly soluble depending how they dissolve in water or not. How do we know which are which?

  6. Predicting precipitation reactions using solubility rules The solubility rules are used to determine whether precipitation reactions occur or not. A precipitate is an insoluble compound formed during a chemical reaction in solution. Observe carefully: It is possible to have a coloured solution without the presence of a precipitate

  7. The solubility rules

  8. Molecular, ionic and net ionic equations Molecular Equations-complete formulas are written for all the reactants and products, no ions are written. Ionic equations-all strongly soluble electrolytes are written in their dissociated (ionized) forms. Net Ionic equations-only involve those chemical species which are involved in a chemical reaction. All spectator ions are eliminated. Spectator ions- do not participate in the chemical reaction but are present in the reaction mixture.

  9. Molecular, ionic and net ionic equations Write the ionic equation for the reaction of an aqueous solution of CaCl2 and an aqueous solution of Na2CO3. • molecular equation: CaCl2(aq) + Na2CO3(aq)CaCO3(s)+2NaCl(aq) 2)ionic equation: Ca2+(aq) + 2 Cl-(aq) + 2 Na+(aq) + CO32-(aq) CaCO3(s) + 2 Na+(aq) + 2 Cl-(aq) 3)net ionic equation: Ca2+(aq) + CO32-(aq) CaCO3(s)

  10. If we add a solution of KCl to a AgNO3 solution will a precipitate form? First write an equation. KCl(aq) + AgNO3(aq) KNO3 + AgCl Look at the solubility rules to see if any of the products are insoluble in water. AgCl is insoluble - all chlorides are soluble except for Ag+ and Pb2+ Balanced molecular equation: KCl(aq) + AgNO3(aq) KNO3(aq)+AgCl(s) KCl(aq) + AgNO3(aq) KNO3(aq)+ AgCl(s) The net ionic equation is: Cl-(aq) + Ag+(aq)  AgCl(s)

  11. Molecular, ionic and net ionic equations If we add a solution of NaNO3 to an NH4Cl solution will a precipitate form? NaNO3(aq) + NH4Cl(aq) NaCl + NH4NO3 Solubility rules: Nitrates and most chlorides are soluble so mixing these two solutions gives no precipitates. What will result if we add a solution of Pb(NO3)2 to a solution of KCl?

  12. 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+

  13. Ammonia Solution • NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) • The equilibrium means that all species are present in solution at any one time • In the presence of a cation precipitates could form with either NH3 orOH-. • When a few drops of ammonia is added to a solution of Zinc cations a white precipitate is seen. • Zn2+(aq) + 2OH-(aq) Zn(OH)2(s) • But if excess ammonia is added the precipitate dissolves again as a complex ion is formed • Zn(OH)2(s)+ 4NH3(aq) [Zn(NH3)4]2+(aq)+ 2OH-(aq)

  14. Complex Ion (exception) When a few drops of aqueous ammonia are added to silver cations a brown precipitate is formed 2Ag+(aq)+ 2OH-(aq) Ag2O(s) + H2O(l) But if excess ammonia solution is added the precipitate will redissolve Ag2O(s) + H2O(l)+ 4NH3(aq) 2[Ag(NH3)2]+(aq) + 2OH-(aq)

  15. Dilute acid solutions • Basic precipitates formed from carbonate or hydroxide ions will redissolve in the presence of dilute hydrochloric acid • Zn(OH)2(s) + 2H+ Zn2+ + 2H2O(l) • MgCO3(s) + 2H+ Mg2+ + CO2(g) + H2O(l)

  16. Complex ions

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