QA_for students

Chapter 11 Qualitative Analysis

Chapter 11 Qualitative Analysis 11.1 Test for Cations 11.2 Test for Anions 11.3 Test for Gases

11.1 Test for Cations ● Learning Outcome(s) ● Describe the use of aqueous sodium hydroxide and/or aqueous ammonia to identify aqueous cations through the formation of precipitates and their subsequent solubility.

11.1 How Do We Test for Cations? ● The Singapore Food Agency (SFA) constantly checks the amount of heavy metals in the food we eat in Singapore to make sure it’s safe. Textbook Page 190

11.1 Test for Cations Reagents used to identify cations: ● Aqueous sodium hydroxide, NaOH(aq) ● Aqueous ammonia, NH3(aq) ❑ A cation can be identified by noting: ● the colour of the precipitate produced, if any, when the reagent is added (add dropwise first); ● whether the precipitate is soluble or insoluble in an excess of the reagent; and ● whether ammonia gas is evolved on the addition of aqueous sodium hydroxide solution on warming. ❑ These are also the observations we note down when we add NaOH and NH3. Textbook Page 190

11.1 Test for Cations ❑ Reagents used to identify cations: ● aqueous sodium hydroxide, NaOH(aq) ● aqueous ammonia, NH3(aq) ❑ Most cations give precipitates with alkalis, NaOH(aq) and NH3(aq), except Na+, K+ and NH4 +. ❑ When NaOH or NH3 is added to a solution containing cations, the precipitate formed is the hydroxide of the metal ion. ❑ e.g. Cu2+(aq) + 2OH−(aq)→ Cu(OH)2(s) ❑ Recall most hydroxides are insoluble except those of Na+, K+ and NH4 +. Textbook Page 190

11.1 Test for Cations ❑ Reagents used to identify cations: ● aqueous sodium hydroxide, NaOH(aq) ● aqueous ammonia, NH3(aq) ❑ Most cations give precipitates with alkalis, NaOH(aq) and NH3(aq), except Na+, K+ and NH4 +. ❑ When NaOH or NH3 is added to a solution containing cations, the precipitate formed is the hydroxide of the metal ion. ❑ e.g. Cu2+(aq) + 2OH−(aq)→ Cu(OH)2(s) ❑ Recall most hydroxides are insoluble except those of Na+, K+ and NH4 +. Textbook Page 190 How does NH3 form hydroxides with metal ions? Aqueous ammonia partially ionises in water NH3(aq) + H2O(l) ⇌ NH4 + (aq) +OH−(aq)

11.1 Test for Cations Cation Aqueous Sodium Hydroxide, NaOH(aq) Aqueous Ammonia, NH3(aq) On Adding a Few Drops On Adding Excess On Adding a Few Drops On Adding Excess Aluminium ion, Al3+ White precipitate of aluminium hydroxide, Al(OH)3 Precipitate dissolves in excess → colourless solution White precipitate of aluminium hydroxide, Al(OH)3 Precipitate is insoluble in excess. Zinc ion, Zn2+ White precipitate of zinc hydroxide, Zn(OH)2 Precipitate dissolves in excess → colourless solution White precipitate of zinc hydroxide, Zn(OH)2 Precipitate dissolves in excess → colourless solution Lead ion, Pb2+ White precipitate of lead(II) hydroxide, Pb(OH)2 Precipitate dissolves in excess → colourless solution White precipitate of lead(II) hydroxide, Pb(OH)2 Precipitate insoluble in excess Textbook Page 191 Al3+ (aq) + 3