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

Sodium Na+ Lead (II) Pb2+

Potassium K+ Zinc Zn2+

Silver Ag+ Iron (II) Fe2+

ammonium NH4+ Manganese (II) Mn2+

Magnesium Mg2+ aluminium Al3+

Calcium Ca2+ chromium Cr3+

Barium Ba2+ Iron (III) Fe3+

Copper (II) Cu2+

Cations

Chloride Cl- chromate CrO42-

Bromide Br- sulfate SO42-

Iodide I- Oxide O2-

hydroxide OH- Sulfide S2-

nitrate NO3- Carbonate CO3

2-

permanganate MnO4- dichromate Cr2O7

2-

Anions

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?

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

The solubility rulesThe solubility rulesNitrates NO3

-

Chlorides and Iodides

Cl- and I-

Sulfates SO4

2-

Carbonates CO3

2-

Hydroxides OH-

All soluble

All soluble except

All soluble except

All insoluble except

All insoluble except

AgCl

AgI

PbSO4 Na2CO3 NaOH

PbCl2Pbl2

CaSO4 K2CO3 KOH

BaSO4 (NH4)2CO3

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.

1)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) + CO3

2- (aq) CaCO3(s)

Write the ionic equation for the reaction of an aqueous solution of CaCl2 and an aqueous solution of Na2CO3.

Molecular, ionic and net ionic equations

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)

The net ionic equation is:

Cl-(aq) + Ag+(aq) AgCl(s)

KCl(aq) + AgNO3(aq) KNO3(aq)+ AgCl(s)

If we add a solution of KCl to a AgNO3 solution will a precipitate form? First write an equation.

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?

Molecular, ionic and net ionic equations

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+

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 or OH-.

• 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)

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)

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)

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+

Complex ions