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Notes on Qualitative Analysis
Qualitative analysis involves separating a mixture of cations based on their solubilities. In the
traditional QA scheme, the Group 1 cations (not the periodic table group 1) are separated from a
mixture of dozens of cations based on their insolubility as chloride salts. Hence the Group 1
cations consist of silver, lead(II) and mercury(II) ions which all form insoluble chlorides. The
Group II cations are those that form insoluble sulfides in acidic solution, while the Group III
cations form insoluble sulfides in basic solution, and so forth. We will not follow the tradition
scheme but use parts of it.
Let’s first review some basic ideas on salts and solubility. Recall that salts are ionic compounds.
Ionic compounds are made from metals reacting with nonmetals. In this reaction, a metal loses
electron(s) and is thus oxidized to form a positive ion. Positive ions are termed cations.
Nonmetals gain electron(s) are thus reduced to form negative ions. Negative ions are termed
anions. Hence salts are combinations of cations and anions held together by electrostatic forces
of attraction (the positive and negative charges of the ions) in a crystal lattice structure in the
solid state.
What happens when we dissolve a salt in water?
Recall that water is a polar molecule with the oxygen end having a partial negative charge and
the hydrogen end having a partial positive charge because of the polarity of the H-O bond.
Hydrogen and oxygen are both nonmetals and nonmetals react by sharing electrons to form
bonds called covalent bonds. Since the oxygen atom has a higher electronegativity compared to
hydrogen, the electrons are shared unequally with oxygen “hogging” the shared pair resulting in
a polar covalent bond. Since the shape of the molecule is asymmetric (bent) the molecule as a
whole is polar (also called a dipole).
When ionic salts dissolve in water, the polar water molecules break apart the ionic lattice
crystal such that the partial negative oxygen ends of water molecules surround the cations while
the partial positive hydrogen ends surround the anions. This results in hydrated or aqueous ions,
which allow the resulting solution to conduct electricity. For example, let’s write the reaction for
the dissociation of a salt in water:
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NaCl (s)  Na+(aq) + Cl-(aq)
Let’s review writing equations for the dissociation of salts in water:
a) MgCl2(s) 
b) AlBr3(s) 
c) (NH4)2SO4(s) 
Those salts that dissociate into aqueous ions when placed in water are said to be soluble. Those
salts that do not dissociate into aqueous ions (or dissociate to a very small extent – not enough to
conduct an electric current) are said to be insoluble. For now we won’t be too concerned about
the solubility rules but you should know for starters that all group 1 salts and all nitrate salts are
soluble. All chloride salts are soluble except those containing Ag+, Pb2+ and Hg2+.
Double Replacement Reactions
Recall from sophomore chemistry that mixing two aqueous solutions will result in a double
replacement reaction (switch dance partners) if a solid forms (an insoluble salt termed a
precipitate), or if water or a gas forms. For now we will concern ourselves with precipitates.
Let’s try the following:
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a) NaCl(aq) + AgNO3(aq) 
b) BaCl2(aq) + Na2SO4(aq) 
c) Cu(NO3)2(aq) + NaOH (aq) 
Complex Ions
Let’s start with a reaction and review as we go along. Recall the ammonia (NH 3) is considered a
weak base. According to one definition of a base, the Bronsted-Lowery definition, a base is a
proton (H+) acceptor. What does ammonia do in water?
NH3(g) + HOH(l) =
It accepts a proton from water to form ammonium ion and hydroxide ion. Since ammonia
produces hydroxide ions (OH-) in water, the solution has an excess of OH- ions and is thus
considered to be basic. Notice this is an equilibrium reaction, but when equilibrium is
established the reactant side is much more favored. That is, the reaction between ammonia and
water occurs only to a very small extent, resulting in a fairly low concentration of ammonium
ions and hydroxide ions. That is why ammonia is considered a weak base (and a weak
electrolyte).
Now to this ammonia solution we add a solution of copper(II)nitrate. Aqueous copper(II)nitrate
consists of what aqueous ions?
Now the Cu2+(aq) ions will combine with the OH-(aq) ions to form an insoluble solid:
Cu2+(aq) + 2OH-(aq)  Cu(OH)2(s)
This is as expected. What happens to the NH4+ ions and the NO3- ions? What are they called?
Now if we continue to add aqueous ammonia in excess something unexpected and interesting
happens!
WOW!
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The solid precipitate dissolves! How can this be?
To answer this we need to consider another model of acids and bases – the Lewis model. In the
Lewis model acids are considered to be electron pair acceptors and bases are electron pair
donors.
How can we recognize a Lewis acid? These are cations and electron deficient species (most
typically boron trichloride or boron triflurode). Examples are Al3+, Cu2+, Zn2+, Fe3+, etc and
BCl3 or BF3. We need not concern ourselves with the boron compounds.
How can we recognize a Lewis base? These are anions (like Cl-, OH-, CN-) or molecules with
lone electron pairs to donate, typically H2O and NH3. (Recall the oxygen in water has two lone
pairs while the nitrogen in ammonia has one lone pair.
To understand what happens in the reaction we need to also review LeChatelier’s principle for
systems at equilibrium: If a stress is applied to an equilibrium system, the system moves (shifts)
in the direction that relieves the stress. Consider our equilibrium system:
Cu(OH)2(s) = Cu2+(aq) + 2OH-(aq)
The addition of excess ammonia results in a Lewis acid-base reaction in which the Cu2+ (Lewis
acid) now reacts with the NH3 (the Lewis base) to form the complex ion Cu(NH3)42+ which is
soluble. As the NH3 reacts with the Cu2+, the concentration of Cu2+ decreases and the above
equilibrium will shift right to produce more Cu2+. This means that the solid Cu(OH)2 must
dissociate; in other words the precipitate dissolves as a result of the addition of excess ammonia.
The Lewis acid-base reaction is:
Cu2+(aq) + 4NH3(aq) = Cu(NH3)42+(aq)
For our purposes in our qualitative analysis scheme, the formation of complex ions can be used
to selectively dissolve solids (precipitates).
How do we know the formula of the complex ion? As a rule of thumb the number of Lewis
bases that reacts with the Lewis acid is double the charge on the Lewis acid (cation). In the
above case the charge on the Lewis acid Cu2+ is +2 so we double that and add four ammonia.
Ammonia is a neutral molecule so the overall charge on the complex ion is +2 (from the charge
of the copper ion). The Lewis base in the complex ion is also referred to as a ligand. By the
way, a common chemical test for the presence of copper(II) ion is the addition of ammonia. If
copper(II) ion is present, it will complex with the ammonia to form the Cu(NH3)42+ complex
ion, which has a characteristic deep blue color. Note: not all cations form complex ions.
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Naming Complex Ions
In naming complex ions,
a) prefix for the number of Lewis bases (anion) – e.g. tetra
b) designation for Lewis base - e.g amine for ammonia
c) name and charge of Lewis acid (cation) – e.g copper(II) ion
Hence the name for our complex ion, Cu(NH3)42+, is tetraamine copper(II) ion.
The following are the names of some common Lewis bases (as ligands):
ligand
name
H2O
aqua
NH3
amine
F-
fluro
Cl-
chloro
OH-
hydroxo
CN-
cyano
Earlier when we wrote the dissociation equation for some salts we showed the cation in the
aqueous phase. For example,
AlCl3(s)  Al3+(aq) + 3Cl-(aq)
Al3+(aq) actually exists in the form of a complex ion: Al(H2O)63+(aq)
Al3+ + 6H2O  Al(H2O)63+
name this complex ion: ______________________
Write the chemical formula of the complex ions formed by the following cations in water and
name them:
Fe3+
Cu2+
Co3+
Ni2+
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