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 (NH3) 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 Cu 2+ (Lewis acid) now reacts with the NH3 (the Lewis base) to form the complex ion Cu(NH3)4

2+ 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(NH 3)4

2+ 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. tetrab) 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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