THE OXIDATION STATES OF TIN
Purpose
There are two main objectives for this experiment:
1.
2.
The preparation of a tin iodide—tin(II) or tin(IV).
The analysis of the compound to determine the oxidation state of tin in the
compound.
Background
Like the transition metals, many of the main group elements also exhibit multiple
oxidation states. An example of this occurs with the element tin, which has two common
oxidation states: Sn(IV) (commonly named stannic) and Sn(II) (stannous). They are of
approximately equal stability. Comparing the oxidation states of tin with those of other Group 14
elements, we find that carbon, silicon, and germanium are nearly always found in the IV
oxidation state. Lead, however, is most often found in the II oxidation state, with the IV state
being fairly unstable. A similar trend is found in the oxidation states of other main group families
(Groups 13-16), where the primary oxidation number decreases by two going down the group. It
should be pointed out that in all cases here, we are not referring to ionic charges on the elements,
as the compounds are appreciably covalent.
The trend can be rationalized in the following way, using the Group 14 elements as
examples. The electronic configuration of the family is ns2np2, with the electrons being arranged
in orbitals as follows:
In this electronic state, the element can form two covalent bonds, using the two unpaired
electrons in the px and py orbitals. The two electrons in the s orbital are not used in bonding and
are sometimes termed an “inert pair.” This bonding is seen for Sn(II) and Pb(II).
Alternatively, one of the s electrons can be promoted to the empty pz orbital at the cost of
an absorption of energy by the element, resulting in the following electronic arrangement.
s
3
px
py
pz
Four covalent sp hybridized bonds can now be formed (which through hybridization are
equivalent—sp3) by the Group 14 element. Bond formation occurs with the release of energy.
The promotion of the s electron is thereby compensated for, to some extent, by the “return” of
bond energy.
The likelihood of achieving the IV oxidation state is therefore related to two quantities:
1.
The ease of electronic promotion: Easiest for the heavier family members, because of
the greater distance of the electron from the nucleus.
and more importantly,
2.
Bond strength: Strongest for the lighter family members, because of compact orbital
size, short bond lengths, and good orbital overlap.
In the case of carbon, silicon, and germanium, the “cost” of electron promotion is more than
compensated by the “return” of energy from the additional two bonds formed. These elements
are thus found in the IV oxidation state. In the case of tin, the “cost” and “return” are of similar
magnitude, hence, both states are of approximately equal stability. In the case of lead, the “cost”
of electron promotion is not compensated for by the small energy return from the two additional
bonds. Lead is therefore generally found in the II oxidation state, with Pb(IV) compounds being
easily reduced.
Safety
Tin (CAS No. 7440-31-5): Tin is not generally considered to be a dangerous material;
however, the usual safety precautions should be taken.
Iodine (CAS No. 7553-56-2): Iodine is harmful if swallowed, inhaled, or absorbed
through the skin. It is a lachrymating agent (makes you cry). Ingestion of 2-3 g has been fatal.
Acetic anhydride (CAS No. 108-24-7): This compound is corrosive and causes burns to
any area of contact. The liquid and vapor are flammable. It is harmful if swallowed or inhaled. It
is water reactive.
Acetic acid (CAS No. 64-19-7): This compound is corrosive and can cause severe burns.
It may be fatal if swallowed. It is harmful if inhaled. It is combustible.
Chloroform (CAS No. 67-66-3): This product may be irritating to the eyes, skin, and
respiratory tract.
Hydrochloric acid (CAS 7647-01-0): This compound is harmful if swallowed, inhaled, or
absorbed through skin. It causes burns.
Potassium iodate (CAS 7758-05-6): Strong oxidizer. Contact with other material may
cause fire. Harmful if swallowed or inhaled. Causes irritation to skin, eyes, and respiratory tract.
May affect central nervous system, blood, and kidneys with chronic exposure.
Experimental
Preparation of Tin(?) Iodide: Prepare the solvent for the reaction by adding 25 mL of
acetic anhydride to 25 mL of acetic acid in a 100-mL round-bottom flask. Add 1.0 g of granular
tin and 2.0 g of iodine. Fit the flask with a reflux condenser and gently reflux until there is no
observable violet color of iodine vapor in the flask or condenser (30-45 minutes). Rapidly filter
the hot solution away from the excess tin using a glass wool plug and a small glass funnel. Then
cool the filtrate in ice. Filter off the orange tin iodide with suction in a Buchner funnel, using
cold acetic acid as wash solution. Dry the crystals on the aspirator. Weigh your product.
Characterization of Product: (1) Obtain the MP of the product and confirm its identity
by comparing to literature values. (2) Calculate the theoretical and percent yield of the products.
(3) Prepare a 250 mL of 0.100 M solution of KIO3. Calculate its exact molarity. Add 50 mL of 6
M HCl to a 125 separatory funnel and add an amount of tin iodide which you calculate will
require about 30 mL of the iodate solution. The titration reaction is:
IO3- + 2 I- + 6 H+ + 3 Cl-  3ICl + 3 H2O
An intermediate step in this reaction involves the production of molecular iodine (so you won’t
see iodine until the titration starts). We will take the absence of iodine as the end point of the
titration. Because the color of the iodine is hard to see in a very dilute solution, we will identify
the end point by extraction. Add a few mL of chloroform to the separatory funnel. Titrate with
the iodate almost to the end point and then add the iodate slowly, shaking and venting the
separatory funnel after each addition. The end point is identified by the absence of the violet
iodine color in the chloroform layer. This is a highly sensitive but inconvenient, endpoint. We
cannot use starch as an indicator because the iodine-starch complex does not form in highly
acidic solution. Use the titration results to calculate the percent of iodine in your product and
compare this percent with the theoretical value.
Thoughts/Reflections
1.
2.
Write the oxidation and reduction half-reactions for this experiment (synthesis).
Prepare a table comparing the physical properties (MP, BP, and density) of the
chlorides of C, Si, Ge, Sn, and Pb. Are there any general trends in the table? Explain.
References (in addition to any Inorganic textbook)
1.
Moeller, T.; Edwards, D. C. Inorg. Syn. 1953, 4, 119.
Assignment
An experimental synopsis (including an abstract, experimental data, and answer from
thoughts/reflections above) is due Oct 5.