Oxidation States of Tin

Introduction

The elements of Group IV A exhibit two common oxidation states (+2 and +4) with the lower

oxidation state (ll) becoming increasingly stable as the group is descended. The relative stability

of the oxidation states is the result of two factors:

(a) The energy required to promote one of the paired s valence electrons on the group IV A

element to the empty p orbital decreases down the group.

ns2 np

2 ns

1 np

3

(b) Bond strength as the group is descended, orbital overlap is less effective as the valence

orbitals become more diffuse resulting in longer and weaker covalent bonds.

For the first three group IV A elements the +IV oxidation state is predominant since the bond

energy term compensates for the required promotion energy. At tin, both factors are finely

balanced while for the lead the second factor is the dominant one.

In this experiment, tin (II) will be prepared by a metathetical reaction

Sn2+

(aq) + 2I-(aq) SnI2(s)

while tin (IV) iodide is prepared by direct

reaction.

Sn(s) + 2I2(aq) SnI4(s)

Materials

Reflux condenser

Water bath

0.8 g tin foil

3.5 g iodine

100 mL round bottom flask

Melting point apparatus

1.6 g tin, granular

25 mL of dichloromethane

PROCEDURE PART A:

Tin (IV) Iodide by direct reaction

Place 0.6g of tin granular and 2.4g of iodine crystals into a 100 mL round-bottom flask

containing a boiling stone equipped with a reflux condenser. Add 20 mL of dichloromethane

through the condenser. Gently heat the flask and contents so that mild reflux is maintained for

30-40 minutes and the violet colour of iodine vapour in the condenser disappears. Rapidly filter

the warm solution through a loose glass wool plug using a normal gravity funnel. Rinse the

reaction flask with 3 mL warm dichloromethane, pour through the same glass wool plug and

combine the dichloromethane filtrates.

Concentrate the dichloromethane to a volume of 5 10 mL (fume hood), cool the resulting

solution in an ice-water bath and collect the orange-red crystals of tin (IV) iodide. Wash with

two 2 mL portions of cold dichloromethane.

PROCEDURE PART B:

Tin (II) Iodide by metathesis

(To commence after refluxing begins in Part A)

Into a 100 mL beaker place 1.0g of powder zinc and 5 mL of distilled water. Add 1.0g of iodine

crystals and cool the beaker in an ice-water bath. Stir the reaction mixture with a glass rod. After

a few minutes a brown colour develops because of dissolved iodine. Continue cooling (the

reaction is exothermic) and stirring until all the zinc dissolves. At the end of the reaction some

unreacted zinc may remain but the solution become light yellow.

Filter the ZnI2 solution (through a plug of glass wool) into another 100 mL beaker. Rinse the

contents of the original beaker with 1 mL distilled water and transfer the washings (via the

glass wool plug) into the second beaker.

While the zinc-iodine reaction is in progress set up the preparation of the tin (II) chloride as

follows:

Add 0.8g of tin foil into a 50 mL beaker. Add 5 mL conc HCl and 1 mL 0.1M CuSO4 solution.

Heat the beaker and contents on a water bath until all the tin dissolves. Filter the tin (II) solution

(glass wool plug) and add it dropwise to the beaker containing the previously prepared ZnI2

solution. An orange yellow precipitate of SnI2 will be obtained. Cool in ice to complete

precipitation and collect the solid on a Buchner funnel. Calculate the % yield of the product and

determine its melting point.

Carry out the procedure in question 2 below before leaving the lab and clearly record your

observations in your note book!

Questions

1. Why is Tin (II) iodide a negligible product in the direct synthesis of tin (IV) iodide?

(4 marks)

2. To a mixed solution of FeCl3 and K3Fe(CN)6 add a freshly prepared solution of SnI2 and

explain your observations.

(5 marks)

3. Define the term metathetical reaction with the aid of balanced equations and an example

other than tin.

(6 marks)