Chapter 9 Laboratory: Introduction to Chemical Reactions and Stoichiometry 163LABORATORY 9 .1:
oBSERvE A CompoSITIoN REACTIoN
The reaction of iron and sulfur to form iron(II)
sulfide is an example of the simplest type
of composition reaction, one in which two
elements react to form a compound. In its
simplest form, the balanced equation can be
represented as:
s) + S(s) fe( → feS(s)
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a large bolt and pipe cap (from the
hardware store) for the mortar and pestle. - You may substitute an equal mass of steel wool for the
iron filings. If necessary, wash the steel wool first to
remove soap or other contaminants. Fine steel wool
may react very vigorously. - You may substitute a propane torch for the gas burner.
- You may substitute a pair of pliers or similar gripping
tool for the beaker tongs. - You may substitute an old kitchen knife or putty knife
for the spatula.
RIREEqU d EqUIpmENT ANd SUppLIES£ fume hood or exhaust fan (or perform this
laboratory session outdoors)£ goggles, gloves, and protective clothing£ balance and weighing paper£ ring stand with ring£ large tin can lid£ beaker tongs£ test tube£ sand bed (large cake pan half filled with sand)£ large pail at least half full of water£ gas burner£ mortar and pestle£ spatula£ magnet£ iron filings (~3 g)£ sulfur (~2 g)£ concentrated hydrochloric acid (~1 mL)In fact, it’s a bit more complicated. Although one iron atom can
react with one sulfur atom to yield one molecule of iron(II) sulfide,
solid sulfur exists as a molecule that comprises eight bound
sulfur atoms. The balanced equation therefore becomes:
8 fe(s) + S 8 (s) → 8 feS(s)
But we’re not finished yet. The reaction of iron and sulfur is
an example of a class of reactions called nonstoichiometric
reactions. In a stoichiometric reaction, the proportions of the
reactants are fixed. For example, exactly one mole of sodium
(Na) reacts stoichiometrically with exactly one mole of chlorine
(Cl) to yield exactly one mole of sodium chloride (NaCl), or two
moles of aluminum (Al) react with six moles of hydrogen chloride
(HCl, hydrochloric acid) to yield two moles of aluminum chloride
(AlCl 3 ) and three moles of molecular hydrogen gas (H 2 ). In a non-
stoichiometric reaction, the proportions of the reactants are not
fixed relative to those of the product. In other words, different
amounts of iron can react with different amounts of sulfur to
yield a product in which the proportions of iron and sulfur may
vary according to the available quantities of the reactants and the
conditions under which the reaction takes place.
Three products are possible, which may occur in relatively pure
form or as a mixture. All three of these products occur naturally
as minerals, and can be produced in the lab by adjusting the
proportions of the reactants and the reaction conditions.
Troilite
Troilite is the most common natural form of iron(II) sulfide. It
has the formula FeS, and contains stoichiometrically equivalent
amounts of iron and sulfur.
pyrrhotite
Pyrrhotite has the general formula Fe(1–x)S, which indicates
that it contains a stoichiometric deficiency of iron and
therefore a stoichiometric excess of sulfur. In other words,
even though there is insufficient iron present to react