In this
experiment you will examine two fundamental laws of chemistry: the law of constant composition (Proust,
1799) and the law of mass conservation (Lavoisier, 1789). Although nowadays we take them for granted,
these ideas lay the foundation for Dalton’s atomic theory and chemistry as we
know it.
The
Synthesis of Zinc Iodide from Zinc and Iodine
The
synthesis of zinc iodide from its elements begins by adding 0.3-0.5 g of iodine
crystals to about the same amount of granular zinc (10-50 zinc mesh mixture) in
a small beaker. Five drops of 6 M
acetic acid is added to 10 mL of distilled water. Add about 3mL of this
slightly acidified water to the zinc-iodine mixture and gently swirl the beaker
to mix the reactants. In the presence of water, zinc reacts exothermically with
iodine producing zinc iodide. During this oxidation-reduction reaction, some of
the zinc atoms lose electrons and form cations while the iodine atoms gain
electrons and form anions. Solid zinc iodide cannot be seen because water
immediately dissolves the soluble product yielding aqueous zinc and iodide ions
also in an exothermic reaction.
Once
the reaction is over (it takes about 10 min), separate the unreacted zinc metal
from the aqueous solution of zinc iodide (don’t forget to save the solution –
that’s where the product is!). Make
sure all the zinc is removed from the beaker by washing with a small amount of
water which can be added to the zinc iodide solution. Try to use the minimum volume of wash water to minimize the
amount of time spent boiling off the water in the next step. Once the zinc is isolated, it can be dried
in the oven to remove all trace of water and weighed.
The
dissolved zinc iodide is isolated by boiling off the water. This should be done in an evaporating
dish. (Beakers and flasks can crack
when heated to dryness.) Be careful of
spattering that may occur near the end of the evaporation, as crystals begin to
form, trapping water underneath.
Swirling the dish to spread the water around may reduce the chance of spattering. Let cool just enough to weigh the product,
but don’t wait too long because zinc iodide will absorb water from the air,
forming a hydrate.
The
Decomposition of Zinc Iodide
Electrolysis
A
spatula tip of the zinc iodide product is placed on a watch glass and dissolved
with a few milliliters of water. Two metal electrodes connected to a 9-V
battery are then placed into the zinc iodide solution for 1-2 min. Electrolysis produces zinc metal at one
electrode while at the other a red-brown color characteristic of
iodine-iodide-triiodide is formed. Acquiring electrons from the battery, the
reduction of zinc ion occurs at the cathode to produce elemental zinc.
Zn2+(aq) + 2 e- à Zn(s)
The
oxidation of iodide ion occurs at the anode, where electrons are lost producing
aqueous molecular iodine.
2 I-(aq) à I2 (aq) + 2 e-
Oxidation
(optional)
This
should be done in the hood because you are producing iodine gas! One way to recover a sample of the original
iodine crystals is by heating solid zinc iodide in a small Erlenmeyer flask
which contains a smaller test tube filled with ice. Acting as a cold finger,
the small test tube is held in place by a Buchner funnel filter adapter which
rests on the mouth of the flask. It may also be necessary to manually hold the test
tube using a clamp, so that you can vent the flask slightly to prevent a
buildup of pressure. With heat in the
presence of air, the iodide is oxidized to gaseous violet-colored iodine and
its crystals are collected on the surface of the cold finger. (This may not be
possible because of water condensation on the cold finger). Zinc metal is not
formed because zinc reacts with oxygen to produce zinc oxide.
Synthesis
of Zinc Iodide
Data
workup and report considerations
The
purpose of this experiment is ultimately to determine the empirical formula for
zinc iodide, or, put another way, x and y in the compound ZnxIy.
This
can be done two ways based on the laws of conservation of mass and constant
composition. One way is
graphically. In the Matter and Motion
folder on masu, you’ll find a spreadsheet named ZnI_exp.xls. Before the end of lab, enter the following
data in this spreadsheet along with the rest of the class: mass zinc iodide
produced, mass zinc consumed, mass iodine consumed, and percent yield of zinc
iodide (more on this later). After
everyone has entered their data, create two graphs of the mass of zinc iodide
produced: one as a function of the mass of zinc consumed, the other as a
function of the mass of iodine consumed.
What does the slope of these lines represent?
The
other way is algebraically. By knowing
the mass of zinc and iodine consumed (there’s an important assumption
here – what is it?) and the mass of zinc iodide produced, you can
determine the mass percent of each element in the compound. From this, with consideration for the atomic
mass of each element, the empirical formula can be found.
Using
your experimentally determined empirical formula, calculate the theoretical
yield and percent yield of zinc iodide. (If you’re unfamiliar with these terms, don’t
worry – we’ll discuss the concepts in class on Thursday) Pool your percent yield with the rest of the
class in a histogram. Comment on trends
and discuss a few factors that might account for these (ignoring calculation
errors).
In your
report you should emphasize: (a) how this experiment relates to the
conservation laws, being as quantitative and specific as possible; (b) a comparison of your graphical and
algebraic results, addressing any questions brought up here. This includes expected results, whether they agree
with actual results, and possibilities why.
(Be conscious and clear in how you present your data!)
When
writing your report, consider the following statement by Tufte: “In scientific
work, both order-of-magnitude reasoning and precise measurements are
pervasive. How are such quantities
represented in visual [or written] expression of ideas, experience,
evidence? How are…diagrams, and charts
to be scaled and labeled? And what
makes images [and words] quantitatively eloquent?”