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Reaction of zinc with iodine
This experiment involves the synthesis of a metal salt by direct reaction of a metal and a
non-metal. Zinc powder is added to a solution of iodine in ethanol. An exothermic redox
reaction occurs, forming zinc iodide, which can be obtained by evaporating the solvent.
Zn + I2 → ZnI2
The experiment can be extended to show the decomposition of a compound into its
elements by electrolysing the zinc iodide.
Lesson organisation
This experiment can be used to illustrate the differences between metallic and non-metallic
elements and their reaction to form a compound – a metal salt – with new properties.
The reaction can be easily reversed using electrolysis to decompose the compound back
into its elements. These are easily recognisable from their distinctive appearances.
Both parts of the experiment can be done either as demonstrations or as class experiments.
Each part should take about 10 mins as a demonstration; longer as a class experiment.
•
Apparatus and chemicals
Eye protection
Each group (or demonstration) requires:
Test-tubes (100 x 16 mm), 3
Test-tube bung
Test-tube rack
Measuring cylinder (10 cm3)
Small filter funnel
Filter paper
Teat pipette
Thermometer (0–100 °C)
Spatula
Watchglass
Weighing boat or suitable container for zinc powder
Chemicals are for one demonstration or one group of students.
Solid iodine (Harmful, Dangerous for the Environment), about 0.5 g (see note 1)
Zinc powder (Highly Flammable. Contact with water can also release a flammable gas,
dangerous for the environment), about 0.5 g (see note 2)
Ethanol (Highly Flammable) or IDA (Industrial Denatured Alcohol) (Highly Flammable,
Harmful) about 5 cm3
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For the extension work
Beaker (100 cm3)
Pair of graphite electrodes mounted
in a rubber bung
Electrical leads and crocodile clips
Source of 3–6 V DC, either battery
or power supply
Torch bulb in a suitable holder
Distilled water, about 20 cm3
Spatula
Access to a little dilute (about 1 M)
hydrochloric acid (Low Hazard at this
concentration) or sulfuric acid (Irritant at
this concentration)
– +
zinc iodide
solution
graphite
electrodes
Technical notes
Solid iodine (Harmful, Dangerous for the Environment) Refer to CLEAPSS® Hazcard 54A
Zinc powder (Highly Flammable, Contact with water can also release a flammable gas,
Dangerous for the environment) Refer to CLEAPSS® Hazcard 107
Ethanol (Highly Flammable) Refer to CLEAPSS® Hazcard 40A or IDA (Highly Flammable,
Harmful) Refer to CLEAPSS® Hazcard 40A
Zinc iodide (Irritant, Dangerous for the environment) No CLEAPSS® Hazcard but similar to 108A
Hydrochloric acid (Low Hazard at concentration used) Refer to CLEAPSS® Hazcard 47A and
Recipe card 31
Sulfuric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 98A, Recipe card 69
and L195 Safer chemicals, safer reactions
1 The solid iodine should be powdered by grinding in a mortar in a fume cupboard. For a
class experiment a stoppered test-tube containing 0.5 g of powdered iodine should be
supplied to each group of students.
2 For a class experiment each group of students should be supplied with a pre-weighed
sample of 0.5 g zinc powder in a weighing boat or a test-tube.
Procedure
HEALTH & SAFETY: Wear eye protection
Synthesis of zinc iodide
•
a Measure out 5 cm3 of ethanol using a measuring cylinder. Place a thermometer in the
ethanol and record the temperature.
b Add the ethanol to 0.5 g of powdered iodine in a test-tube. Stir carefully, using the
thermometer, to dissolve the iodine. The solution should be dark brown. Note the
temperature.
c When all the iodine has dissolved, slowly add the zinc powder using a spatula and
stir the mixture with the thermometer. The temperature should rise, indicating an
exothermic reaction. When the reaction is finished, the colour of the iodine should have
faded and excess zinc will be left. If not, add further small amounts of zinc powder and
stir until the brown colour due to iodine has gone.
d Filter the solution into another test-tube. Using a teat pipette, transfer a few drops of the
filtrate on to a watchglass and allow the solvent to evaporate. This can be speeded up by
placing the watchglass on a beaker containing some hot water. Zinc iodide will be left as
a white solid.
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Decomposition
aPour the remainder of the solution containing the zinc iodide into a 100 cm3 beaker. Add
about 20 cm3 of distilled water and stir to mix.
bClamp the bung carrying the two graphite electrodes over the beaker, so that the
bottoms of the electrodes are immersed as far as possible in the solution. It may be
easier just to rest the bung in the beaker so that the electrodes touch the bottom.
cUsing the leads and crocodile clips, connect the electrodes and the bulb in series and
then to the power supply as shown in the diagram on this page. The bulb should glow to
show that the circuit is complete, and that electrolysis is occurring.
dIf the bulb does not glow, raise the bung out of the solution and check the connections
by touching both electrodes at once with a metal spatula. If the bulb lights up, put the
electrodes back into the solution. If there is still no indication of electrolysis, add a small
amount of zinc iodide from the watchglass to the solution and stir. Repeat until the bulb
starts to glow.
eAllow electrolysis to continue for a few minutes. Note any changes occurring around
the electrodes in the solution – a brown colour (due to iodine) should develop in the
solution around the positive electrode. There may be some effervescence at the negative
electrode.
fDisconnect the power supply. Lift the electrodes out of the solution. Wash them under
a tap. The bottom of the negative electrode should be covered with a silver-grey layer of
zinc metal.
The zinc deposit can be tested (and removed) by immersing the tip of the electrode in a
little dilute acid. It reacts, giving off a colourless gas (hydrogen).
Teaching notes
This reaction shows the synthesis of a compound from two elements, each with their own
distinctive appearance and properties. (A practical worksheet could involve drawing up a
table of properties (type of element, appearance, and so on) for each of the elements and
the compound formed.)
The reaction can also be used to illustrate the direct reaction of a typical metal and nonmetal. It is one of the few reactions of the halogens (Group 7) with a metal that students can
do safely themselves.
A useful extension of this experiment is the decomposition, by electrolysis, of the compound
formed back into its elements.
Reference
This experiment has been reproduced from Practical Chemistry:
http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/reaction-of-zinc-with-iodine,119,EX.html
Useful resource
At this website, you can find images and a movie showing this experiment:
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA1/R1MAIN/CD1R1260.HTM#1320
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Health & Safety checked, October 2007
Updated 29 Oct 2008