Reactivity of Halide Ions
Chapter 5 Lab
Objectives
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Observe the reactions of halide ions with different reagents.
Analyze the data to determine characteristic reactions of each of the halide
ions.
Infer the identity of an unknown solution based on your observations of a
variety of known reactions.
Introduction
The four halide salts used in this experiment are found in your body. Although
sodium fluoride is poisonous, trace amounts seem to help strengthen tooth enamel and
prevent tooth decay in humans. Sodium chloride is added to most of our food to enhance
flavor while masking sourness and bitterness. Sodium chloride is essential for life
processes, but excessive intake of this salt appears to be linked to high blood pressure in
certain segments of the population. Sodium bromide is distributed throughout body
tissues, and in the past it was used as a sedative. Sodium iodide is necessary for the
proper function of the thyroid gland, which controls cell growth. The concentration of
sodium iodide is almost 20 times greater in the thyroid than in the blood. To help meet
the body’s need for this halide salt, about 10 ppm of NaI is added to packages of table
salt that are labeled “iodized”.
The principal oxidation state of the halogens is -1. However, all halogens except
fluorine may exist in other oxidation states. The specific tests you will develop in this
experiment involve the production of recognizable precipitates and complex ions. You
will use you observations to determine the halide ion present in an unknown solution.
In this lab, you will be in charge of doing two basic things. First, you will
determine the halide ion present in the unknown solution. Secondly, you will rank the
four known halides according to their reactivity based on what they do when the reagents
are added to them (the more reactive the halide salt is, the more ‘stuff’ happens to it when
the reagents are added). Make sure and include both of these things in your lab report.
Safety
Always wear safety goggles, disposable gloves and a lab apron to protect
your eyes, hands and clothing. If you get a chemical in your eyes, immediately flush
the chemical out at the eyewash station while calling to your teacher. Know the location
of the emergency lab shower and eyewash station and the procedures for using them.
Do not touch any chemicals. If you get a chemical on your skin or clothing,
wash the chemical off at the sink while calling to your teacher. Make sure you carefully
read the labels and follow the precautions on all containers of chemicals that you use. If
there are no precautions stated on the label, ask your teacher what precautions you should
follow. Do not taste any chemicals or items used in the laboratory. Never return leftover
chemicals to their original containers so you do not contaminate the entire container; take
only small amounts to avoid wasting supplies.
Call your teacher in the event of a spill. Spills should be cleaned up promptly,
according to your teacher’s directions.
Never put broken glass into a regular waste container. Broken glass should be
disposed of properly in the broken glass waste container.
Materials
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0.1 M AgNO3
0.1 M NaCl
0.1 M NaF
0.2 M KBr
0.2 M KI
0.2 M Na2S2O3
0.5 M Ca(NO3)2
3% starch solution
5% NaOCl solution (commercial bleach)
1 well-plate
eye droppers
Procedure
1. Place 4-5 drops of NaF, NaCl, KBr and KI solutions into separate wells on the
well-plate going across. Label the wells to identify their contents. Add 1-2 drops
of 0.5 M Ca(NO3)2 to each of the wells going down the first column. Record the
color of any precipitates that form in the data table on the next page. Indicate
which, if any, did not form a precipitate.
2. Add 1-2 drops of 0.1 M AgNO3 to each of the wells going down the next column.
Record your observations in the data table.
3. Add 4-5 drops of 0.2 M Na2S2O3 solution to each of the wells where a precipitate
had formed from step #2. Record your observations in the data table.
4. Add 4-5 drops of the 3% starch solution to each well going down the next
column. Add a 1-2 drops of the 5% NaOCl (bleach) to each of the wells down the
column. Record your observations in the data table.
5. Save the results of the tests on the known samples for comparison, and obtain an
unknown solution containing one of the halide ions. React the unknown with
each of the reagents as you did in steps 1-4. Record your observations on the data
table. Determine what halide ion is present in the unknown solution.
Cleanup and Disposal
Clean all apparatus and your lab station. Return the lab equipment to its proper
place. Dispose of chemicals and solutions in the containers designated by your teacher.
Do not pour any chemicals down the drain or in the trash unless your teacher directs you
to do so. Wash your hands thoroughly after all lab work is completed and before you
leave the lab.
Data Table – Reagents
Halide Salts
NaF
NaCl
KBr
KI
Unknown
Ca(NO3)2
AgNO3
AgNO3 +
Na2S2O3
NaOCl +
starch
Questions
1. Analyzing Data Which step(s) confirm(s) the presence of (a) F- ions, (b) Cl- ions,
(c) Br- ions, (d) I- ions?
a. ___________________________________________________________
b. ___________________________________________________________
c. ___________________________________________________________
d. ___________________________________________________________
2. Inferring Conclusions What generalizations can be made about silver halides?
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General Conclusions
1.
Inferring Conclusions Identify your unknown, and use your experimental
evidence to support your identification.
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