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EXPT. 7 HALOGEN REACTIONS
PURPOSE
This
experiment provides the students with the opportunity to investigate some of
the characteristic properties of halogens and halide salts and their
interactions with polar and nonpolar solvents.
BACKGROUND
One
of the most useful things about studying chemistry is discovering the amount of
useful information contained in the periodic chart. Many of these periodic
properties of the elements are discussed in Brown, LeMay and Bursten, 8th
Edition, Chapter 7. Electronegativity and bond polarity are discussed in
Chapter 8.
In
today's experiment, some of the properties of the Group 17 (Group VIIA)
elements, known as halogens, and their compounds will be explored. The
solubility properties of the halogens will be used to observe their reactions.
Figure 7.23 on page 246 shows the halogens in their elemental state.
The
more electronegative an element is, the more it wants an electron. Group 17
atoms which have become ions by gaining an extra electron, such as F-,
Cl-, Br-, and I- are called halides. Note that NaCl is a
halide-specifically a chloride. Also note that anions such as halides must
always be paired with a cation when found in the formula for a compound. Group
17 molecules which have not gained extra electrons, such as F2, C12,
Br2, and I2, are halogens.
In this experiment, the relative electronegativities
of the halogens will be determined.
If
a halide solution containing halide, X-, is added to a solution of a
different halogen, Y2,
there are two possibilities. When element Y is more electronegative than
element X, then Y will take the electron from X, leaving X as a halogen. On the other hand, when Y is
less electronegative than X, then no reaction will take place, and Y2
remains as the halogen. In terms of balanced equations:
2 X-
+ Y2 ® 2 Y- + X2 (or) 2 X- + Y2 ® No Reaction
The properties of two solvents, water and hexane,
will be useful in sorting out what happens. Water is a polar solvent and it
will solvate polar or ionic
molecules. This means that a polar molecule (one that has a dipole) or an ionic
compound may dissolve in water. A diatomic molecule is polar if the two atoms
have different electronegativities. Identify which of the halogens and halides
in the above equations are ionic and which are non-ionic.
Nonpolar solvents solvate nonpolar molecules. Hexane
is an organic molecule that is nonpolar. Since water is polar and hexane is
nonpolar, the two do not mix. When added together, two distinct, colorless
layers are formed. The heavier liquid, water, is on the bottom.
If colored compounds are added to a test tube
containing water and hexane, the polarity of the compounds can be determined.
If they are nonpolar, the will color the hexane layer. If the colored compounds
are polar, the color is observed in the water layer.
For the reaction postulated above, 2 X- +
Y2 ® 2 Y- + X2,
if Y2 is green before any reaction takes place, the hexane layer is
green because nonpolar compounds will be in the hexane layer. As the reaction
proceeds, the green disappears from the hexane layer because the Y2
molecules are reacting and disappearing. The hexane will then take on the color
of X2. On the other hand, if X is more electronegative than Y, the
more electronegative atom already has the electron, so no reaction would occur.
Since no reaction occurs, the hexane layer would remain green, the color of Y2.
It
should be noted that the reaction will only occur if the two reactants come
into contact. Although each of the reactants prefers the solvent most like
itself, each is at least slightly soluble in the other solvent. In order to
increase the likelihood that they will interact, the test tube must be shaken
in order for the reaction to occur. The proper method of shaking is given in
the procedure below.
Whenever
a color change occurs, this is a clue that a reaction is taking place. Three
halogens in aqueous (water) solutions will be available today: chlorine,
bromine and iodine. Each of these halogens has a distinctly different color in
hexane. Therefore, by observing the color of the hexane layer, the halogen
which is present can be determined.
PROCEDURE
At
least 12 clean test tubes will be needed for today's experiment. Students may
work in pairs, but each must do all steps of the procedure.
Solubility Testing
This
portion of the experiment tests the solubilities of the halides. Two sets of halide salts are available, sodium salts (Na+)
and potassium salts (K+). Choose one of the sets. Get together with
another pair of students who will test the other set to share data.
Add
a small amount of the solid chloride salt to a test tube. Note
the appearance and other properties in your notebook. Now add 2 ml of distilled
water and agitate the test tube. Record your observations. Repeat for each of
the other solid halide salts in the
set.
Repeat
the above process using hexane as the solvent instead of water. Any solution
that contains hexane should be disposed of in the waste jar under the hood.
The
solubilities of the halogens will be
tested next. Add 1 ml of the aqueous (water) solution of iodine (I2)
to a test tube. Iodine is a solid at room temperature. Can you hear the solid
crystals rattling in the bottom of the dropper bottle? Do not measure the
amount in a graduated cylinder. An approximate amount is good enough, and 20
drops is roughly 1 ml. Be careful not to spill any of the halogen solutions on
your skin or clothes, as it will stain. Record the appearance of the aqueous
solution. Now add 1 ml hexane and gently tap the bottom of the test tube to
mix. Be sure it mixes, it may take a more vigorous shake, but start gently.
Don't spill, but do not put your
finger over the top and shake. Record all observations of both layers. When
your observations are complete, put the solutions in the labeled waste jar in
the hood. Rinse the test tube and put the rinsing in the jar too.
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Bromine
is a liquid at room temperature, and is quite volatile (that is, it evaporates
easily). The aqueous solution of bromine (Br2) is under the hood. DO
NOT TAKE ANY TEST TUBE CONTAINING BROMINE OUT OF THE HOOD. When finished with
the bromine solutions, pour them in the waste jar in the hood, then rinse the
test tube with water from a wash bottle.
Add
1 ml of the bromine solution to a test tube. Record the appearance of the
aqueous I solution. Just as before, add 1 ml of hexane and mix. Record all
observations.
Cl2
is normally a gas at room temperature. Since aqueous solutions of chorine are
not stable for very long (It's the equivalent of a soda going flat due to the
loss of CO2.), each student will make a solution of chlorine (Cl2).
Add 0.5 ml Clorox to 0.5 ml I M HCl. Record the appearance of the aqueous
solution. Do not breathe the bubbles that are given off -- this is chlorine
gas. Now add 1 ml hexane. Note the color of each layer. When finished, pour the
mixture in the waste jar and rinse the test tube.
Electronegativity Testing
Now
that baseline observations of what each of the ions and molecules looks like in
water and in hexane, this set of reactions will look at all possible
combinations of two reactants. This should allow relative electronegativities
of the halogens to be determined.
Add
0.5 ml Clorox to 0.5 ml IM HCl to make Cl2 (aq), then add 1 ml
hexane and mix. To this, add 1 ml of the water solution of I-. Mix
well and record your observations. Dispose of the solution in the waste jar.
Repeat
using the water solution of Br- instead of the I-.
Next,
add 1 ml of the Br2(aq) to 1 ml hexane, mix, then add 1 ml of the
water solution of I-. Mix well and record all observations. Again,
dispose of the solution in the waste jar.
Repeat
using the water solution of Cl- instead of the I-.
Finally, add 1 ml of the I2(aq) to 1 ml
hexane, mix, then add 1 ml of the water solution of Cl-. Record all
observations. Mix well and record all observations. Again, dispose of the
solution in the waste jar.
Repeat
for the water solution of Br-instead of the Cl-.
You
have now added each of the other halides to
each of the halogen solutions. By
noting the products of the reaction (or if none occurred) you can organize a
scale of the electronegativities of the halogens.
SAFETY POINTS
ALL HALOGEN SOLUTIONS CAN BE CONSIDERED SOMEWHAT DANGEROUS. IODINE CAN BURN THE SKIN AND CHLORINE AND BROMINE ARE VERY CORROSIVE. THE VOLATILITY OF CHLORINE AND BROMINE INCREASES THE DANGER OF INHALING FUMES. AVOID ALL CONTACT WITH THESE COMPOUNDS. EF A SPILL OF ANY OF THE HALOGEN SOLUTIONS OCCURS, CALL THE TA DIATELY.
Any
solution containing bromine must be used only under the hood and should be
disposed of in the waste jar. The only solutions that may be disposed of in the
sink are the water solutions of the halide salts. All other solutions must be
discarded in the waste jar.
Hexane
is not considered a toxic substance, but it is an organic solvent which cannot
be disposed of via the sewer. Therefore, all solutions which contain hexane
must be disposed of in the waste jar.
REPORT HINTS
The
most efficient method to present the data collected this experiment is in
tables. A data table showing the results of the solubility testing as well as a
table showing the results of the electronegativity experiments will present the
material in a very concise, easy to follow manner. Ask the TA for assistance if
more suggestions would help.
Comment
on the results of your experiments. What did the solubility testing tell you?
What was the general feature of water soluble species? hexane soluble species?
From
your observations in the electronegativity testing, write balanced equations
for each of the reactions which occurred. If no reaction occurred, write 'no
reaction' as the product. What conclusions can be made from the
electronegativity experiments?
From
the data, arrange the halogens in order of their reactivity. Indicate which is
most reactive and which is least reactive. What experimental evidence supports
the order? Where would fluorine fit into this order? Why?
Similarly,
arrange the halides in order of their reactivity. Indicate which is most
reactive and which is least reactive. What experimental evidence supports the
order? Where would fluoride fit into this order? Why? What is the relationship
of this reactivity order to the reactivity order for the halogens?
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Pre-Laboratory Assignment
HALOGEN REACTIONS
1. Describe the differences
between halides and halogens. Why is a halogen less soluble in water than in
hexane?
2. In a test tube containing both water and hexane, which layer will
be on top? In order to get good observations, the two layers must be mixed
thoroughly after a reaction has taken place. How will you determine which
halogen is in the hexane layer after a reaction?
3. Complete and balance the following reaction,
X- + Y2
®
Using chlorine and bromine,
write a reaction for each of the two combinations which are possible. Will both
of them occur spontaneously?
4. What solutions in today's lab are safely poured down the sink?
Where are the rest disposed of?
