CH-180
Properties of the Halogens
The halogens make up the group of elements next to the Noble Gases in the Periodic Table:
fluorine, chlorine, bromine, iodine, and astatine. Because the atoms of these elements have seven
valence electrons, and because they have high electronegativities, the halogens appear as diatomic
molecules: F2, Cl2, Br2, I2, and At2. A single covalent bond (-bond) joins the atoms in each
molecule.
The halogens are nonmetals, so it isn't surprising that they react with both metals and nonmetals,
forming ionic compounds with the metals, and covalent compounds with the nonmetals. The
halogens also form a number of polyatomic anions, the most important of which are the ions
composed of a halogen and one or more oxygen atoms: XO-, XO2-, XO3-, and XO4- (where X = Cl,
Br, or I).
In this experiment, we investigate some chemical and physical properties of Cl2, Br2, and I2. We
won't use F2 because of its extremely high reactivity: F2 reacts with all elements except some of the
noble gases, and it will react with most compounds, including the glass of your beakers, test tubes,
and flasks. We won't use At2 in this experiment because astatine is radioactive. Besides, At2 is rare
and almost impossible to buy.
Experimental Procedure:
1. Examine the containers of the elements located in one of the wall-mounted fume hoods. Note
the color and physical state of the elements at room temperature. Look up the physical properties of
Cl2, Br2, and I2 in a reference, such as the CRC Handbook (available in MG 214 and in the
Stockroom).
2. Using small amounts (less than one shake of a salt shaker) and clean 10  75 mm test tubes,
determine whether solid NaCl, NaBr, or NaI dissolve in water. Next, determine whether or not
hexane (C6H14) dissolves in water. Finally, test solid NaCl, NaBr, and NaI to see if these ionic
compounds will dissolve in hexane. Water is polar; hexane is nonpolar: what can you conclude
about the ability of the three salts to dissolve in polar and nonpolar liquids?
3. Although diatomic molecules like Cl2, Br2, and I2 are considered by chemists to be nonpolar,
these substances do have a limited ability to dissolve in water. Where do you think Cl2, Br2, or I2
would end up, if given the chance to dissolve in either water or hexane? Design and perform a
procedure to find out whether Cl2, Br2, and I2 (the halogens) will dissolve readily in hexane. Are
your observations consistent with the conclusions about solubility that you obtained above?
You might be wondering why we are attempting to make observations on the interactions of
halogens and halides with hexane. The reason becomes apparent with a little study: the halogens
produce different colors when dissolved in hexane: Cl2 is colorless, Br2 is yellow-orange, and I2 is
purple (recall that Br2 and I2 produce the same color in water.) Therefore, by introducing hexane to
receive the Cl2, Br2, or I2 that is produced in a chemical reaction, we can quickly and conveniently
2
know which halogen was produced. The hexane does not react with the halogens; rather, the
hexane merely provides a place for the halogen to be observed. All of the chemical reactions take
place in the water layer.
4. Now we will look at reactions between halogens (Cl2, Br2, I2) and halide ions (Cl-, Br-, I-). Our
classroom theory suggests that the element having the highest electronegativity will be the one
that gains electrons in a chemical reaction. So, what would happen if we mixed some chlorine
water (a source of Cl2) with some NaI (a source of I-)? The reaction equation is:
Cl2 + 2 I-  2 Cl- + I2
Do you see how the more electronegative chlorine atoms gained electrons to become ions, while the
iodine atoms lost electrons? Now perform the following procedures, in order to see if the halogens
and halides behave in a manner consistent with our theory's predictions.
a. Into a 13  100 test tube shake together (use a clean cork!) 10 drops of 0.2 M NaI solution and 10
drops of hexane. Note the color of the hexane layer. Why is there no color? Now add 20 drops of
chlorine water to the test tube. Stopper the tube and shake vigorously for 15 seconds. Record the
color of the hexane layer. Why is the hexane now colored? We added chlorine water, but Cl2 is
colorless in hexane as we observed in step 3 of the procedure. The Br- and I- ions were also
colorless before we added the chlorine water. Look at the test tube with NaI in it. Compare the
color of the hexane layer to the color of the hexane layer for iodine water, I2, in step 3 of the
procedure. If a reaction occurred, the following equation would describe the reaction:
Cl2 + 2 I-  I2 + 2 Clb. Repeat the procedure for step 4(a), but use 0.2 M NaBr instead of 0.2 M NaI. Be sure to record
the color of the hexane layer. Did you see evidence of a reaction? What reaction equation would
describe the reaction, if one did occur?
c. Repeat the procedure for step 4(a), but use bromine water instead of chlorine water. Here, the
reaction equation would be:
Br2 + 2 I-  I2 + 2 BrDid you see evidence of such a reaction? Record your observations. Would the reaction be
consistent with our classroom theory, with its emphasis on electronegativity?
d. Mix NaCl solution and bromine water together, patterning your procedure after your work in step
4(a). Did you see evidence of a chemical reaction of Br2 with Cl- ions? Record observations.
e. Repeat the procedure again. This time use 0.2 M NaBr and iodine water. Once again, write the
reaction equation that would describe the reaction, if it should occur. Was there a reaction? Note
the hexane layer's color.
f. Finally, repeat the procedure again, using 0.2 M NaCl and iodine water. Was there a reaction?
Note the hexane layer's color.
3
5. For many years, people have known that Ag can react with Cl , Br , or I to form silver halide
salts: AgCl, AgBr, or AgI. Surprisingly, the salts aren't soluble in water. To chemists, the salts are
very useful in analysis of samples for the presence of Cl-, Br-, or I- ions. Two properties -- color and
solubility in ammonium hydroxide -- are used to tell the halide salts apart.
+
-
-
-
a. Mix together equal volumes of 0.2 M NaCl and 0.2 M AgNO3 to make about 1 mL of solution in
a 13  100 mm test tube. Do the same in a second test tube, using 0.2 M NaBr and 0.2 M AgNO3;
and likewise in a third test tube, using 0.2 M NaI and 0.2 M AgNO3. Record the colors of the silver
chloride, silver bromide, and silver iodide precipitates that form.
b. Use about 1 mL of dilute (5 M) ammonium hydroxide solution per test tube to test the solubility
of the silver salts in dilute ammonium hydroxide. If any of the salts fail to dissolve after shaking,
add 1 mL of concentrated (15 M) ammonium hydroxide and shake well. Caution: use a fume
hood for 15 M ammonium hydroxide! If any precipitates still have not dissolved, add another mL
of 15 M ammonium hydroxide. Can you draw any conclusions about the salts' solubility? Record
your observations and conclusions.
6. Obtain an unknown salt (NaCl, NaBr, NaI) solution from the stockroom window. Be sure to
record the sample's ID number. Using what you have learned about halogens and halide ions in
parts 3, 4, and 5, perform tests to let you determine which halide ion (Cl-, Br-, or I-) is present in
your sample. Remember, two different tests leading to the same conclusion are better than one;
three are better than two, etc. Be sure to record the procedure that you used. Return the 13  100
test tube in which the unknown salt solution came to the stockroom window after you clean it.
7. Use the report form provided to complete your report.
4
Electronegativity -- A Key Property in Chemical Reactions
We've seen that atoms achieve lowest energy when the electrons are all in completely filled
sublevels. But, reactions often occur between atoms that already have all electrons in completely
filled sublevels. Why do these reactions occur? Well, the property of electronegativity provides
us with an explanation. Electronegativity (en) is the tendency of an atom to attract electrons to
itself. As a periodic trend (seen in the "Properties of the Elements" lab), the en increases from left
to right across the Periodic Table's rows, and increases from bottom to top of the Table's groups. If
a high-en element comes in contact with an element of lower en, the high-en element may be able
to take electrons away from the low-en element.
Question: Predict whether or not the following chemical species can react when placed together.
Show your reasoning.
a. F2 & Ca
b. Cl2 & H+
c. Li & Na
Reaction?
Reaction?
Yes
Yes
No
No
Reaction?
Yes
No
d. Br- & Cl2 Reaction?
Yes
No
The reactions described above are called oxidation-reduction reactions, nicknamed redox
reactions. A redox reaction always involves the transfer of control of electrons from one chemical
species to another. Electronegativity can explain ionic bonding, but we can also use
electronegativity to explain covalent bonding:
CH4
+
2 O2
+
electrons equally shared


CO2
+
2 H2O
+
unequal sharing of electrons
The high-electronegativity O bonds with the lower-electronegativity C and H, so that electrons are
attracted more to the O atoms than they are to C and/or H. Think of covalent bonding as a
partnership: O seeks to be the majority partner, while C and H are content to be minority partners.