Using Periodic Properties to Identify Group 2A
Cations and Group 7A Anions
The objectives of this lab are as follows:
• To observe the solubility properties of various ionic compounds containing alkaline earth
• To observe the relative abilities of the halogens to be reduced to halides, or act as
• To use the above observations to identify an unknown salt consisting of an alkaline earth
metal cation and a halide anion.
Elements within a given column of the periodic table tend to have similar properties due to their
similar valence electron configurations. Because of this, columns of elements are often referred
to as “groups” or “families” of elements. These families include the alkali metals, alkaline earth
metals, halogens, and noble gases. The physical and chemical properties of the elements within
a given family tend to change gradually as one goes from one element in the column to the
next. In this experiment the properties of elements in the alkaline earth metal and halogen
families will be studied and this data used to identify an unknown salt consisting of an alkaline
earth metal cation and a halide anion.
Group 2A—The Alkaline Earth Metals
The alkaline earth metals— beryllium, magnesium, calcium, strontium, barium, and radium—are
all moderately reactive. Beryllium compounds are quite rare and often very poisonous and
radium compounds are highly radioactive; thus, neither of these will be studied. Alkaline earth
metals lose two electrons to make ions with a +2 charge and can thus be represented
generically as M2+. When solutions containing these cations are mixed with solutions containing
anions such as CO32-or SO42-, ionic compounds of the general form MX will precipitate if the
compound MX is insoluble under the reaction conditions used, as shown in the net ionic
M2+ (aq) + X2- (aq) MX (s) if MX is insoluble
In this experiment M2+ = Ba2+, Ca2+, Mg2+, or Sr2+;
X2- = SO42-, CO32-, C2O42-, or CrO42-
No precipitate will be observed if the compound MX is soluble.
When the solubilities of compounds containing various cations combined with a given anion are
compared, a solubility trend that follows the order in the periodic table is expected. For example,
for the solubilities of the sulfate salts, the solubility is expected either to increase or decrease as
we go down the alkaline earth family. These solubility properties will be used to identify an
unknown compound containing a Group 2A cation.
Group 7A—The Halogens
The elementary halogens are also relatively reactive. They include fluorine, chlorine, bromine,
iodine, and astatine. We will not study astatine or fluorine since the former is radioactive and the
latter is too reactive to be safe. Unlike the alkaline earth metals, the halogens tend to gain
electrons to form anions, such as Cl- and Br-. Since they are reduced when this occurs, the
halogens are oxidizing agents, species that tend to oxidize (remove electrons from) other
species. Thus it is possible for some halogens (Cl2, Br2, I2) to react with halide ions (Cl-, Br-, I)
a single replacement reaction. Taking X2 to be a halogen, and Y- to be a halide ion, the reaction
would be as follows:
X2 + 2 Y- 2 X- + Y2
The reaction will only occur if X2 is a better oxidizing agent than Y2, since X2 has to remove
electrons from the Y- ions. If Y2 is a better oxidizing agent than X2 then no reaction will occur.
When comparing two or more halogens, the one that is a better oxidizing agent is considered to
be “more active.” Solutions of halogens and halide ions will be combined to determine the
relative oxidizing abilities of the halogens. These should show a trend as one goes from one
halogen to the next in the Periodic Table.
Since the halogens have characteristic colors in non-polar organic solvents, such as hexane,
while the halide ions are colorless we can use the color changes that occur (or don’t occur) to
determine whether or not one halogen has displaced another in the above reaction. The relative
oxidizing strengths of the halogens will be determined from the reactivity patterns. This data will
then be used to determine the identity of the halide anion in an unknown compound.
Example. Suppose that an aqueous solution of bromine (Br2) is mixed with hexane, which is
less dense than and insoluble in water. The hexane layer will be the top layer due to the relative
densities of hexane and water. The bromine is much more soluble in hexane than in water and
goes into the hexane layer if you shake the mixture well, giving the hexane layer an orange
color. Now suppose we add a solution containing chloride ion (Cl-), to the bromine mixture and
mix well. There are two possible results:
a. If bromine (Br2) is a better oxidizing agent (more easily reduced) than chlorine (Cl2), it will
take electrons from the chloride (Cl-) ions to be converted to bromide (Br-) ions:
Br2 + 2 Cl- 2 Br- + Cl2
If this reaction occurs the color of the hexane layer will change from orange (the color of Br2
in hexane) to that of a solution of Cl2 in hexane.
b. On the other hand, if no reaction occurs:
Br2 + 2 Cl- No Reaction
There will be no color change; the hexane layer will remain orange. This implies that
bromine is not a better oxidizing agent than chlorine or, in other words, that chlorine is a
better oxidizing agent than bromine.
In summary, no color change means that no reaction occurred and a color change means that a
reaction occurred. In either event, the halogen that remains after the halogen and halide are
mixed is the less active halogen.
It is important to keep in mind the difference between the halogen elements and
Halogens Halide Ions
Bromine, Br2 Bromide ion, Br-
Chlorine, Cl2 Chloride ion, Cl-
Iodine, I2 Iodide ion, I-
The halogens are molecular substances, oxidizing agents, have odors, and are distinct colors.
They are only slightly soluble in water and much more soluble in hexane.
The halide ions are soluble only in water, have no color or odor, and are not oxidizing agents.
They do not dissolve in hexane.
I. Relative Solubilities of Alkaline Earths
Place about 15 drops of 1 M Na2PO4 in each of 4 wells in row A of a 24 well plate. Then place
the same amount of 0.1 M Na2S2O3 in 5 wells of row B, 1 M NaHCO3 in row C, and 0.1 M
K2Cr2O7 in row D.
Now add the chlorine salts of magnesium (Mg), calcium (Ca), and barium (Ba) so that each
alkaline earth cation is tested with each of the anions from above. Be sure to note how many
drops are added to get any precipitates and the appearance of each. Stop adding after 12 drops if
there is still no precipitant, but check after a few minutes to see if there is any delayed reaction.
Leave the last well of each row for the unknown.
Sample well plate.
II. Relative Oxidizing Powers of the Halogens
Add a small amount of aqueous chlorine to one of 3 medium test tubes and then add 1 mL of
cyclohexane. Stopper and shake well. Allow the layers to reappear and note the appearance of the
top layer. Repeat with the aqueous bromine and iodine. These are your color standards for
reference. See the standards.
To each of 3 more medium test tubes add about 1 mL of aqueous bromine and 1 mL
cyclohexane. Then add 1 mL of 0.1 M NaCl to the first test tube, and the same amount of NaBr to
the second, and the same amount of NaI to the third. Stopper and shake well. Compare the color
of the top layer to that in the reference test tubes from above. Decide if Br2 is a stronger oxidizing
agent than the halogen that was produced.
Repeat the above procedure twice, first using aqueous chlorine and then aqueous iodine. Again,
decide whether or not Cl2 or I2 are stronger oxidizing agents than the halogen that was produced.
III. Identification of the Unknown Alkaline Earth Halide
Select unknown A, B, or C, and using the solubility tests from Part I and the oxidizing tests
from Part II, determine the identity of your unknown alkaline-earth halide.