Lab #8 – Periodic Properties
Background
In this lab we will study two properties – the solubility of alkaline earth salts and the oxidizing strength of the
halogens – to determine if either qualifies as a periodic property. That is to say, is there a regular trend in the
solubilities of the alkaline earth metals Mg, Ca, Sr, and Ba, and if so, is the trend consistent with the sequence
of the metals on the Periodic Table? Similarly, is there a trend in the oxidizing strengths of the halogens Cl2,
Br2, and I2, and if so, does the trend reflect their sequence on the Table?
We will rank the solubilities of the alkaline earth metals by attempting to form four different precipitates and
seeing which metal forms the most precipitates. Recall that a precipitate is an insoluble, and therefore visually
apparent, salt formed during an aqueous chemical reaction. We will consider the metal that forms the most
precipitates to be the least soluble, and the metal that forms the fewest precipitates to be the most soluble.
We will rank the oxidizing strengths of the halogens by combining the anion halide form of one element with
the diatomic halogen form of another. Recall that oxidizing strength is the ability to remove electrons from
another species. For example, we will mix a halide solution containing I- with a solution containing the halogen
Cl2. If a reaction occurs, then we know that Cl2 was able to pull the extra electron from I-, meaning chlorine has
a greater oxidizing strength than iodine. If no reaction occurs, than we know no electron transfer occurred and
iodine must have the greater oxidizing strength. We can confirm this by mixing I2(aq) with Cl-(aq). In either case,
we will be able to tell whether a reaction has occurred because there will be a color change. All of the halogen
waters have a faint but unique color. If we begin with the color of I2 and end up with the color of Cl2, then we
know an electron transfer reaction occurred. [In the procedure, we will concentrate the colors by using an
intentionally small quantity of non-polar hexane to preferentially dissolve the remaining halogen.]
Having established the precipitating properties of the alkaline earth metals and the oxidizing properties of the
halogens using known solutions, we will test for the same characteristics in an unknown alkaline earth halide
salt. By determining the precipitates formed by the unknown salt, we can identify the alkaline earth metal in the
unknown. Similarly, by determining the relative oxidizing strength of the halogen in our unknown we can
determine its identity.
Prelab summary
Procedural Tips: Place a labeled piece of paper under the test tube rack to keep track of the solutions.
Make sure the halide salt (clear bottle) is in excess.
Use a minimum of hexane in order to concentrate the faint halogen colors
Don’t forget to record your Unknown Number.
Safety: Halogen waters must be use in the FUME HOOD only. Report any spills to the instructor.
Disposal: Waste from alkaline earth solubility tests can be disposed of in the sink.
The halogen waste must stay in the fume hood. Pour all waste in the labeled waste beaker.
Procedure
Solubility tests for Mg, Ca, Sr, and Ba: Set up a test tube rack with four small test tubes. You will run
sixteen different reactions, four at a time. In each set of tests you will combine 1 mL (approximately 10 drops)
of each alkaline earth nitrate solution – Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, and Ba(NO3)2 – with one of the
precipitating agents as listed in the table below. Mix with several finger flicks of the test tube. Record your
observations in the appropriate box in the table on the next page; indicate whether a precipitate forms using the
abbreviation PPT/no PPT, and briefly describe the appearance of the precipitate (color, cloudy or gelatinous).
1 mL 0.1 M Mg(NO3)2
1 mL 0.1 M Ca(NO3)2
1 mL 0.1 M Sr(NO3)2
1 mL 0.1 M Ba(NO3)2
1 mL 1 M H2SO4
1 mL 1 M Na2CO3
1 mL 0.25 M
(NH4)2C2O4
1 mL 1 M K2CrO4
plus
1 mL 1 M HCH3COO
Oxidizing strengths of the halogens: CAUTION: Halogens must be used in the fume hood only. Report any
spills in the fume hood to your instructor immediately.
Take three small test tubes with you to the fume hood. Labeled test tube racks will be in the fume hood for your
use. Combine 1 mL’s of a halogen water (brown bottles) with 3 or 4 drops of hexane. Stopper, shake, and
record the initial color of the hexane layer. Add 1 mL of a halide salt solution (clear bottles), stopper, shake,
and record the final color of the hexane layer. Repeat, making sure to react each halogen with the other two
halides. Record the initial and final colors in the table below, and record R or NR to indicate a reaction or no
reaction.
Cl-
Br-
I-
Cl2
Br2
I2
Identification of an unknown Alkaline Earth Halide: Obtain an unknown salt solution and record the
unknown number. Repeat the solubility and oxidizing strengths with your unknown solution and record the
results below:
Unknown #
Precipitates formed with:
H2SO4
Na2CO3
(NH4)2C2O4
Color change occurred with:
Cl2
Br2
I2
K2CrO4
Conclusions
1. Based on the number of precipitates formed by each of the known alkaline earth nitrate solutions, list the
alkaline earth metals in order of solubility, starting with the most soluble.
Does this order match their order on the Periodic Table?
Suggest an explanation for this trend, considering that Mg is the smallest metal and Ba is the largest.
2. List the halogens in order of increasing oxidizing strength.
Does this order match their order on the Periodic Table?
Suggest an explanation for this trend, considering that Cl is the smallest halogen and I is the largest.
3. What is the identity of your unknown alkaline earth halide salt?
Explain why you chose that alkaline earth metal:
Explain why you chose that halogen:
Prelab Questions
1. Write the balanced half-reaction showing halogen X2 oxidizing halide ion A-.
2. Halogen A2 reacts with halide ion C-. Halogen B2 reacts with halides A- and C-. Halogen C2 reacts with
neither A- or B-. Write the halogens in order of increasing oxidizing strength.
3. According to the data in #2 above, which halide ion is the strongest reducing agent?