15: Using Periodic Properties to Identify Group 2A Cations and Group 7A Anions (Experiment)


Objectives
• To observe the solubility properties of various ionic compounds containing alkaline earth metal cations.
• To observe the relative abilities of the halogens to be reduced to halides, or act as oxidizing agents.
• 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 $$\ce{M^{2+}}$$. When solutions containing these cations are mixed with solutions containing anions such as $$\ce{CO3^{2-}}$$ or $$\ce{SO4^{2-}}$$, ionic compounds of the general form $$\ce{MX}$$ will precipitate if the compound $$\ce{MX}$$ is insoluble under the reaction conditions used, as shown in the net ionic equation below.

$\ce{M^{2+} (aq) + X^{2-} (aq) -> MX (s)} \quad \quad \text{if } \ce{MX} \text{ is insoluble}$

In this experiment $$\ce{M^{2+}}$$ = $$\ce{Ba^{2+}}$$, $$\ce{Ca^{2+}}$$, $$\ce{Mg^{2+}}$$, or $$\ce{Sr^{2+}}$$; and $$\ce{X^{2-}}$$ = $$\ce{SO4^{2-}}$$, $$\ce{CO3^{2-}}$$, $$\ce{C2O4^{2-}}$$, or $$\ce{CrO4^{2-}}$$.

No precipitate will be observed if the compound $$\ce{MX}$$ 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 $$\ce{Cl^-}$$ and $$\ce{Br^-}$$. 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 ($$\ce{Cl2}$$, $$\ce{Br2}$$, $$\ce{I2}$$) to react with halide ions ($$\ce{Cl^-}$$, $$\ce{Br^-}$$, $$\ce{I^-}$$) in a single replacement reaction. Taking $$\ce{X2}$$ to be a halogen, and $$\ce{Y^-}$$ to be a halide ion, the reaction would be as follows:

$\ce{X2 + 2Y^- -> 2X^- + Y2}$

The reaction will only occur if $$\ce{X2}$$ is a better oxidizing agent than $$\ce{Y2}$$, since $$\ce{X2}$$ has to remove electrons from the $$\ce{Y^-}$$ ions. If $$\ce{Y2}$$ is a better oxidizing agent than $$\ce{X2}$$ 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 15.1

Suppose that an aqueous solution of bromine ($$\ce{Br2}$$) 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 ($$\ce{Cl^-}$$), to the bromine mixture and mix well. There are two possible results:

$\ce{Br2 + 2Cl^- -> 2Br^- + Cl2} \nonumber$

If this reaction occurs the color of the hexane layer will change from orange (the color of $$\ce{Br2}$$ in hexane) to that of a solution of $$\ce{Cl2}$$ in hexane.

$\ce{Br2 + 2 Cl^- ->} \text{No Reaction} \nonumber$

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 the halide ions:

Halogens Halide Ions
Bromine, $$\ce{Br2}$$ Bromide ion, $$\ce{Br^-}$$
Chlorine, $$\ce{Cl2}$$ Chloride ion, $$\ce{Cl^-}$$
Iodine, $$\ce{I2}$$ Iodide ion, $$\ce{I^-}$$

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.

Identification of a Salt Containing an Alkaline Earth Cation and a Halide

Given the observed solubility properties of the alkaline earth cations and the oxidizing ability of the halogens, it is possible to develop a systematic procedure for determining the identities of the alkaline earth cation and halide present in an unknown. You will devise such a procedure and then carry it out on an unknown substance containing an alkaline earth cation and a halide.

Procedure

Materials and Equipment

Reagents: 0.1 M $$\ce{Ba(NO3)2}$$, $$\ce{Ca(NO3)2}$$, $$\ce{Mg(NO3)2}$$, and $$\ce{Sr(NO3)2}$$; 1 M $$\ce{H2SO4}$$, 1 M $$\ce{Na2CO3}$$, 0.25 M $$\ce{(NH4)2C2O4}$$, 1 M $$\ce{K2CrO4}$$, 1 M acetic acid, bromine water (aqueous $$\ce{Br2}$$), chlorine water (freshly prepared is best), iodine water, 0.1 M $$\ce{NaCl}$$, $$\ce{NaBr}$$, and $$\ce{NaI}$$; hexane

Equipment: four or more small test tubes, test tube rack, stirring rod, and cork stoppers(*) for test tubes

Safety

Hexane is flammable. Do not use it anywhere near an open flame. All waste must be poured into the waste container. Avoid breathing the halogen vapors. Don’t use your finger to stopper the tubes. Notify your instructor if you spill any bromine solution; it can be cleaned up with sodium thiosulfate solution. All reaction mixtures should be placed in the waste container when you are done with them.

Part A: Relative Solubilities of Some Salts of the Alkaline Earth Metals

1. Each of the following reactions is performed in a small test tube, using about 1 mL (12-15 drops from the reagent bottle pipet) of each solution. The reaction mixtures must be mixed well by using a stirring rod or a method described by your instructor. The stirring rod should be rinsed in a beaker of deionized water between uses. The test tubes must be washed and then rinsed them with distilled water before you use them and before you reuse them for another reaction.
2. The results are to be recorded in the data table provided, noting whether a precipitate forms, and any characteristics—as color, odor, amount, size of particles, and settling tendencies— that might distinguish it.

$\ce{H2SO4 + Ba(NO3)2}$

$\ce{H2SO4 + Ca(NO3)2}$

$\ce{H2SO4 + Mg(NO3)2}$

$\ce{H2SO4 + Sr(NO3)2}$

$\ce{Na2CO3 + Ba(NO3)2}$

$\ce{Na2CO3 + Ca(NO3)2}$

$\ce{Na2CO3 + Mg(NO3)2}$

$\ce{Na2CO3 + Sr(NO3)2}$

$\ce{(NH4)2C2O4 + Ba(NO3)2}$

$\ce{(NH4)2C2O4 + Ca(NO3)2}$

$\ce{(NH4)2C2O4 + Mg(NO3)2}$

$\ce{(NH4)2C2O4 + Sr(NO3)2}$

$\ce{K2CrO4 + Ba(NO3)2 +} \text{acetic acid}$

$\ce{K2CrO4 + Ca(NO3)2 +} \text{acetic acid}$

$\ce{K2CrO4 + Mg(NO3)2 +} \text{acetic acid}$

$\ce{K2CrO4 + Sr(NO3)2 +} \text{acetic acid}$

Part B: Relative Oxidizing Abilities of the Halogens

1. The color of the bromine in hexane is determined by placing 1 mL of bromine-saturated water in a small test tube, adding 1 mL of hexane, and mixing the contents well until the bromine color is mostly in the hexane layer. The color of the hexane layer should be noted at this point. Although the colors of the two layers may be different it is the color in the hexane layer that will be used throughout this experiment.
2. The colors of chlorine and iodine in hexane are determined as for bromine. Chlorine water is combined with hexane and then iodine water is combined with hexane, noting the color of the hexane layer in each case.
3. Each of the following reactions is to be performed in a small test tube by combining 1 mL of hexane with 1 mL of each of the two reagents listed. The contents should be mixed well and the color of the (upper) hexane layer noted. The test tubes must be washed and then rinsed with distilled water before you use them and before you reuse them for another reaction.

$\ce{Br2 + NaCl}$

$\ce{Br2 + NaBr}$

$\ce{Br2 + NaI}$

$\ce{Cl2 + NaCl}$

$\ce{Cl2 + NaBr}$

$\ce{Cl2 + NaI}$

$\ce{I2 + NaCl}$

$\ce{I2 + NaBr}$

$\ce{I2 + NaI}$

Part C: The Unknown

The solubility properties of the alkaline earths, the oxidizing properties of the halogens, and the methodology of this experiment will be used to devise a systematic procedure for determining the chemical formula of an unknown that contains one alkaline earth metal cation and one halide ion. A written procedure or flow chart may be required before an unknown is issued. Appropriate tests will be performed on the unknown so as to ascertain the cation and anion present. It may not be necessary to perform all of the reactions used in parts A and B to identify your unknown.

Clean Up

Reaction mixtures should be disposed of in the waste container.

Pre-Laboratory Assignment: Using Periodic Properties to Identify Group 2A Cations and Group 7A Anions

1. Suppose that substances $$\ce{X}$$, $$\ce{Y}$$, and $$\ce{Z}$$ each potentially react with the anion ($$\ce{X^-}$$, $$\ce{Y^-}$$, or $$\ce{Z^-}$$) of another species in the series. In solution $$\ce{X}$$ is yellow, $$\ce{Y}$$ is blue, and $$\ce{Z}$$ is red. The anions are all colorless.

When a solution of $$\ce{Z}$$ is reacted with $$\ce{X^-}$$ the color changes from red to yellow, meaning that the reaction $$\ce{Z + X^- -> Z^- + X}$$ has occurred.

• Which reactant was oxidized? ___________
• Which reactant was reduced? ___________
• What is the better oxidizing agent, $$\ce{Z}$$ or $$\ce{X}$$? ___________

When a solution of $$\ce{X}$$ is reacted with $$\ce{Y^-}$$, the color changes from yellow to blue.

• Write an equation for the reaction that occurred, assuming it is the same type of reaction as above.
• Which reactant was oxidized? ___________
• Which reactant was reduced? ___________
• What is the better oxidizing agent, $$\ce{X}$$ or $$\ce{Y}$$? ___________

Rank X, Y, and Z in order of decreasing oxidizing ability.

_________________ > _________________ > _________________

From best oxidizing agent to worst oxidizing agent

Would you expect a reaction when $$\ce{Y}$$ is combined with $$\ce{Z^-}$$? Why or why not?

1. Hexane and water form two layers when they are combined. What layer is on the bottom and on the top? Why?
1. Silver ion reacts with chloride ion to form insoluble $$\ce{AgCl}$$. Aluminum ion does not form a precipitate when reacted with chloride ions. Lead (II) ion reacts with chloride ion to form insoluble $$\ce{PbCl2}$$. $$\ce{PbCl2}$$ is soluble in hot water whereas $$\ce{AgCl}$$ is not. Use this information to outline a procedure that would determine whether $$\ce{Ag^+}$$, $$\ce{Pb^{2+}}$$, or $$\ce{Al^{3+}}$$ is present in an unknown that contains one, and only one, of these three ions. Use the back of the page for your answer.

Lab Report: Using Periodic Properties to Identify Group 2A Cations and Group 7A Anions

Part A: Relative Solubilities of Some Salts of the Group 2A Cations

Data and Observations

Indicate whether a precipitate forms (“ppt”) or there is no precipitate (“no ppt”). Note any distinguishing characteristics of the precipitates.

 1 M $$\ce{H2SO4}$$ 1 M $$\ce{Na2CO3}$$ 0.25M $$\ce{(NH4)2C2O4}$$ 1 M $$\ce{K2CrO4}$$ 1 M Acetic Acid $$\ce{Ba(NO3)2}$$ $$\ce{Ca(NO3)2}$$ $$\ce{Mg(NO3)2}$$ $$\ce{Sr(NO3)2}$$

Questions and Conclusions

1. Consider the relative solubilities of the Group 2 cations in the various precipitating reagents. On the basis of the trends you observed, list the four alkaline earth metal ions in order based on the solubility of the salts formed—the one that forms the greatest number of soluble compounds is the most soluble and the one that forms the greatest number of precipitates is the least soluble

________ > ________ > ________ > ________
From most soluble to least soluble

1. Is this order consistent with the data from all of the precipitation reagents? If not, why did you order them as you did?
1. Is this order consistent with a periodic trend? If no, how is it different? If yes, does solubility increase or decrease down the column?

Part B: Relative oxidizing powers of the halogens

Data and Observations

Indicate the color of the hexane layer after mixing hexane and each halogen in the space provided in the header row of the following table. Indicate the color of the hexane layer after addition of each halide reagent and the corresponding halogen that is present in the top portion of each box. If a reaction occurred, write a net ionic equation in the lower portion of the box. If no reaction occurred, write “NR” instead of a reaction equation

 $$\ce{Br2}$$ color ______________ $$\ce{Cl2}$$ color ______________ $$\ce{I2}$$ color ______________ $$\ce{Cl^-}$$ Color: ____________ Halogen: __________ Reaction: Color: ____________ Halogen: __________ Reaction: Color: ____________ Halogen: __________ Reaction: $$\ce{Br^-}$$ Color: ____________ Halogen: __________ Reaction: Color: ____________ Halogen: __________ Reaction: Color: ____________ Halogen: __________ Reaction: $$\ce{I^-}$$ Color: ____________ Halogen: __________ Reaction: Color: ____________ Halogen: __________ Reaction: Color: ____________ Halogen: __________ Reaction:

Questions and Conclusions

1. Rank the halogens in order of their decreasing oxidizing power: ___________ > ___________ > ___________

From best oxidizing agent to worst oxidizing agent

1. Does the order above follow a periodic trend? If no, how is it different? If yes, does oxidizing ability (“activity’) increase or decrease down the column?

Part C: The Unknown

ID Code: ____________________

Outline the procedure you will use to identify the ions present in the unknown.

Cation:

Anion:

Give the observations made when the above procedure was done. Explain how the data and observations lead to your conclusion.

Based on the work detailed above:

Cation Present ____________

Anion Present ____________

Formula of Unknown ____________________