Gravimetric determination of phosphorus in plant food answers

QUALITATIVE OBSERVATIONS OF CHEMICAL CHANGES

Introduction

The Periodic Table arranges the elements in order of increasing atomic number such that elements with similar properties recur periodically. That is they fall directly beneath each other in the Table. The columns are sometimes referred to as families or groups. Chemists refer to some of these families by name. Three such names are; the alkali metals (Group IA), the alkaline earth metals (Group IIA), and the halogens (Group VIIA). Physical and chemical properties of the elements in a given column usually change in a regular fashion as one goes down a column. By observing the trends in properties the elements can be arranged in the order in which they appear in the Periodic Table. In this experiment we will study the properties of the alkaline earth metals and the halogens.

The alkaline earths are all moderately reactive metals; this reactivity generally increases down the column. We will study only the middle four members of this family. Alkaline earth metals exist in their compounds and in solution as M2+ cations (Mg2+, Ca2+, etc). If a solution containing one of these cations is mixed with one containing an anion (CO3

2-, SO4

2-, IO3 -, etc.) a solid, called a precipitate, may form.

For example:

M2+.(aq) + SO4 2- (aq)  MSO4 (s) if MSO4 is insoluble (1a)

M2+ (aq) + 2IO3 - (aq)  M(IO3)2 (s) if M(IO3)2 is insoluble (1b)

We would expect that the solubilities of the salts of the alkaline earth cations with any one of the given anions shows a smooth trend as one goes down the column. That is, as we go from magnesium down to barium the solubilities of the sulfate salts either gradually increase or decrease. We would expect to find similar trends for the carbonates, oxalates, and iodates formed by those cations. You will test this hypothesis by carefully observing the results obtained when you mix a small quantity of a solution containing an alkaline earth metal with a small quantity of a solution containing one of the four anions.

In elemental form the halogens are also relatively reactive. We will study only the middle three members of this family. Like other non-metal elements the halogen atoms tend to gain electrons, forming X- anions (Cl-, Br-, etc.). The halogens are oxidizing agents, that is, species that tend to remove electrons from other species.

When dissolved in water the halogens do not exhibit striking differences in color. When dissolved in hexane, on the other hand, each of the halogens exhibits a distinctive color. We will examine the reaction between halogen molecules (e.g. Cl2) and solutions containing halide ions (e.g. Br¯). We will be able to determine if a reaction has occurred by using hexane to extract the halogen from the solution.

Cl2 (aq) + 2 Br¯ (aq)  2Cl¯ (aq) + Br2 (aq)

The reaction will occur if Cl2 is a better oxidizing agent than Br2, that is, Cl2 can produce Br2 by removing electrons from the Br¯ ions. If a reaction occurs, we expect the hexane layer to exhibit the color of bromine. If no reaction occurs, we expect the hexane layer to exhibit the color of chlorine. We predict that we will observe a consistent trend in the oxidation strength of the halogens as we go down the column.

One difficulty students tend to have in this experiment involves terminology rather than chemistry. You must learn to distinguish the halogen elements from the halide ions, since the two kinds of species are not at all the same, even though their names are similar:

Halogen (elements) Halides (ions)

Chlorine, Cl2 Chloride, Cl¯

Bromine, Br2 Bromide, Br¯

Iodine, I2 Iodide, I¯

Given the solubility properties of the alkaline earth cations, and the oxidizing power of the halogens, it is possible to develop a systematic procedure for determining the presence of any alkaline earth cation and any halide in a solution. In the last part of this experiment, you will be asked to establish the identity of an unknown solution containing a single alkaline earth halide.

Procedure

NOTE: It is important that you use a clean test tube for each test. If you need to reuse test tubes, dispose of the material in the appropriate waste container in the hood. Then rinse each test tube 3-4 times with distilled water. Shake out as much of the distilled water as possible. There is no need to completely dry the test tube before the next use.

1. Relative Solubilities of Alkaline Earth Compounds

To each of four test tubes add about 1 mL of 1 M H2SO4 using a graduated cylinder. Don’t forget to rinse the graduated cylinder before using it for a different solution. Then add 1 mL of 0.1 M Mg(NO3)2 solution to the first test tube, 1 mL of 0.1 M Ca(NO3)2 solution to the second test tube, 1 mL of 0.1 M Sr(NO3)2 solution to the third test tube, and 1 mL of 0.1 M Ba(NO3)2 solution to the fourth test tube. You do not need to measure each solution. Just add about the same amount as is already in the test tube. Stir each mixture with a glass stirring rod, rinsing the rod in a beaker of distilled water between stirs. Record the results on the solubilities of the sulfates of the alkaline earths in the Table, noting whether a precipitate forms, and any characteristics (such as color, amount, size of particles, and settling tendencies) that might distinguish it.

Rinse out the test tubes, and to each add 1 mL 1 M Na2CO3. Then add 1 mL of the same 0.1M solutions of the alkaline earth nitrates, one solution to a tube, as before. Record the observations on the solubility properties of the carbonates of the alkaline earth cations. Rinse out the tubes, and test for the solubilities of the oxalates of these cations, using 0.25 M (NH4)2C2O4 as the precipitating reagent with the 0.1M alkaline earth nitrate solutions. Finally, determine the relative solubilities of the iodates of the alkaline earths, using 1 mL 0.1 M KIO3 as the test reagent.

2. Relative Oxidizing Powers of the Halogens In a test tube place a 1-2 mL of bromine-saturated water and add 1 mL of hexane. Stopper the test tube and shake until the bromine color is mostly in the hexane layer. (CAUTIONS: Avoid breathing the halogen vapors. Don’t use your finger to stopper the tube, since a halogen solution can give you a chemical burn.) Repeat the experiment using chlorine water and iodine water with separate samples of hexane, noting any color changes as the bromine, chlorine, and iodine are extracted from the water layer into the hexane layer. Set these three test tubes aside as color references.

To each of three large test tubes add 1 mL bromine water. Then add 1 mL 0.1 M NaCl to the first test tube, 1 mL 0.1 M NaBr to the second, and 1 mL 0.1 M NaI to the third. Stopper each tube and shake it. Now add 1mL of hexane to each tube. Stopper the tube and shake it. Note the color of the hexane phase above each solution. If the color is not that of Br2 in hexane, a reaction occurred, and Br2 oxidized the anion you had placed in to that tube producing the corresponding halogen. In such a case, Br2 is a stronger oxidizing agent than the halogen that was produced. Carefully note the color of the hexane layer in each tube and record it in the table.

Rinse out the tubes, and add 1 mL chlorine water to each tube. Then add 1 mL of the 0.1 M solutions of the sodium halide salts one solution to a tube, as before. Stopper each tube and shake. Now add 1 mL of hexane to each tube, stopper the tube and shake again. Note the color of the hexane layer after shaking. Depending on whether or not the color of the hexane layer is that of Cl2 in hexane, decide whether Cl2 is a better oxidizing agent than Br2 or I2. Carefully note the color of the hexane layer in each tube and record it in the table.

Rinse out the tubes, and add 1 mL iodine water to each tube. Then add 1 mL of the 0.1 M solutions of the sodium halide salts one solution to a tube, as before. Stopper each tube and shake. Now add 1 mL of hexane to each tube, stopper the tube and shake again. Note the color of the hexane layer after shaking. Depending on whether or not the color of the hexane layer is that of I2 in hexane, decide whether I2 is a better oxidizing agent than Br2 or I2. Carefully note the color of the hexane layer in each tube and record it in the table.

3. Identification of an Alkaline Earth Halide

Your observations on the solubility properties of the alkaline earth cations should allow you to develop a method for determining which of those cations is present in a solution containing one alkaline earth metal cation and no other cations. The method will involve testing samples of the solution with one or more of the reagents you used in Part 1. Indicate on the Data page how you would proceed.

In a similar way you can determine which halide ion is present in a solution containing only one such anion and no others. Here you will need to test a solution of a halogen (e.g. bromine water) with your unknown to see how the halide ion is affected. From the behavior of the halogen-halide ion mixtures you studied in Part 2 you should be able to identify the particular halide that is present. Describe your method on the Data page. Discuss your methods with your instructor if you are unsure of how to proceed. Once you are satisfied with your methods, obtain an unknown solution of an alkaline earth halide, and then use your procedures to determine the cation and anion that it contains. Note that you will need to use a separate portion of your unknown (about 1 mL) for each test you will perform. Be sure to save some of your unknown in case you need to redo one or more of these tests.

Date ___________ Section _________ Name __________________________

Qualitative Observations of Chemical Changes

1M H2SO4 1M Na2CO3 0.25 M (NH4)2C2O4

0.1 M KIO3

0.1 M Mg(NO3)2

0.1 M Ca(NO3)2

0.1 M Sr(NO3)2

0.1 M Ba(NO3)2

Key: ppt = precipitate; sol = soluble (i.e. no ppt); be sure to note any identifying characteristics of any precipitates that form

Cl2 Br2 I2

Color of halogen in hexane

Br¯ Cl¯ I¯

Br2

Cl2

I2

Fill in the color of the hexane layer for each combination. Indicate that a reaction occurred by writing R in the box. Write NR if no reaction occurred.

Alkaline Earth Metal Cation

1. To determine the identity of your alkaline earth cation, place one carefully

chosen reagent along with your unknown solution in a test tube, and

note the result.

2. Based on that result, add a second carefully chosen reagent to a sample of

your unknown in a second test tube. With these two observations, you

should have enough information to identify your unknown cation.

What reagents should you add, and in what order? Explain your reasoning.

Halide Anion

1. To determine the identity of your halide anion, mix one carefully

chosen reagent along with your unknown solution in a test tube (don’t

forget the hexane), and note the result. You should have enough

information to identify your unknown anion.

What reagent should you add? Explain your reasoning.

Unknown Number: ________

Cation present in your unkown: _______

Anion present in your unknown: _______

Post Lab Questions

1. Are the trends that you observe consistent with how the alkaline earth metals are arranged in the Periodic Table? Explain.

2. Are the trends that you observe consistent with how the halogens are arranged in the Periodic Table? Explain.

Date ___________ Section _________ Name __________________________

ADVANCED STUDY ASSIGNMENT: Qualitative Observations of Chemical Changes

1. What does a chemist mean by the term periodicity?

2. What does a chemist mean by the term oxidizing agent?

3. What does a chemist mean by the term oxidized?

4. What does a chemist mean by the term precipitate?