Iodine (I2) and bromine (Br2) are soluble in mineral oil and slightly soluble in water. Why?

Experiment 7. Qualitative Analysis of Anions

LEARNING OBJECTIVES

The students will be able

1. to observe the chemical behavior of various anions in solutions.

2. to apply various laboratory techniques.

BACKGROUND

This experiment is an exercise in qualitative analysis where you will be analyzing solutions to determine the presence of anions. Certain techniques will be used to separate the ions from one another. A good starting point for this type of analysis is the observation of the color and solubility of a substance. Therefore, we will rely on more sophisticated analyses such as precipitation, acid-base, and electron transfer reactions. Again, the goal will be to correctly identify the anions (two of the five investigated) present in an unknown solution.

· In the first part, you will analyze a “known” solution which contains all five of the anions; these procedures will enable you to see the color changes, gas evolution, precipitation, and any other special effects of each reaction.

· In the second part, you will analyze an “unknown” solution using the same techniques, to determine the presence of two of these anions.

· The plan should be simple.

First, you will investigate the precipitation reactions given in Table 1. Remarkably, the AgNO3 solution will give precipitates with all of the anions present in the solution. The only anion that gives gas evolution is carbonate. At some point during the analysis each anion will react specifically with the reagent added to the sample. Therefore, you will develop a schematic flowchart called “Flowcharting Qualitative Analysis”.

MATERIALS

Known and unknown solutions: 0.10 M NaCl, 0.10 M KI, 0.50 M Na2CO3, 0.10 M Na2SO4, and 0.10 M

NaBr.

Test Reagent:

Silver nitrate, 0.10 M AgNO3, nitric acid, 6.0 M HNO3, Ammonia, 6.0 M NH3, iron(III) nitrate, 0.10 M Fe(NO3)3 in 6.0 M HNO3, Potassium permanganate, 0.020 M KMnO4, Barium chloride, 0.10 M BaCl2, mineral oil or Hexane, baking soda (to neutralize spills)

Equipment: small test tubes (12), test tube holder, test tube rack, Beral pipets, centrifuge.

SAFETY ALERT:

· Most of the acids and bases are very concentrated and can cause chemical burns.

· Acid spills can be neutralized with baking soda, NaHCO3.

· Neutralize base spills with vinegar solution (dilute acetic acid, HC2H3O2)  Some compounds are poisonous; wash your hands.

· Solutions containing silver and potassium permanganate can cause stains which do not appear immediately.

NOTE: DISPOSE OF THE MINERAL OIL or HEXANE LAYER IN THE ORGANIC WASTE CONTAINER.

PRELAB QUESTIONS NAME:

1. Identify the five possible anions which are to be identified in this experiment.

2. How many of these five anions will appear in your unknown sample?

3. Four of the chemicals used to make the known and unknown samples contain the sodium ion. How will this affect your results in this experiment?

4. What purpose does the centrifuge have in this experiment?

5. Four of positive tests for an unknown ion involve the formation of a precipitate. Which ion has a positive test that involves the formation of gas?

PART A. KNOWN SOLUTION OF THE ANIONS

PART I

1. Aqueous solutions of all of the anions to be tested are colorless. The cation associated with each of the anions will be either sodium or potassium ion.

Using a Beral Pipet, place 40 drops of the known sample (colorless solution) into a small test tube.

2. Add 2-3 drops of 6 M HNO3 solution. Evolution of CO2 gas indicates the presence of carbonate ion. The gas evolution could be difficult to observe. Record your observations. The corresponding net ionic equation representing the chemical reaction is: CO32-(aq) + 2 H+(aq)  CO2(g) + H2O(l)

Figure 1. CO2 evolution when carbonate ions are present.

3. Add 10 drops of 0.20 M of Ba(NO3)2 solution to the same test tube. A white precipitate will form. This is a positive identification for the sulfate ion. Observe the reaction. The corresponding net ionic equation representing the chemical reaction is:

Ba2+(aq) + SO42-(aq)  BaSO4(s)

Figure 2. Precipitation of Barium cations with sulfate ions.

4. Place the test tube in the centrifuge and counterbalance it with a test tube of the same size filled with water to the same level (this is very important otherwise the centrifuge can move and cause undesired consequences). Centrifuge for 5-10 minutes at 2000 rpm. DO NOT OPEN

THE COVER OF THE CENTRIFUGE UNTIL THE CENTRIFUGE COMES TO A FULL STOP.

Remove the test tube from the centrifuge. You will see a solid at the bottom of the test tube. This a positive identification of the presence of the sulfate ion.

5. Add the supernatant solution to a different test tube. Add 20 drops of 0.10 M of AgNO3 solution. Centrifuge the test tube for 5-10 minutes at 2000 rpm. (Counterbalance the test tube using another test tube of the same size filled with water to the same level; this is very important otherwise the centrifuge can move and cause undesired consequences). You will observe white yellow precipitate at the bottom of the test tube. The chloride, bromide, and iodide ions are all precipitated. Observe the reaction and write the corresponding equations representing the chemical reactions given below (see the note to the students).

Figure 3. Swinging basket centrifuge (left) and colors of the halide precipitates. (Courtesy of edusanjalbiochemist.com)

6. Add 6.0 M NH3 solution to dissolve AgCl. AgBr and AgI will stay in the solid. Carefully remove the liquid form the test tube and place into another test tube. Add HNO3, test with litmus or pH paper to see if the solution is acidic. If it is not, add more HNO3 until the solution is acidic. This will precipitate AgCl and positively confirm the presence of the chloride ions. PART II

7. In a clean test tube add 20 drops of the known solution. Add 10 drops of 0.10 M Fe(NO3)3 solution. Swirl the mixture. Add 10 drops of mineral oil and shake the mixture. The mineral oil or hexane layer (the upper layer) will turn purple indicating the presence of the iodide ions.

8. In a clean test tube add 20 drops of the known solution. Add 2 drops 6 M HNO3 solution and 10 drops of 0.020 M KMnO4 solution. Swirl the mixture. Add 10 drops of mineral oil (or hexane) and shake the mixture. The mineral oil or hexane layer (the upper layer) will turn yellow-orange indicating the presence of the bromide ions.

Note to students: The halides all form insoluble silver compounds. Silver chloride is a white solid, silver Bromide is pale cream−colored solid, and the solid silver iodide is light yellow in color. The net ionic reactions are given below:

Ag+ + Cl-  AgCl(s) Ag+ + Br-  AgBr(s) Ag+ + I-  AgI(s)

Silver chloride is the only silver halide that dissolves in 6.0 M ammonia, NH3, forming the colorless, [Ag(NH3)2]+ complex ion. If nitric acid, HNO3, is added to a solution containing this ion,

the ammonia in the complex reacts with hydrogen ions to form ammonium ions, and the silver recombines with the chloride ions that are still in solution.

AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Cl-(aq)

[Ag(NH3)2]+(aq) + Cl-(aq) → AgCl(s) + 2NH4 +(aq)

In acid solution, iron(III) ion, Fe3+, is a weak oxidizing agent capable of oxidizing the easily oxidized iodide ion to iodine. Bromide and other ions present will not interfere. The nonpolar iodine will preferentially dissolve in nonpolar mineral oil, where it can be identified by its pink to violet color.

2I-(aq) + 2Fe3+(aq) → I2(aq) + 2Fe2+(aq)

KMnO4 is a stronger oxidizing agent than the iron(III) nitrate and will oxidize bromide, Br- to bromine, Br2. Other ions present will not interfere. The nonpolar bromine can be extracted into nonpolar mineral oil where it can be identified by its characteristic yellow to brown color.

10Br-(aq) + 2MnO4 -(aq) + 16H+(aq) → 5Br2(aq) + 2Mn2+(aq) + 8H2O(l)

PART B. FLOWCHART FOR THE UNKNOWN ANALYSIS

UNKNOWN #

DATE: NAME:

Figure 4. Flowchart of the anion analysis.

NOTE:

NOT

REPORT: The unknown sample # contains and ions.

POSTLAB QUESTION NAME:

1. Write the name of the following ions.

a. Cl-

b. Br-

c. I-

d. SO42-

e. NO3-

f. C2H3O2-

g. CO32-

h. NH4+

i. Ba2+

j. Cu2+

k. Fe3+

2. Write the formula of the following ions.

a. hypochlorite

b. chlorate

c. magnesium

d. sulfite

e. bicarbonate

f. aluminum

g. sulfide

h. fluoride

i. nitride

j. phosphate

k. cobaltic or cobalt(III)

3. Write the formulas of the following compounds.

a. magnesium acetate

b. sodium sulfate

c. iron(III) sulfite

d. potassium bicarbonate

e. zinc oxide

f. copper(II) oxide

g. sulfur dioxide

h. dinitrogen monoxide

i. aluminum phosphate

j. magnesium nitride

k. sodium oxalate

4. Write the name of the following formulas.

a. Mg3P2

b. Ca(HCO3)2

c. CaF2

d. Al2(CO3)3

e. K3PO3

f. FeCl3

g. Ni2S3

h. CaC2O4

i. Zn(C2H3O2)2

j. CrO3

k. MnO2

5. A student analyzed an unknown sample that contained a single anion. The sample gave a yellow precipitate upon addition of a solution of AgNO3 which did not dissolve in ammonia. Which anion is present in the unknown solution? Write the corresponding net ionic equation.

6. A student analyzed an unknown sample that contained a single anion. The sample gave a white precipitate upon addition of a solution of AgNO3 which dissolved in ammonia. Which anion is present in the unknown solution? Write the corresponding net ionic equation.

7. Iodine (I2) and bromine (Br2) are soluble in mineral oil and slightly soluble in water. Why? Suggest another solvent, hexane or alcohol, which can be used instead of mineral oil? Explain.

CH3CH2CH2CH2CH2CH3 CH3CH2OH

Hexane Alcohol

8. Write the net ionic equations for the following reactions.

a. An aqueous solution of sodium carbonate is added to an aqueous solution of calcium chloride.Calcium carbonate is precipitated.

b. An aqueous solution of sodium phosphate is added to an aqueous solution of strontium chloride.Strontium phosphate is precipitated.

c. An aqueous solution of potassium sulfate is added to an aqueous solution of barium chloride.Barium sulfate is precipitated.

d. An aqueous solution of ammonium iodide is added to an aqueous solution of silver nitrate. Silveriodide is precipitated.