1 Qualitative Analysis II - Anions Purpose: To study the chemistry of a number of anions and identify an anion in an unknown salt sample Introduction In this lab experiment you will learn how to identify a number of anions in aqueous solution based on their chemistry. Unlike the last (cation) lab in which you separated cations from a mixture before identifying each one, in this lab you will deal with a single anion at a time in solution. The anions to be studied are SO4-2, SO3-2, CO3-2, NO2 -, Cl -, Br -, and I -. Most of these ions will yield an insoluble precipitate upon addition of an appropriate reagent. Procedurally, this lab consists of four parts, each examining a certain aspect of the chemistry of the ions involved. Part A examines those ions which form an insoluble barium salt, Part B examines those ions which give off a gas upon addition of an acid to the salt, and Part C examines those ions which form an insoluble silver salt. Once the identity of an unknown anion is narrowed down by the results of the reactions in Parts A, B, and C, some other confirmatory reactions will be performed in order to confirm the identity of the anion. We will be working with a number of different precipitates in this experiment, and frequently testing to see whether or not the precipitate dissolves in acid. It is important to realize that a precipitate will dissolve in acid solution only if the precipitate contains the anion of a weak acid. Chemistry of Specific Ions Part A, Insoluble Barium Salts Only three of the above ions will produce an insoluble barium salt upon addition of barium chloride, BaCl2, and those three are SO4-2, SO3-2, and CO3-2. The other anions will not yield a precipitate with BaCl2. So addition of aqueous BaCl2 to a solution of the salt containing one of these four anions will immediately produce a visible (white) precipitate that will eventually settle to the bottom of the container. The equilibria involved are all similar, and are described by solubility equilibrium equations; BaSO4(s) Ba +2 (aq) + SO4-2 (aq) (eq. 1) BaSO3(s) Ba +2 (aq) + SO3-2 (aq) (eq. 2) BaCO3(s) Ba +2 (aq) + CO3-2 (aq) (eq. 3)
2 Notice that even though it is the solid that is forming, it is shown on the left side of the equilibrium equation since that is the way Ksp is defined. Of the three insoluble barium salts shown in eqs. 1-3, two of them contain the anion of a weak acid (SO3-2 or CO3-2 ), and so they will dissolve when the solution is made acidic since the anion prefers to associate with hydrogen ion to form the weak acid. (BaSO4 will not dissolve in acidic solution.) SO3-2 (aq) + 2H + (aq) H2SO3(aq) (eq. 4) CO3-2 (aq) + 2H + (aq) H2CO3(aq) (eq. 5) When the equilibria in eqs. 4 or 5 occur, the excess acid (present as H + ) is forcing the reactions to the right, effectively removing the anion from solution (to form undissociated acids). LeChâtelier s Principle suggests that the related equilibria in eqs. 2 and 3 would then be shifted to the right, causing the solids in those equilibria to dissolve. One additional aspect of this chemistry is that the acids shown in eqs. 4 and 5 are unstable in aqueous solution, and breakdown into other substances. H2SO3(aq) H2O(l) + SO2(g) (eq. 7) H2CO3(aq) H2O(l) + CO2(g) (eq. 8) Chemistry of Specific Ions Part B, Gas Evolution Three of the nine anions studied in this experiment give off a gas when a solution of the anion is made acidic; SO3-2, CO3-2, and NO2 -. All three of these ions are the anion of a weak acid, and as such, they will prefer to associate with H + ions from the added acid to form the corresponding weak acid. However, all three weak acids formed are unstable in solution and break down, producing a gaseous product; SO3-2 (aq) + 2H + (aq) H2SO3(aq) H2O(l) + SO2(g) (eq. 9) CO3-2 (aq) + 2H + (aq) H2CO3(aq) H2O(l) + CO2(g) (eq. 10) NO2 - (aq) + H + (aq) HNO2(aq) (eq. 11) In eq. 11, the unstable HNO2 produced decomposes in water to produce NO gas; 3HNO2 2NO(g) + NO3 - (aq) + H3O + (aq) (eq. 12) but the NO produced is quickly air-oxidized yielding orange-brown NO2 gas;
3 2NO(g) + O2(g) 2NO2(g) (eq. 13) The three gases, SO2, CO2, and NO2 can all be distinguished from one another based on their properties. Sulfur dioxide, SO2, is colorless but has a sharp, choking odor. Carbon dioxide, CO2, is colorless and odorless. Nitrogen dioxide, NO2, is brownish-orange (one of only a few gases that has a color) and has a strong antiseptic-like odor. Chemistry of Specific Ions Part C, Insoluble Silver Salts The halide ions, Cl -, Br -, and I -, all form an insoluble precipitate upon addition of a source of silver ions, such as aqueous silver nitrate, AgNO3. AgCl(s, white) AgBr(s, cream) Ag + (aq) + Cl - (aq) (eq. 14) Ag + (aq) + Br - (aq) (eq. 15) AgI(s, pale yellow) Ag + (aq) + I - (aq) (eq. 16) All three solids, AgCl, AgBr, and AgI, are insoluble even when the solution is made strongly acidic. That s important because there are other anions examined in this experiment which form a precipitate with Ag +, but those ions form insoluble silver salts which will dissolve in acidic solution. Most notably, CO3-2 and NO2 - will form Ag2CO3 and AgNO2, both insoluble silver salts, but both will dissolve upon acidification of the solution. It is also worth noting here that silver chloride uniquely forms as a bright white precipitate which changes to a pale lavender-gray color after a few minutes exposure to light. No other substance behaves this way. Chemistry of Specific Ions Confirmatory Reactions In testing for the presence of an anion, the results of Parts A, B, and C should allow you to narrow down the possible anions to one or, at most, two. You will then do a confirmatory reaction in order to verify the presence of the suspected anion. In some cases the confirmatory reaction just repeats what was done earlier in Parts A, B, and/or C to confirm that the results are, in fact, reproducible. This is the case for sulfate, sulfite, carbonate and nitrite ions. In other cases a unique reaction is performed to either confirm or deny the presence of a particular anion. The anions requiring a unique confirmatory test are the halide ions, Cl -, Br - and I -. So let s examine those confirmatory reactions. Halide Ions, Cl -, Br -, I - Confirmatory reactions for the halides are based upon their very different colors when dissolved in an organic solvent in the form of the free halogens, Cl2, Br2, or I2. In solvent like hexane, Cl2 is colorless to pale yellow, depending on the concentration; Br2 is orange; and I2 is a gorgeous purple. So if chlorine water (Cl2 dissolved in water) is added to a solution containing bromide ion, Br -, in the presence of
4 hexane, the bromide is converted into Br2, which is obvious in the organic layer due to its orange color. Cl2(aq) + 2Br - (aq) Br2(aq,) + 2Cl - (aq) (eq. 19) Similarly, iodide ion, I -, would be converted into I2 upon addition of chlorine water in the presence of hexane, and the I2 would be confirmed as a purple color in the organic layer. Cl2(aq) + 2I - (aq) I2(aq,) + 2Cl - (aq) (eq. 20) Note that chlorine water added to a solution containing the chloride ion, Cl -, would result in no reaction, and is thus considered a negative confirmation for the presence of chloride. This is still a valid confirmation, however, as only chloride ion would give an acidinsoluble precipitate in Part C which then gives a colorless hexane layer in the confirmatory reaction. (Of course, the presence of a white precipitate in Part C which turns lavender-gray in a few minutes strongly suggests the presence of chloride ion.) In the halide confirmatory tests, you will be combining aqueous solutions with hexane. Hexane does not mix with water, and since the density of hexane is less than that of water, the hexane layer will always be the top layer of the two. So you will be looking for any color imparted to the top (hexane) layer in each case. Procedure In the following procedures, when told to dissolve an amount of a given substance, that should always be taken to mean an amount approximately equal in volume to a grain of uncooked rice (enriched, long grain, of course). That is important, because if too much sample is used, the limits of solubility of that salt in water could be exceeded, producing a solid which might be erroneously interpreted as a newly formed precipitate when, in fact, it is just an undissolved portion of the original sample. You will first go through all the tests using known samples containing the various anions. This will give you a detailed base of observations showing what should happen after each step. After you have analyzed all known samples, then you will analyze two unknown samples for the identity of their anion, by comparing observations for the unknowns with the observations made for the knowns.
5 Part A, Insoluble Barium Salts 1. Dissolve an amount of sodium sulfate (Na2SO4) in 10 drops deionized water in a test tube. 2. Add 0.1 M barium chloride (BaCl2) solution to the test tube dropwise until a precipitate appears or up to a maximum of 5 drops, and mix gently. Record on Data Sheet 1 your observations regarding the presence or absence of a precipitate and its color and texture (e.g. thin cloudiness, clumpy white, curd-like lumps, etc.). SAFETY NOTE!! Nitric acid, HNO3, is a strong acid, and is a corrosive and toxic substance which can cause severe burns and discoloration of the skin. Avoid contact with your skin, eyes, and clothing. If you do spill some on yourself, wash immediately with plenty of cold water. Notify your instructor. 3. If no precipitate formed in step 2, proceed to step 4. If a precipitate formed in step 2, add 5 drops of 3M nitric acid (HNO3) to the test tube and mix gently. Check cautiously for the evolution of any odorous gas by using your hand to gently waft some of the fumes near the opening of the test tube toward your nose. Record your observations on Data Sheet 1 regarding whether or not any precipitate from step 2 dissolved upon addition of the acid. 4. Repeat steps 1-3 with samples of solid sodium sulfite (Na2SO3), and sodium carbonate (Na2CO3). Record all observations on Data Sheet 1. 5. Obtain two unknown samples from your instructor. Record the unknown numbers on Data Sheet 1. Each sample will contain only one anion. Repeat steps 1-3 using each unknown, one at a time, in place of a known salt. Record your observations on Data Sheet 1. Part B, Gas Evolution 6. Starting with a fresh sample, dissolve an amount of solid sodium sulfite (Na2SO3) in 5 drops of deionized water in a test tube. SAFETY NOTE!! Sulfuric acid, H2SO4, is a strong acid, and is a corrosive and toxic substance which can cause severe burns. Avoid contact with your skin, eyes, and clothing. If you do spill some on yourself, wash immediately with plenty of cold water. Notify your instructor.
6 7. Add 5 drops of 3M sulfuric acid (H2SO4) to the tube and mix gently. Check carefully for the presence of any gas, either by visible bubbling, by odor, and/or by gas color. Record your observations on Data Sheet 2. 8. Repeat steps 6 and 7 with a solid sample of sodium carbonate (Na2CO3) and again with a solid sample of sodium nitrite (NaNO2). Record all observations on Data Sheet 2. 9. Repeat steps 6 and 7 using each unknown, one at a time, in place of a known salt. Record your observations on Data Sheet 2. Part C, Insoluble Silver Salts 10. Starting with a fresh sample, dissolve an amount of sodium chloride (NaCl) in 10 drops deionized water in a test tube. SAFETY NOTE!! Silver nitrate, AgNO3, is a toxic substance which can cause burns and discoloration of the skin. Avoid contact with your skin, eyes, and clothing. If you do spill some on yourself, wash immediately with plenty of cold water. Notify your instructor. 11. Add 5 drops of 0.10 M silver nitrate (AgNO3) solution and mix gently. Record any observations on Data Sheet 3 regarding the presence or absence of a precipitate and its color and texture. 12. If no precipitate formed in step 11, proceed to step 13. If a precipitate formed in step 11, add 5 drops of 3 M nitric acid (HNO3) to the test tube and mix gently. Record all observations on Data Sheet 3. 13. Repeat steps 10-12 with solid samples of sodium bromide (NaBr) and sodium iodide (NaI). 14. Repeat steps 10-12 using each unknown, one at a time, in place of a known salt. Record your observations on Data Sheet 3. Confirmatory Reactions Do all the following tests for knowns. For your unknowns, compare your observations with those for the knowns, and perform only those confirmatory tests
7 suggested by the results of Parts A, B, and C. For unknowns you should only have to perform one or two confirmatory reactions to verify which anion is present. THERE IS NO NEED to do all confirmatory reactions for your unknowns. Sulfate ion, SO4-2 Dissolve an amount of solid sodium sulfate, Na2SO4, (or an unknown salt) in 5 drops deionized water in a test tube. Add 0.1 M barium chloride (BaCl2) solution to the test tube dropwise until a precipitate appears or up to a maximum of 5 drops, and mix gently. If a precipitate formed, add 5 drops of 3 M HNO3 to the test tube, mix gently, and record all observations. Sulfite ion, SO3-2 Add an amount of solid sodium sulfite, Na2SO3, (or an unknown salt) to a test tube. Add 5 drops of 3 M H2SO4 and mix gently. Cautiously note the odor and color of any gas evolved. Record your observations. Nitrite ion, NO2 - Add an amount of solid sodium nitrite, NaNO2, (or an unknown salt) to a test tube. Add 5 drops of 3 M H2SO4 and mix gently. Cautiously note the odor and color of any gas evolved. Record your observations. Carbonate ion, CO3-2 Add an amount of solid sodium carbonate, Na2CO3, (or an unknown salt) to a test tube. Add 5 drops of 3 M H2SO4 and mix gently. Cautiously note the odor and color of any gas evolved. Record your observations. Halide ions, Cl -, Br -, I - Dissolve an amount of sodium chloride, NaCl, (or an unknown salt) in 5 drops of deionized water in a test tube. Add 10 drops of chlorine water, then 20 drops of hexane. Use a vortex mixer to thoroughly mix the contents of the tube. Note the color of the top (hexane) layer after mixing. Record your observations. Repeat this procedure using first NaBr, then NaI. Record all observations.
8 You may find the following flowcharts helpful in determining which ions may or may not be present in your unknown. Part A Insoluble Barium Salts SO4-2, SO3-2, CO3-2, NO3 -, NO2 -, Cl -, Br -, or I - 0.1M BaCl2 yes ppt.? no ppt. does not dissolve SO4-2 3M HNO3 ppt. dissolves could be SO3-2 or CO3-2 could be NO3 -, NO2 -, Cl -, Br -, or I -
9 Part B Gas Evolution SO4-2, SO3-2, CO3-2, NO2 -, Cl -, Br -, or I - 3M H2SO4 yes got gas? no could be SO3-2, CO3-2, or NO2 -, could be SO4-2, Cl -, Br -, or I - brown gas NO2 - present colorless, odorless gas CO3-2 present sharp smell, SO3-2 present Part C Insoluble Silver Salts SO4-2, SO3-2, CO3-2, NO2 -, Cl -, Br -, or I - 0.1M AgNO3 yes ppt.? no ppt. dissolves add 3M HNO3 ppt. doesn t dissolve could be SO4-2 NO 2 - or CO3-2 may be present contains Cl -, Br -, or I -
10 Qualitative Analysis II - Anions Data Sheet 1 Observations Part A Insoluble Barium Salts Substance after BaCl2 after HNO3 Na2SO4 Na2SO3 Na2CO3
11 Qualitative Analysis II Anions Data Sheet 2 substance Observations Part B Gas Evolution observations after H2SO4 Na2SO3 Na2CO3 NaNO2
12 Qualitative Analysis II Anions Data Sheet 3 Observations Part C Insoluble Silver Salts Substance after AgNO3 after HNO3 NaCl NaBr NaI
13 Qualitative Analysis II Anions Data Sheet 4 ion known Confirmatory Reactions SO4-2 SO3-2 CO3-2 NO2 - Cl - Br - I -
14 Qualitative Analysis II - Anions Post-lab Questions 1. Give the identity of both your anion unknowns, and write balanced, net ionic equations for any reactions which confirmed the presence of the anions you reported for both your unknowns. Include not only equations for any confirmatory reactions, but also any equations from Parts A, B, and C that prove the presence of the ion(s) you are reporting. Unknown # is Unknown # is Equations: 2. A student added aqueous BaCl2 to a sample of an anion unknown. No precipitate formed. Next the student added 3M H2SO4, and no gas was evolved. Then the student added aqueous AgNO3 to a sample of unknown in water and a precipitate formed. Then the student did the halide confirmatory reaction and concluded that the unknown anion was chloride since the hexane layer was colorless. Comment on the student s conclusion. 3. Another student got a precipitate upon addition of aqueous BaCl2 to a sample of an anion unknown. The precipitate dissolved in HNO3, along with effervescence. Upon addition of 3M H2SO4 to another fresh sample, a colorless, odorless gas was produced. When AgNO3 was added to a third fresh sample of the unknown, a white precipitate formed which dissolved, with bubbling, upon addition of HNO3. What anion was present in the sample?
15 Qualitative Analysis II - Anions Pre-laboratory Assignment 1. Circle any of the following salts which should be soluble in acidic solution; BaF2 Ag3PO4 PbS Sn(ClO2)4 CaCO3 2. Describe any safety precautions necessary in working with the reagents in this experiment. 3. A student has a mixture of Cl - and Br - in the same aqueous solution. Is it possible to positively identify both ions? Explain your answer in detail. 4. For which anions studied in this experiment are the confirmatory tests exactly the same as one of the tests performed in Part A, B, or C? Last revised 11/8/2016 DN