New version page

Oneonta CHEM 112 - Silver Group

Upgrade to remove ads
Upgrade to remove ads
Unformatted text preview:

ANALYSIS OF THE SILVER GROUP CATIONSAg+ Pb2+ Hg22+Analysis of a Mixture of CationsOne problem often faced in qualitative analysis is to test for one ion in a mixture of many ions. To find a test for one ion that is not interfered with by another ion is nearly impossible. Therefore, if one has a mixture of a large number of ions, the usual approach is to use a chemical method to separate the mixture into subgroups that consist of just a few ions. Then it may be possible to test for one particular ion in the presence of just one or two others. Alternatively, each subgroup of just a few ions may be separated further so that each ion in the subgroup ends up in a different test tube where its presence can be confirmed by other chemical tests. The chemical reactions encountered in qualitative analysis fall conve-niently into four categories: (i) acid-base (proton transfer), (ii) precipitation, (iii) complex formation, and (iv) oxidation-reduction (electron transfer). Precipitation reactions are of particular importance in qualitative analysis (as you have already seen in the analysis of anions), and they are important in the silver group. In addition, acid-base, complexation, and even oxidation-reduc-tion reactions are useful. The silver group of ions — silver(I) (Ag+), lead(II) (Pb2+), and mercury(I) (Hg22+) — is a chemically related subgroup of ions. In this experiment we want to focus on this small group to show how to use the basic reaction types—especially precipitation, acid-base, and complex formation—to sepa-rate one ion from another and to confirm the presence of that ion.CHEMISTRY OF THE SILVER GROUP CATIONSSilver(I), lead(II), and mercury(I) are grouped together in qualitative analy-sis schemes because they are the only common metal cations that form insoluble precipitates with chloride ion. For example, Ag+(aq) + Cl-(aq) → AgCl(s)This means that, in a mixture of metal cat-ions, these three metal ions can be separat-ed from all others by precipitating them as their insoluble chlo-rides, usually with 6 M HCl. Once the precipi-tates of AgCl, PbCl2, and Hg2Cl2 have been Chemistry 112 Laboratory: Silver Group Analysis Page 11Revised: December 2005Other metal cationsdissolved in water PrecipitatesAgCl, PbCl2, Hg2Cl2+ HCl(aq)Mixture of metal cations including Ag+, Pb2+, Hg22+The structure of solid AgCl. Note that this model of the unit cell of AgCl has a net of 4 Ag+ ions and 4 Cl- ions. As you will see in Chapter 13 of Chemistry & Chemical Reactivity, the structure can be thought of as a face centered cubic lattice of Cl- ions with Ag+ ions in the octahedral holes (like NaCl). See the Models folder on the General Chemistry Interactive CD-ROM.isolated from the solution containing the other metal cations, the three insol-uble chlorides can be separated from one another by chemical means. To do this, we exploit differences in the chemistry of the three ions according to the separation scheme given on a separate sheet and in the table. As you can see in the table of properties of the three silver group chlorides, PbCl2 is by far the most soluble of the three in water. Therefore, the first step in separating the three chlorides is to treat the solid mixture with hot water to selectively dissolve PbCl2 and to leave AgCl and Hg2Cl2 as a solid mixture. Once Pb2+ is back in aqueous solution, the presence of this ion in this solu-tion can be confirmed by the addition of potassium chromate, K2CrO4. The Pb2+ ion and the chromate ion, CrO42-, combine to form the bright yellow, insoluble solid lead(II) chromate, PbCrO4.Pb2+(aq) + CrO42-(aq) → PbCrO4(bright yellow solid) Silver(I) and mercury(I) chlorides are too insoluble in water to be redis-solved in water, even when the water is boiled. Therefore, we turn to another useful trick for dissolving precipitates: we take advantage of the tendency of transition metal ions such as Ag+ to form water-soluble complex ions with ammonia, NH3. Here the ammonia uses the lone pair of electrons of the N atom to form a bond with the Ag+ ion.[H3N : → Ag+ ← : NH3] , a water-soluble complex ion When a large enough concentration of NH3 is added to an insoluble precipi-tate of AgCl, the ammonia binds to the Ag+ ion and forms the complex ion. The net result is that the AgCl dissolves. AgCl(s) + 2 NH3(aq) Æ [Ag(NH3)2]+(aq) + Cl–(aq)In this way, the silver(I) ion is separated from the mercury(I) ion, as noted on the attached separation scheme. When relatively concentrated ammonia is added to the mixture of sol-ids AgCl and Hg2Cl2, the silver chloride precipitate is dissolved. However, Hg2Cl2 also reacts with NH3, but in a different manner. Mercury(I) chloride can undergo what is known as a disproportionation reaction. That is, the mercury(I) ion is both oxidized [to mercury(II), Hg2+] and reduced (to metallic mercury).Hg2Cl2(s) → Hg(liq) + Hg2+(aq) + 2 Cl–(aq)This reaction is induced by the presence of ammonia. In addition, when ammonia and Cl– ion are present, the Hg2+ ion forms a rather strange com-pound, an amido salt HgClNH2. The latter is an insoluble white solid. Hg2+(aq) + NH3(aq) + Cl–(aq) + H2O(liq) → HgClNH2(s) + H3O+(aq) Chemistry 112 Laboratory: Silver Group Analysis Page 12Revised: December 2005 Compound Color Solubility (20 ˚C) Solubility (100 ˚C) Reaction with Ammonia per 100 g H2O per 100 g H2O AgCl white 0.00015 g 0.0021 g forms colorless soluble complex PbCl2 white 1.0 g 3.3 g forms white ppt. Pb(OH)Cl Hg2Cl2 white 0.0002 g 0.001 g redox reaction; see textThe reaction of aqueous ammonia with AgCl is illustrated and described on page 779 of Chemistry & Chemical Reactivity (6e).Properties of the Chlorides of Silver Group CationsThe silver-ammonia complex ion, Ag(NH3)2+.Therefore, when NH3 is added to the AgCl/Hg2Cl2 mixture the AgCl dis-solves, as described above, and the Hg2Cl2 turns into black or gray finely-divided mercury metal and the white insoluble solid HgClNH2. The net reac-tion for Hg2Cl2 is Hg2Cl2 (s) + 2 NH3(aq) → HgClNH2(s) + Hg(0)(s) + NH4Cl(aq)Notice that a second molecule of the base NH3 is used to “collect” the H+, an acid, that is produced when Hg2+ reacts with NH3; the product is of course the salt NH4Cl. The importance of this reaction is that it provides confirma-tion of the presence of the mercury(I) ion, Hg22+, in a solution of unknown composition. If we had


View Full Document
Download Silver Group
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Silver Group and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Silver Group 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?