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Experiment 7
Qualitative Analysis of Group I Cations- The Silver Group
Pre-Lab Assignment
Before coming to lab:
Read the lab thoroughly.
Answer the pre-lab questions that appear at the end of this lab exercise. The questions should be answered on a separate (new) page of your lab notebook. Be sure to show all work, round answers, and include units on all answers.
Follow the guidelines in the "Lab Notebook Policy and Format for Lab Reports" section of the lab manual to complete in your lab notebook the following sections of the report for this lab exercise: Title, Lab Purpose, and Procedure and Data Tables. Your Data Table section will include a flow chart that summarizes the directions and results of the experiment.
Purpose
In this lab, you will gain experience with qualitative analysis by identifying whether an unknown sample contains Ag
, Hg
2+ and/or Pb
2+ .
Background
Qualitative analysis is a branch of analytical chemistry that identifies particular substances in a given sample of material. In this experiment, you will analyze a known solution that contains all the Group I cations—silver, lead, and mercury(I)—and an unknown solution to determine which of these ions are present and which are absent. These three cations are grouped together because they are the only common cations that form insoluble precipitates when reacted with chloride. Therefore, they can be removed as a group from solution by the addition of HCl. The reactions that occur are simple precipitations and can be represented by the equations:
Hg 2 2+(aq) + 2 Cl-(aq) → Hg 2 Cl 2 (s) (1)
Ag+(aq) + Cl-(aq) → AgCl(s) (2)
Pb2+(aq) + 2 Cl-(aq) → PbCl 2 (s) (3)
It is important to add enough HCl to ensure complete precipitation but not too large an excess. In concentrated HCl solution, these chlorides tend to dissolve producing soluble chloro-complexes such as AgCl 2 - (aq) and PbCl 4 2-(aq).
Referring to the Ksp values of AgCl and PbCl 2 , note that PbCl 2 is significantly more soluble than AgCl and Hg 2 Cl 2. In addition, the solubility of PbCl 2 increases approximately threefold as the temperature of the solution increases from 20°C to 100°C. The solubilities of AgCl and Hg 2 Cl 2 increases very little over this temperature range. Thus, PbCl 2 can be separated from the other two chlorides by adding hot water. In hot water, PbCl 2 will dissolve while AgCl and Hg 2 Cl 2 remains insoluble.
Ksp of Group I Chlorides at 25 oC
Hg 2 Cl 2 1.1 x 10 -^18
AgCl 1.8 x 10 -^10
PbCl 2 1.7 x 10 -^5
PbCl 2 (s) → Pb2+(aq) + 2 Cl-(aq) (4)
Once Pb2+^ has been put into solution, we can check for its presence by adding a solution of K 2 CrO 4. The chromate ion, CrO 4 2-, gives a yellow precipitate with Pb2+:
Pb2+(aq) + CrO 4 2-(aq) → PbCrO 4 (s, yellow) (5)
The other two insoluble chlorides, AgCl and Hg 2 Cl 2 , can be separated by adding aqueous ammonia. Silver chloride dissolves, forming the soluble complex ion Ag(NH 3 ) 2 +:
AgCl(s) + 2 NH 3 (aq) → Ag(NH 3 ) 2 +(aq) + Cl-(aq) (6)
Ammonia also reacts with Hg 2 Cl 2 via a rather unusual oxidation-reduction reaction. The products include finely divided metallic mercury, which is black, and a compound with formula HgNH 2 Cl, which is white:
Hg 2 Cl 2 (s) (white) + 2NH3(aq) → Hg(s, black) + HgNH 2 Cl (^) (s, white) + NH 4 +(aq) + Cl-(aq) (7)
As this reaction occurs, the solid appears to change color, from white to black or grey which indicates the presence of mercury.
To establish the presence of silver, the solution containing Ag(NH 3 ) 2 +^ needs to be further tested The addition of a strong acid (HNO 3 ) to the solution destroys the complex ion and re-precipitates silver chloride. We may consider that this reaction occurs in two steps:
Ag(NH 3 ) 2 +(aq) + 2H+(aq) → Ag+(aq) + 2 NH 4 +(aq)
Ag+(aq) + Cl-(aq) → AgCl(s)
Ag(NH 3 ) 2 +(aq) + 2 H+(aq) + Cl-(aq) → AgCl(s, white) + 2 NH 4 +(aq) (8)
The formation of a white precipitate indicates the presence of silver in the solution.
Heating Due to the small quantity of material being heated, test tubes containing samples should NEVER be heated directly in a flame. A solution in test tube can reach its boiling point within a few seconds, and may be ejected violently from the test tube. All heating should be done using a water bath on a hot plate. Be careful that the tops of the test tubes are well above the water. The water may be boiling at times and could spatter into the test tubes, contaminating the contents.
Testing pH When directed to check the pH of a solution, stir the solution thoroughly with a clean glass stirring rod and then touch the tip of the rod to a piece of litmus paper. Several such tests may be performed on each strip of paper. Never insert the test paper into the test tube since the chemicals on the paper could contaminate the contents. Red litmus paper will turn blue in basic solutions; blue litmus paper will turn red in acidic solutions.
General Safety Tips Add all reagents gradually. Heat may be evolved, and the solution could become hot enough to boil. This is most likely to occur when neutralizing strong acids and bases. If a gas is evolved, such as when dissolving a carbonate solid in acid, the solution could bubble out of the test tube. Never hold a test tube so the open end is pointing at anyone. Never smell the contents of a test tube directly. If directed to check an odor, hold the test tube about 15 cm from your face, and gently waft any fumes from the top of the test tube toward your nose.
How to Describe Mixtures Always describe the color and clarity of mixtures and reagents before mixing and what the mixture looks like after mixing, heating, centrifuging, etc. For example the following might be recorded for the first steps of this experiment.
Starting unknown solution-clear and colorless
6 M HCl-clear and colorless
Add 8 drops of the HCl to the unknown solution and stir mixture. A cloudy white,
precipitate formed.
After centrifuging, white precipitate settles to bottom, clear and colorless supernatant.
Common Reagents Some common reagents and their uses in qualitative analysis are listed Table 1. You should become familiar with these reagents and their uses.
Table 1: Common Reagents in Qualitative Analysis Reagent Effect on System and Uses
6 M HCl
Raises [H+]; lowers [OH–]; dissolves insoluble hydroxides, carbonates, chromates and some sulfides; destroys hydroxo and NH 3 complex ions; increases [Cl
- ] causing precipitation of insoluble chlorides. 6 M NaOH Raises [OH
]; lowers [H
]; precipitates insoluble hydroxides; forms hydroxo- complex ions.
6 M NH 3
Raises [OH–]; lowers [H+]; forms NH 3 complex ions; precipitates insoluble hydroxides; forms a basic buffer solution with NH 4 +.
6 M HNO 3
Raises [H+]; lowers [OH–]; dissolves insoluble hydroxides, carbonates and chromates; destroys hydroxo and NH 3 complex ions; a good oxidizing agent when hot; dissolves insoluble sulfides by oxidation of sulfide ion.
Procedure
Safety: Wear your safety glasses while performing this experiment. Lead and mercury salts are toxic, and chromates are known to be carcinogenic. Silver ion is corrosive and leaves a black stain on the skin. HCl, NH 3 and HNO 3 are irritants. Avoid contact and wash immediately if any is spilled or splashed on you. Make certain to wash your hands thoroughly when you leave the laboratory.
Waste: As you perform the experiment, collect all waste solutions in a waste beaker. This mixture should then be discarded in the appropriate waste container. DO NOT POUR ANY OF THE SOLUTIONS DOWN THE DRAIN.
Note: The known is a mixture of equal volumes of 0.1 M AgNO 3 , 0.2 M Pb(NO 3 ) 2 and 0.1 M Hg 2 (NO 3 ) 2. Testing known samples is helpful in this analysis since doing so will allow you to observe what a positive test looks like. It is usually convenient to test a known sample simultaneously with your unknown.
Your unknown may contain Ag+, Hg 2 2+^ and/or Pb2+^ ions.
Record your unknown number in your notebook.
Precipitation of Group I Chlorides
- Add 8 drops of 6.0 M HCl to 2.0 mL of the sample. Shake the sample gently for two minutes to mix the sample. A white precipitate will form since Group I ions are present.
Group I ion (aq) + Cl-(aq)→Group I Chlorides (s)
Centrifuge the solution, being careful to balance the centrifuge by placing test tubes containing equal volumes on opposite sides of the centrifuge.
After centrifuging your sample, add one more drop of the 6 M HCl to the solution to test for completeness of precipitation. If the solution turns cloudy (indicating that the precipitation is not complete), add 2 more drops of 6.0 M HCl, stir, centrifuge, and again test for completeness of precipitation. The supernatant must be free of Group I ions before moving on to the next step.
Be careful not to add too much HCl since it is possible to form chloro-complexes which are soluble. PbCl 2 , in particular, tends to do this.
PbCl2(s) + 2Cl-^ PbCl 4 2-(aq)
- Decant the supernatant solution from the chloride precipitate. The solution can be discarded into a waste container since all of the ions we are interested in this experiment should be in the precipitate.
Separation and Identification of Pb2+
- Half-fill a 100 mL beaker with distilled water and heat to near boiling.
- Using a glass pipette and using caution with the hot distilled water, add 2.0 mL of the hot water to the test tube containing the precipitate.
Data Tables
It is possible to summarize the directions for analysis of the Group I cations in what is called a flow diagram. In the diagram, vertical lines link successive steps in the procedure. Reactants are at the top end of each vertical line and products formed are at the bottom end. On the product end, a horizontal line separates the solid products on the left and the solution products on the right. Reagents and conditions used to carry out each step are placed alongside the lines. The chart has been filled in for step 1 of the procedure as an example.
Before coming to lab , study the procedure and then copy into your notebook the flow diagram shown below in the Procedure section of your lab notebook. Complete the flow diagram for the entire lab. Every line and box should be filled in.
In addition, under the diagram (or on the following page in your notebook), write the correct balanced reactions for each step of the lab for the known solution. Label them with the corresponding step in the lab and include phases for each substance (s, aq etc).
Finally, in your notebook organize two spaces where you will record your observations for the experiment- one space for the known and one for the unknown. An example of what you might record is on pg 4 of this lab under “How to Describe a Mixture”.
Group I Flow Diagram Ag+, Pb+2, Hg 2 +
Solution contains other ions
Add 6 M HCl
White precipitate AgCl, PbCl 2 , Hg 2 Cl 2
White precipitate AgCl, Hg 2 Cl 2
Solution contains _____________
indicates ________
Solution contains indicates ________
indicates ________
Post-Lab Questions
For numerical problems, you must show all work for credit!
- Using the Ksp values given in the table on the first page of the lab, calculate the molar solubility of
a) AgCl b) PbCl 2
- A 0.50 gram sample of AgCl(s) is shaken with 5.0 mL of 6.0 M NH 3 until there is no more net reaction. (Kf for Ag(NH 3 ) 2 +^ = 1.7 x 10^7 ) a) Write the net ionic equation, including phase symbols, for the chemical reaction that occurs.
b) Does any solid AgCl remain? If so, what mass remains? (Hint: this is a solubility-complex ion question, NOT a limiting reactant problem!)
- Suppose 6 M NH 3 is accidentally added instead of hot water in the step 4. Is it still possible to identify the ions present using this sample? If identification is still possible, use a flow diagram to show the steps needed to complete the analysis.
