Qualitative Analysis: Cations - Group I



AP Chemistry

Qualitative Analysis: Isolation of Group I Cations

Objective:

To separate and identify Group I ions using qualitative analysis techniques.

Purpose:

We will use a computer simulation () to separate and identify the ions Ag+, Pb2+, and Hg22+. Known solutions of the three ions will first be identified. Two unknown solutions will then be identified using the same procedure.

Discussion:

In classic qualitative analysis, transition metal ions are separated into various groups by their properties. These group numbers do not correspond with the element’s location in the periodic table. The ions in Group I are not found in group I of the periodic table.

If a dilute solution of HCl is added to a solution containing the common metal ions of lead, mercury I and silver, a precipitate will be formed. If this solution is centrifuged to collect the solid material at the bottom of a test tube, the supernatant liquid can be removed (decanted). The remaining solid can be washed with distilled water so that the ONLY metal ions that remain are Ag+, Hg22+, and Pb2+, which are designated as Group I Cations. The reactions are conducted in acidic solution, to avoid the possibility that these and other metal ions might be precipitated as hydroxides or oxides.

If addition of dilute HCl to a solution of metal cations results in a precipitate, this is conclusive evidence that at least one - and possibly all - of the Group I Cations were are present in the original solution. Additional tests are necessary to determine which of these three cations are present in the precipitate.

Testing for Pb2+

While these three chlorides are relatively insoluble in cold water, lead chloride is found to be quite soluble in hot water. Therefore, cold water may be added to the precipitate containing any or all of the three chlorides without any of them re-dissolving. However, heating this mixture will cause lead chloride to dissolve, if any is present. While still hot, the tube can be centrifuged and the supernatant solution can be decanted into another test tube. If any lead ion is present, addition of cold HCl solution to the liquid will cause white lead chloride to precipitate.

Testing for Ag+

Silver ion has a special property which distinguishes it from the other two cations of Group I. When mixed with ammonia in basic solution, complex ions are formed (these complexes are not formed in acidic solution in which the ammonia exists as ammonium ion): Ag+(aq) + 2NH3(aq) ( Ag(NH3)2+(aq)

The formation of these complex ions lowers the concentration of free silver ion, allowing precipitated silver chloride to dissolve: AgCl(s) + 2 NH3(aq) ( Ag(NH3)2+(aq) + Cl-(aq)

Thus, when the Group I cations are precipitated with HCl, ammonia may be added to the precipitate until litmus indicates that the solution is basic. If the precipitate contains the silver ion, it will dissolve and may be removed from the others by centrifuging and decanting the supernatant solution. Addition of HCl to this solution until litmus indicates that the solution is acidic will re-precipitate silver ion, if present.

Testing for Hg22+

Mercurous chloride may be distinguished from the other two Group I chlorides by its ability to resist re-dissolving by either heating or the addition of ammonia.

Once the Group I cations have been separated by heating and the addition of ammonia, their presence(s) may be confirmed by their precipitates with chromate ion:

• Ag+(aq) + CrO42-(aq) ( Ag2CrO4(s) Red precipitate

• Hg22+(aq) + CrO42-(aq) ( Hg2CrO4(s) Yellow-brown precipitate

• Pb2+(aq) + CrO42-(aq) ( PbCrO4(s) Yellow precipitate

Procedure

Observations with solutions containing known cations:

1. Place a drop or two of a sample of Ag+ ion in one test tube, a sample of Hg22+ in a second tube, and a sample of Pb2+ in a third tube.

2. Add 2 drops of dilute HCl (3 M) to each tube and observe the precipitates.

3. Centrifuge the tubes. Decant and discard the supernatant liquid from each tube.

4. Wash the precipitates by adding 2 drops of distilled water to each tube. Centrifuge, decant and discard the supernatant liquids.

5. Add 2 drops of distilled water to each tube, then heat the tubes in a boiling water bath. Note what happens to the tube containing Pb2+. (This tube may be centrifuged and no solid will appear.) If the solution is cooled by adding a drop of distilled water (or simply by standing for a while) the white precipitate will reappear.

6. Add two drops of ammonia (NH3) solution to each tube, and test the acidity of each solution by dipping a clean stirring rod into the liquid and touching it to litmus paper. If the solutions are not basic, add more ammonia until they turn red litmus blue. Note any change in the precipitates. Note specifically the results for the tube that contains the solution with Ag+. Again, this tube may be centrifuged.

7. Add a drop of dilute HCl (3 M) to the clear solution, and test with litmus. If the solution is not acidic, add more HCl dropwise until the solution turns blue litmus red. Note any changes in the tube.

8. Add a drop or two of sodium chromate solution (Na2CrO4) to each tube. Note the colors of the precipitates.

9. Discard all of the solutions in the appropriate waste containers.

10. Repeat the experiment with two drops of each ion in the same test tube.

11. Unknowns:

There is a trial unknown and an assigned unknown. The trial unknown is randomly generated by the program. After performing the experiment, click on Check Unknown. Click on the buttons to indicate whether each cation is present or not, then click on Check Answer. Your results will be "graded".

If your Instructor has assigned you a code number, select assigned unknown. When you Check Unknown, your results will not be graded. You will be able to print out your results.

*The procedure for analyzing an unknown is given as a Flow Sheet. When you click on Flow Sheets, a large schematic diagram appears. When this window is closed, step-by-step instructions are given on the right side of the screen.

Write down all observations. Include results from trial unknown and assigned unknown. You can do more than one trial unknown if time permits. Make sure to record the number and identity of your unknown!!!!

Tips: Remember to stir with the stirring rod!

Name_________________________________

Report Sheet

Please only turn in this sheet!

Pre-Lab Questions

1. Please write the reactions that occur when HCl is added to the solution of metals. (Write three equations, one for each metal)

a. ________________________________________________________

b. ________________________________________________________

c. ________________________________________________________

2. If a solution contains the precipitates of silver, mercury I and lead II, what effect will the following procedures have?

a. Heating the solution

b. Adding ammonia to the solution

Observations/Results

|Step |Observations |

|2 | |

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|5 | |

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|7 | |

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|8 | |

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|Trial 2 with two drops of each solution in the same tube. |

|2 | |

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|5 | |

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|6 | |

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|7 | |

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|8 | |

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Assigned Unknown

Please briefly record the steps you took to solve for your unknown in the space below. The flowchart at the end of the page should help you isolate the unknown ion.

My Unknown # ______________

My Procedures

Attach a copy of your printed results for the assigned unknown to this page to receive full credit for this lab.

|UNKNOWN SAMPLE |

|add 1-2 drops HCl (3M) |

|Centrifuge |

|Decant |

|[pic] |

|SOLID 1 | |LIQUID 1 |

|May contain Ag+, Hg22+, Pb2+ | |May contain cations other than Group I |

|Add 2 drops distilled water | |Save for further analysis or Discard |

|Heat | | |

|Centrifuge | | |

|Decant | | |

|[pic] |

|SOLID 2 | |LIQUID 2 |

|May contain Ag+, Hg22+ | |May contain Pb2+ |

|Add NH3 until Basic | |Cool or add HCl |

|Centrifuge | |Precipitate Confirms Pb2+ |

|Decant | |Add Na2CrO4 for further confirmation |

|[pic] |

|SOLID 3 | |LIQUID 3 |

|May contain Hg22+ | |May contain Ag+ |

|Add HCl until Acidic | |Add HCl until acidic |

|Precipitate Confirms Hg22+ | |Precipitate Confirms Ag+ |

|Add Na2CrO4 for further confirmation | |Add Na2CrO4 for further confirmation |

Dispose of all Materials in Appropriate Waste Containers

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