This results in the chemical reaction:
(PbCO3)2 . Pb(OH)2(s) + 6HNO3(aq) 3Pb2+(aq) + 6NO3- (aq)+ 2H2O(aq) + 2H+(aq) + 2HCO3-(aq)
The purpose of this reaction is to produce a solution of reactive lead (Pb2+)
Stop here for next week. you will be assigned a drawer to put your ppt. in to allow it to dry. Write your name on a paper towel, place your watch glass with the filter paper on it on the paper towel.You will be using 200 mg of basic lead carbonate, (PbCO3)2 . Pb(OH)2, rather than galena. The formula weight for basic lead carbonate is 775.63 g mol-1. How many moles of Pb2+ does one mole of (PbCO3)2 . Pb(OH)2 produce? On page 405 line 5 after Preparation of Lead Iodide change "5 mL of dilute (6 M) HCl." to 5 mL of dilute (6 M) HNO3. This results in the chemical reaction:
(PbCO3)2 . Pb(OH)2(s) + 6HNO3(aq)
The purpose of this reaction is to produce a solution of reactive lead (Pb2+)Skip the section between the two caution boxes and begin again at "Transfer the solution..." The last line on page 405 where the test calls for dilute nitric acid to be added dropwise make sure you use the 0.5 M HNO3. This is not an insurmountable error, but it will take longer to complete the experiment. skip the first paragraph after the first caution box on page 406. Continue from the last paragraph on page 405 to the paragraph on page 406 which starts "Dissolve exactly 600 mg KI ..." and change the amount of distilled water used here to 50 mL instead of 5 mL. Use a 150 mL beaker for this step. You will be using a larger Buchner funnel instead of the Hirsch funnel.
Things to do for greater success:Cross out the word flask in the third paragraph on page 407, use a 50 mL beaker. You will be using vacuum filtration rather than gravity filtration. You will not be recovering or recycling the lead as lead carbonate.
1st week:Predict the theoretical yield of lead iodide. (See balanced stochiometric equation above) Predict the amount of NH3 you will need to neutralize the acid you used (See balanced stochiometric equation above) If all the nitric acid is not neutralized the iodide in the next steps will oxidize to iodine (see the competitive balanced stochiometric equation below) and be useless in the experiment. 2I-(aq) + 2NO3-(aq) + 4H+(aq) 2NO2(g) + I2(s) + 2H2O(l) Note color changes and formation of precipitates (This precipitate is very fine and hard to manage so we will be recrystallizing the solution to produce larger more managable crystals). 2I-(aq) + Pb2+(aq) I2(s) Look up the solubility of lead iodide in ice cold, room temperature, and hot water. (The Merck Index) Determine how much hot water you will need to dissolve your predicted yield. Allow the hot lead iodide solution to cool slowly and without shocks. Once a large amount of the lead iodide has recrystallized at room temperature use an ice bath to precipitate out even more. When decanting the lead iodide into the vacuum filtration apparatus transfer as much of the liquid first as you can. Use a glass stir rod held over the mouth of the beaker with your index finger to direct the liquid into the center of the filter paper, and do not add the liquid so quickly that it leaves a pool much larger than a quarter on your filter paper. Chase the final bit of ppt. out of the beaker using very small and very quick jets of ice cold water from your wash bottle. Try and maintain the size of the accumulated ppt. to that of a quarter also.
2nd week:Start your crucible constant weight process early. The lead sulfate is very fine and white. You may lose some in the filtration process if you are not careful. Pb I2(s) + 2HNO3(aq)
(The lead is now again in a reactive form)
Pb2+(aq) + + 2I-(aq) + 2H+(aq) + SO42- (aq)
(The lead is now again in a solid form) When decanting the lead sulfate into the vacuum filtration apparatus transfer as much of the liquid first. Use a glass stir rod held over the mouth of the beaker with your index finger to direct the liquid into the center of the filter paper, and do not add the liquid so quickly that it leaves a pool much larger than a quarter on your filter paper. Chase the final bit of ppt. out of the beaker using very small and very quick jets of ice cold water from your wash bottle. Carefully fold the filter paper to trap the ppt. and to fit into the crucible. Ash the filter paper completely. Achieve a constant weight.
Technique helpers:
When setting up your vacuum filtration apparatus insure its proper operation. After securing the side arm flask to a ring stand attach the hose securely. Turn the water on supplying your aspirator. Liberally add water to your filter funnel containing the filter paper. The water should disappear rapidly and the filter paper should take on a dimpled appearance.