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Preparing Lead(II) Iodide


A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Preparing Lead(II) Iodide
How to make lead(II) iodide from lead(II) nitrate and potassium iodide?

Two soluble metal salts can react with each other to form one soluble salt and one insoluble salt. Lead nitrate solution and potassium iodide solution react to produce solid lead iodide, leaving soluble potassium nitrate in solution.

The yellow precipitate of lead iodide can be recovered by filtration. The precipitate is washed in cold water. If this precipitate is dissolved in hot water and re-crystallised, some quite spectacular crystals can be obtained

Warning! Lead compounds, especially soluble lead compounds (like lead nitrate) are very toxic.

Method
Stage 1: Precipitate lead(II) iodide
1. Dissolve both lead(II) nitrate and potassium iodide in distilled water - 250mL for each solution. (For safety reasons, an excess of potassium iodide is used to ensure all of the soluble lead is reacted away).
2. Add the potassium iodide to the lead(II) nitrate a few drops at a time. As soon as the solutions touch, bright yellow lead iodide is produced. Lead iodide is slightly soluble at room temperature, so after the first precipitate is formed a bit of swirling dissolves it back to a clear solution.
3. Add the rest of the potassium iodide solution to the lead(II) nitrate. A bright yellow precipitate (lead iodide) is produced from two clear solutions. (Pouring the potassium iodide solution into the lead nitrate was a safety measure to minimize handling the lead solution as much as possible).

Stage 2: Lead(II) iodide crystals
4. Take the flask containing the lead iodide and heat it to near boiling on a hot plate until all of the yellow crystals dissolved and the solution is clear. This happens because lead iodide is almost ten times more soluble in hot solution than in cold.
5. Turn off the heat and allowed the flask to cool to room temperature on its own. This causes the lead iodide to precipitate back out of solution. This time, because of the slow cooling, it forms extremely thin hexagonal plate-like crystals that shine and glitter as they move around the solution. (This is the golden rain effect). It is a beautiful sight.
6. Place the flask in the lab fridge overnight. Filter to isolate the lead iodide. Carefully remove the tiny crystals on the filter paper to obtain a very beautiful sheet of golden coloured lead iodide.

Cleanup:
7. Cleanup and disposal of the waste solution is an important point. We need to ensure that all of the soluble lead has been eliminated. Make a hot, saturated solution of bicarbonate and use it to treat anything that came in contact with the lead-bearing solutions. This will cause the solutions to go cloudy with a lead carbonate precipitate. The lead iodide is slightly soluble whereas the lead carbonate is totally insoluble. The lead carbonate precipitate can be filtered off and disposed of as regular garbage. The solution now contains only potassium nitrate and baking soda, and can be poured down the drain with plenty of water.



Questions
1. Write word and balanced chemical equations for the precipitation reaction.

Answers
  • Show Solutions
    1. Lead(II) nitrate + potassium iodide → potassium nitrate + lead(II) iodide
    Pb(NO3)2 (aq) + 2KI (aq) → 2KNO3 (aq) + PbI2 (s)

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