Question

When lead nitrate and potassium iodide are mixed together a yellow precipitate is formed. Name this precipitate and write its formula.

Answer 1

Step 1: Precipitation reactions

Reactions in which aqueous solutions of two soluble metal salts react to form an insoluble precipitate are called precipitation reactions. Precipitation reactions occur by double displacement. The ions are exchanged between the metal ions in an aqueous solution.

Step 2: Reaction of lead nitrate with potassium iodide

When aqueous solutions of lead nitrate $\left[\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\right]$ and potassium iodide $\left(\mathrm{KI}\right)$ are mixed, a yellow precipitate of lead iodide $\left({\mathrm{PbI}}_2\right)$ is formed. Potassium nitrate $\left({\mathrm{KNO}}_3\right)$ is also formed as a product that remains in the solution. Therefore, it is an example of a precipitation reaction.

The reaction occurs by double displacement in which the iodide ions and nitrate ions are exchanged between metal ions. The chemical equation for the reaction is:

$$\begin{gathered} \mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)+2\mathrm{KI}\left(\mathrm{aq}\right)\to {\mathrm{PbI}}_2\left(\mathrm{s}\right)+2{\mathrm{KNO}}_3\left(\mathrm{aq}\right) \\ \mathrm{Lead}\mathrm{nitrate}\mathrm{Potassium}\mathrm{Lead}\mathrm{Potassium}\mathrm{nitrate} \\ \mathrm{iodide}\mathrm{iodide} \end{gathered}$$

Therefore, the yellow precipitate is lead iodide with the chemical formula ${\mathrm{PbI}}_2$.