Chapter 9, Problem 120A

Design an Experiment You suspect that the water in a lake close to your school might contain lead in the form of ${\text{Pb}}^{\text{2+}}\text{(aq) ions}\text{.}$ Formulate your suspicion as ahypothesis and design an experiment to test your theory. Write the net ionic equations for the reactions of your experiment. (Hint: In aqueous solution, Pb2+ forms compounds that are solids with ${\text{Cl}}^{\text{-}}{\text{, Br}}^{\text{-}}{\text{, I}}^{\text{-}}{\text{, and SO}}_{\text{4}}{}^{2-}\text{ ions}\text{.)}$

Interpretation Introduction

Interpretation:

The net ionic equation for the experimental reactions needs to be determined.

Concept introduction:

Chemical equation is the symbolic description of a chemical reaction that products and reactants are indicating in terms of their chemical formula.

Answer to Problem 120A

Net ionic equations for the reaction of lead are as follows:

$\begin{array}{l}{\text{Pb}}^{2+}\left(aq\right)+{\text{SO}}_4{}^{2-}\left(aq\right)\to {\text{PbSO}}_{\text{4}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{I}}^{-}\left(aq\right)\to {\text{PbI}}_{\text{2}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{Cl}}^{-}\left(aq\right)\to {\text{PbCl}}_{\text{2}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{Br}}^{-}\left(aq\right)\to {\text{PbBr}}_{\text{2}}\left(s\right)\end{array}$

Explanation of Solution

Lead forms solid compounds with bromide, chloride, iodide and sulfate in aqueous solutions. Some white solids of lead are lead (II) sulfate, lead bromide and lead (II) chloride. At room temperature lead (II) iodide is a bright yellow solid and on heating it turns brick red.

Iodine is the single most conclusive anion to use in this position. Through heating the compound can be further confirmed and precipitate it self will be a kind of distinct yellow. It could be useful using other anions as well but lead to further hazard to toxic compounds. As additional tests they are described below.

The net ionic equations for the reaction of lead are as follows:

$\begin{array}{l}{\text{Pb}}^{2+}\left(aq\right)+{\text{SO}}_4{}^{2-}\left(aq\right)\to {\text{PbSO}}_{\text{4}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{I}}^{-}\left(aq\right)\to {\text{PbI}}_{\text{2}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{Cl}}^{-}\left(aq\right)\to {\text{PbCl}}_{\text{2}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{Br}}^{-}\left(aq\right)\to {\text{PbBr}}_{\text{2}}\left(s\right)\end{array}$

Conclusion

Water source is polluted with a detectable concentration of aqueous lead ions.

Net ionic equations for the reaction of lead are as follows:

$\begin{array}{l}{\text{Pb}}^{2+}\left(aq\right)+{\text{SO}}_4{}^{2-}\left(aq\right)\to {\text{PbSO}}_{\text{4}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{I}}^{-}\left(aq\right)\to {\text{PbI}}_{\text{2}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{Cl}}^{-}\left(aq\right)\to {\text{PbCl}}_{\text{2}}\left(s\right)\\ {\text{Pb}}^{2+}\left(aq\right)+2{\text{Br}}^{-}\left(aq\right)\to {\text{PbBr}}_{\text{2}}\left(s\right)\end{array}$