Problem 5 - Chemistry: The Central Science

Question

You are presented with a white solid and told that due to careless labeling it is not clear if the substance is barium chloride, lead chloride, or zinc chloride. When you transfer the solid to a beaker and add water, the solid dissolves to give a clear solution. Next a \(\mathrm{Na}_{2} \mathrm{SO}_{4}(a q)\) solution is added and a white precipitate forms. What is the identity of the unknown white solid? [Section 4.2]

Step-by-Step Solution

Answer

The unknown white solid must be either barium chloride (\(\mathrm{BaCl}_{2}\)) or lead chloride (\(\mathrm{PbCl}_{2}\)) since both form a white precipitate upon adding the sodium sulfate solution. Zinc chloride (\(\mathrm{ZnCl}_{2}\)) can be ruled out as it does not form a precipitate.

Step 1: Write down the possible chemical reactions

First, we need to consider the reactions that could occur when adding the sodium sulfate solution to the dissolved unknown white solid. We can represent the three possible compounds as \(\mathrm{MCl}_{2}\), where M is either barium (Ba), lead (Pb), or zinc (Zn). The possible reactions are: 1. \(\mathrm{BaCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{BaSO}_{4}(s) + 2\mathrm{NaCl}(aq)\) 2. \(\mathrm{PbCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{PbSO}_{4}(s) + 2\mathrm{NaCl}(aq)\) 3. \(\mathrm{ZnCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{ZnSO}_{4}(s) + 2\mathrm{NaCl}(aq)\)

Step 2: Check the solubility of the precipitates

Now that we have the reactions, we can check the solubility rules for the precipitates formed: 1. \(\mathrm{BaSO}_{4}\) has low solubility in water and forms a white precipitate. 2. \(\mathrm{PbSO}_{4}\) has low solubility in water and forms a white precipitate. 3. \(\mathrm{ZnSO}_{4}\) is highly soluble in water and does not form a precipitate.

Step 3: Identify the unknown solid based on the given information

According to the information given in the exercise, the unknown white solid dissolves in water and forms a white precipitate when a sodium sulfate solution is added. Based on the solubility rules, we conclude that: - If the unknown solid was \(\mathrm{BaCl}_{2}\), it would form a white precipitate of \(\mathrm{BaSO}_{4}\) upon adding the sodium sulfate solution. - If the unknown solid was \(\mathrm{PbCl}_{2}\), it would form a white precipitate of \(\mathrm{PbSO}_{4}\) upon adding the sodium sulfate solution. - If the unknown solid was \(\mathrm{ZnCl}_{2}\), it would not form a precipitate upon adding the sodium sulfate solution. Since a white precipitate was formed, we can rule out zinc chloride as the unknown solid. Therefore, the unknown white solid must be either barium chloride or lead chloride.

Key Concepts

Precipitation Reactions

Understanding precipitation reactions is essential when working with solutions and mixtures in chemistry. They occur when two solutions containing dissolved ions are mixed and an insoluble compound, called a precipitate, forms. This process can be used to identify unknown substances, separate elements, or even purify compounds.

For instance, in the case of our unknown white solid, when we mix this with an aqueous solution of sodium sulfate, a white precipitate forms if the solution contains barium or lead ions. These reactions are visual and swift, allowing chemists to infer the makeup of the original solution based on the precipitate's appearance and characteristics. To predict when and what type of precipitate will form, we rely on the solubility rules, which are guidelines that dictate whether certain compounds will be soluble or insoluble in water.

Solubility Rules

The solubility rules are a set of guidelines that help predict the solubility of ionic compounds when they are placed in water. An important aspect of these rules is that they list which ionic compounds are generally soluble and which tend to form precipitates.

In the context of our exercise, the solubility rules indicate that compounds containing sulfate ions, such as \(\mathrm{BaSO}_4\) and \(\mathrm{PbSO}_4\), are usually insoluble with few exceptions. These rules suggest that when barium or lead ions are present in a solution and come into contact with sulfate ions, a white precipitate is likely to form. In contrast, \(\mathrm{ZnSO}_4\) is an exception where the zinc ion and sulfate ion remain in solution, signifying that it is highly soluble and no precipitate would be observed. By applying these rules to the observed precipitation reaction, we can deduce the potential identity of the unknown white solid.

Ionic Compounds

Ionic compounds such as \(\mathrm{BaCl}_2\), \(\mathrm{PbCl}_2\), and \(\mathrm{ZnCl}_2\) are formed from the ionic bonds between a metal and a non-metal ion. In a solid state, these compounds create a crystal lattice structure, but when dissolved in water, they separate into their individual ions. The ability of these compounds to dissolve in water and produce ions is what makes solutions of ionic substances able to conduct electricity.

The exercise we are examining demonstrates this property beautifully. Initially, the unknown solid dissolves, suggesting it is an ionic compound that dissociates into its constituent ions. Once dissociated, these ions can interact with other dissolved ions, potentially leading to the formation of a new insoluble ionic compound, or precipitate, as dictated by the solubility rules. This fundamental characteristic of ionic compounds is the foundation of the precipitation reactions described earlier and illustrates the interplay between the physical properties of substances and chemical reactivity.