Question

A solution contains one or more of the following ions: \(\mathrm{Hg}_{2}^{2+}, \mathrm{Ba}^{2+},\) and \(\mathrm{Fe}^{2+} .\) When potassium chloride is added to the solution, a precipitate forms. The precipitate is filtered off, and potassium sulfate is added to the remaining solution, producing no precipitate. When potassium carbonate is added to the remaining solution, a precipitate forms. Which ions were present in the original solution, Write net ionic equations for the formation of each of the precipitates observed.

Short answer

The original solution contained (Hg_2^{2+}) and (Fe^{2+}) ions. The net ionic equations are: (Hg_2^{2+} + 2Cl^{-} -> Hg2Cl2(s)) and (Fe^{2+} + CO3^{2-} -> FeCO3(s)).

Step-by-step solution

Observing the Reaction with Potassium Chloride

When potassium chloride is added to the solution, a precipitate forms. This suggests that one of the ions reacts with chloride to form an insoluble salt. Out of the given ions, (Hg_2^{2+}) and (Ba^{2+}) can form precipitates with chloride, namely mercury(I) chloride (Hg2Cl2) and barium chloride (BaCl2), but the latter is soluble. Therefore, the precipitate must be mercury(I) chloride, indicating the presence of (Hg_2^{2+}) ions.

Analyzing the Reaction with Potassium Sulfate

When potassium sulfate is added to the remaining solution, no precipitate is formed. This indicates that (Ba^{2+}) and (Fe^{2+}) do not form an insoluble sulfate since both barium sulfate (BaSO4) and iron(II) sulfate (FeSO4) can be precipitated. However, because no precipitate forms at this step, we can infer that (Ba^{2+}) was not present in the original solution.

Testing with Potassium Carbonate

Upon adding potassium carbonate to the remaining solution, a precipitate is observed. This suggests that the remaining cation can form an insoluble carbonate. Iron(II) carbonate (FeCO3) is such an insoluble compound, indicating the presence of (Fe^{2+}) ions in the original solution.

Net Ionic Equation for Mercury(I) Chloride Precipitation

The net ionic equation for the precipitation of mercury(I) chloride when potassium chloride is added can be written as: (Hg_2^{2+} + 2Cl^{-} -> Hg2Cl2(s))

Net Ionic Equation for Iron(II) Carbonate Precipitation

The net ionic equation for the precipitation of iron(II) carbonate when potassium carbonate is added is: (Fe^{2+} + CO3^{2-} -> FeCO3(s))

Key concepts

Solubility Rules

Understanding solubility rules is crucial when it comes to predicting whether a precipitate will form in a chemical reaction.

These rules offer a guideline on which ionic compounds are soluble and which are not in water. For instance, compounds containing alkali metal ions and ammonium ion are generally soluble. Nitrates, acetates, chlorates, and perchlorates are also soluble. However, compounds with sulfate ions are soluble, except for those containing lead, barium, calcium, or mercury ions.

In the given exercise, knowledge of solubility rules helped deduce that barium forms a soluble compound with sulfate, thus explaining why no precipitate formed when potassium sulfate was added to the solution.

Net Ionic Equations

Net ionic equations simplify reactions to show only the species that directly participate in the reaction – the formation of the precipitate. Ions that do not participate, known as spectator ions, are omitted.

This makes it easier to focus on the chemistry that is happening. For example, in the given exercise, the net ionic equation for the formation of mercury(I) chloride is:
\( \mathrm{Hg}_2^{2+} + 2\mathrm{Cl}^{-} \rightarrow \mathrm{Hg2Cl2}(s) \)
This equation shows that mercury ions react with chloride ions to form the solid mercury(I) chloride, and similarly for iron(II) carbonate:
\( \mathrm{Fe}^{2+} + \mathrm{CO3}^{2-} \rightarrow \mathrm{FeCO3}(s) \)
These equations help with understanding the specific reaction without the clutter of the other compounds present.

Qualitative Analysis

Qualitative analysis uses observations to determine the presence of particular ions in a solution without precisely measuring their amount.

In the exercise, no precipitate forms when potassium sulfate is added, indicating the absence of ions that form insoluble sulfates. Conversely, precipitation upon adding potassium carbonate indicates the presence of ions that result in insoluble carbonates.

Steps in Qualitative Analysis

The approach taken in the exercise reflects sequential qualitative analysis, where:

• Different reagents are added to find which ions form precipitates.
• Phenomena such as color change, formation of gas, or precipitation reveal the presence or absence of certain ions.
• The order of reagent addition helps to narrow down the possibilities and identify the ions present.

Understanding qualitative analysis allows chemists to identify ions and their reactions without complex equipment.

Insoluble Salts

In the context of precipitation reactions, insoluble salts are those that do not dissolve in water and hence form a solid precipitate.

These salts are pivotal for qualitative analysis as their formation can indicate the presence of specific ions in the solution. For example, in the exercise, mercury(I) chloride and iron(II) carbonate are considered insoluble salts which precipitate out when chloride and carbonate ions, respectively, are added to the solution.

Common Insoluble Salts

Some examples of insoluble salts include carbonates, phosphates, and sulfides of most metals, except for those of alkali metals and ammonium. These insights on solubility are applied in various fields, such as environmental science to remove contaminants and in medicine to understand drug interactions.