Problem 77 - Chemistry The Central Science In Si Units

Question

A solution containing several metal ions is treated with dilute HCl; no precipitate forms. The pH is adjusted to about \(1,\) and \(\mathrm{H}_{2} \mathrm{~S}\) is bubbled through. Again, no precipitate forms. The pH of the solution is then adjusted to about 8 . Again, \(\mathrm{H}_{2} \mathrm{~S}\) is bubbled through. This time a precipitate forms. The filtrate from this solution is treated with \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{HPO}_{4}\). No precipitate forms. Which of these metal cations are either possibly present or definitely absent: \(\mathrm{Al}^{3+}, \mathrm{Na}^{+}, \mathrm{Ag}^{+}, \mathrm{Mg}^{2+} ?\)

Step-by-Step Solution

Answer

The definitely present ion is possibly \(\mathrm{Al^{3+}}\); \(\mathrm{Ag^+}\), \(\mathrm{Na^+}\), and \(\mathrm{Mg^{2+}}\) are absent.

Step 1: Reaction with Dilute HCl

When dilute HCl is added to a solution, metal ions that form insoluble chlorides will precipitate out. However, no precipitate forms, which indicates that none of the ions present form insoluble chlorides with HCl. - **Definitely Absent:** \(\mathrm{Ag}^+\) ions, because \(\mathrm{AgCl}\) is insoluble and would have precipitated. - **Possibly Present:** \(\mathrm{Al}^{3+}\), \(\mathrm{Na}^+\), and \(\mathrm{Mg}^{2+}\), as they do not form insoluble chlorides with HCl.

Step 2: Reaction at pH 1 with H2S

When \(\mathrm{H}_2\mathrm{S}\) is bubbled through the solution at low pH (pH 1), metal ions that form less soluble sulfides will precipitate. Since no precipitate forms, the ions that form sulfides at low pH are absent.- **Definitely Absent:** \(\mathrm{Ag}^+\), which would form \(\mathrm{Ag}_2\mathrm{S}\).- **Possibly Present:** \(\mathrm{Al}^{3+}\), \(\mathrm{Na}^+\), and \(\mathrm{Mg}^{2+}\), as they do not precipitate with \(\mathrm{H}_2\mathrm{S}\) at this pH.

Step 3: Reaction at pH 8 with H2S

At pH 8, \(\mathrm{H}_2\mathrm{S}\) can cause precipitation of hydroxides and some sulfides. A precipitate forms, indicating the presence of ions that precipitate at this pH. These could be hydroxides or sulfides of the remaining ions.- **Possibly Present:** \(\mathrm{Al}^{3+}\), because \(\mathrm{Al(OH)}_3\) is precipitated at this pH.- **Definitely Absent:** \(\mathrm{Na}^+\) and \(\mathrm{Mg}^{2+}\), given that no magnesium precipitate would form and sodium salts are not expected to precipitate.

Step 4: Reaction with (NH4)2HPO4

\((\mathrm{NH}_4)_2\mathrm{HPO}_4\) can precipitate certain metal cations as their phosphates. No precipitate forms, which suggests that the metal cations that normally form insoluble phosphates are absent.- **Definitely Absent:** \(\mathrm{Mg}^{2+}\), as \(\mathrm{Mg}_3(\mathrm{PO}_4)_2\) would normally precipitate. - **Possibly Present:** \(\mathrm{Al}^{3+}\), because aluminum phosphate does not form readily under these conditions.

Key Concepts

Metal Ions

Metal ions are positively charged particles found in solutions, often forming when metals dissolve in water. In chemical solutions, they can react with other elements to form various compounds. These ions can be essential for different chemical analysis processes.
Some typical metal ions include:

• Aluminum (\(\mathrm{Al}^{3+}\))
• Sodium (\(\mathrm{Na}^{+}\))
• Silver (\(\mathrm{Ag}^{+}\))
• Magnesium (\(\mathrm{Mg}^{2+}\))

The presence or absence of these ions can be detected through specific reactions like precipitation reactions. Metal ions play a crucial role in qualitative analysis, allowing chemists to determine which ions are present in a given solution through various chemical reactions and observations.

Precipitation Reactions

Precipitation reactions occur when two solutions that contain soluble salts are mixed, resulting in the formation of an insoluble solid known as a precipitate. This solid can often be seen as a cloudiness or solid in the mixture.
For example, adding dilute hydrochloric acid (HCl) to a solution can trigger precipitation of metal chlorides, but only if they are insoluble in water. In the given chemical process:

• No precipitate with HCl indicates absence of ions that form insoluble chlorides, like \(\mathrm{Ag}Cl\).
• Bubbling \(\mathrm{H}_2\mathrm{S}\) at different pH levels enables precipitation of certain metal sulfides or hydroxides.
• Reactive phosphate compounds can cause some metal cations to form precipitates, depending on their solubility.

Recognizing these reactions helps deduce the presence or absence of specific metal ions by observing whether or not a precipitate forms.

Chemical Equilibrium

Chemical equilibrium refers to a state where the forward and backward reactions occur at the same rate, and the concentrations of reactants and products remain constant. It's critical for understanding how conditions like pH influence the formation of precipitates during reactions.
In the context of metal ions, equilibrium helps dictate solubility and precipitation:

• Equilibrium of \(\mathrm{H}_2\mathrm{S}\) determines how sulfides precipitate under different pH scenarios.
• Shifting equilibrium by changing pH affects the solubility of metal hydroxides and sulfides.
• Equilibrium constants help predict whether a precipitate will form under given conditions.

By adjusting variables like pH, the equilibrium can shift to favor the formation of solids (or keep them dissolved), crucial for successful qualitative analysis.

pH Adjustment

pH is a measure of how acidic or basic a solution is, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. Adjusting pH is a vital process in chemical reactions, influencing the solubility of metals and the formation of precipitates.
In precipitation reactions, pH adjustments can have the following effects:

• At pH 1, only certain metal sulfides can precipitate; higher pH often increases solubility of hydroxides.
• For \(\mathrm{Al}^{3+}\) ions, adjusting pH to around 8 allows \(\mathrm{Al(OH)}_3\) formation, which precipitates out.
• Controlling pH ensures precise manipulation of conditions to achieve desired reactions, like completely removing or retaining specific ions.

Understanding pH adjustments is a powerful tool in qualitative analysis, making it possible to differentiate between potentially present metal ions based on their behavior under various pH conditions.