Problem 94 - Chemistry The Central Science

Question

You choose to investigate some of the solubility guidelines for two ions not listed in Table \(4.1,\) the chromate ion \(\left(\mathrm{CrO}_{4}^{2-}\right)\) and the oxalate ion \(\left(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\right) .\) You are given \(0.01 \mathrm{M}\) solutions (A, B, C, D) of four water-soluble salts: $$ \begin{array}{lll} \hline \text { Solution } & \text { Solute } & \text { Color of Solution } \\ \hline \text { A } & \mathrm{Na}_{2} \mathrm{CrO}_{4} & \text { Yellow } \\ \mathrm{B} & \left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4} & \text { Colorless } \\ \mathrm{C} & \mathrm{AgNO}_{3} & \text { Colorless } \\ \mathrm{D} & \mathrm{CaCl}_{2} & \text { Colorless } \\ \hline \end{array} $$ When these solutions are mixed, the following observations are made: $$ \begin{array}{lll} \hline \text { Expt } & \text { Solutions } & \\ \text { Number } & \text { Mixed } & \text { Result } \\ \hline 1 & \mathrm{~A}+\mathrm{B} & \text { No precipitate, yellow solution } \\\ 2 & \mathrm{~A}+\mathrm{C} & \text { Red precipitate forms } \\ 3 & \mathrm{~A}+\mathrm{D} & \text { Yellow precipitate forms } \\ 4 & \mathrm{~B}+\mathrm{C} & \text { White precipitate forms } \\ 5 & \mathrm{~B}+\mathrm{D} & \text { White precipitate forms } \\ 6 & \mathrm{C}+\mathrm{D} & \text { White precipitate forms } \end{array} $$ (a) Write a net ionic equation for the reaction that occurs in each of the experiments. (b) Identify the precipitate formed, if any, in each of the experiments.

Step-by-Step Solution

Answer

In this exercise, we analyzed the reactions between various ions present in given solutions and identified the interacting ions and precipitates formed in each experiment. The net ionic equations for the reactions are as follows: - Experiment 2: \(2Ag^+_{(aq)} + CrO^{2-}_{4(aq)} \rightarrow Ag_2CrO_{4(s)}\) (Red precipitate: Silver chromate) - Experiment 3: \(Ca^{2+}_{(aq)} + CrO^{2-}_{4(aq)} \rightarrow CaCrO_{4(s)}\) (Yellow precipitate: Calcium chromate) - Experiment 4: \(2Ag^+_{(aq)} + C^{2-}_{2O^2-_4(aq)} \rightarrow Ag_2C_2O_{4(s)}\) (White precipitate: Silver oxalate) - Experiment 5: \(Ca^{2+}_{(aq)} + C^{2-}_{2O^2-_4(aq)} \rightarrow CaC_2O_{4(s)}\) (White precipitate: Calcium oxalate) - Experiment 6: \(Ag^+_{(aq)} + Cl^-_{(aq)} \rightarrow AgCl_{(s)}\) (White precipitate: Silver chloride)

Step 1: (Step 1: Identify the Interacting Ions)

Let's examine the reactions occurring in each experiment and determine which ions are interacting with each other to produce a precipitate. The experiments are: 1. A + B (No precipitate) 2. A + C (Red precipitate) 3. A + D (Yellow precipitate) 4. B + C (White precipitate) 5. B + D (White precipitate) 6. C + D (White precipitate) From these observations, we can see which salts are reacting with each other in each experiment, and it is apparent that the chromate ion is reacting with silver or calcium ions to form precipitates, while the oxalate ion is reacting with silver or calcium ions to form precipitates as well.

Step 2: (Step 2: Write the Ion Interaction Equations)

Now that we have identified the ions interacting with each other, let's write the ion interaction equations for each experiment where a precipitate is formed: - For experiment 2: \(CrO^{2-}_4 + Ag^+ \rightarrow Ag_2CrO_4\) - For experiment 3: \(CrO^{2-}_4 + Ca^{2+} \rightarrow CaCrO_4\) - For experiment 4: \(C^{2-}_2O^2-_4 + Ag^+ \rightarrow Ag_2C_2O_4\) - For experiment 5: \(C^{2-}_2O^2-_4 + Ca^{2+} \rightarrow CaC_2O_4\) - For experiment 6: \(Cl^- + Ag^+ \rightarrow AgCl\)

Step 3: (Step 3: Write the Net Ionic Equations)

Now, let's write the net ionic equations for each experiment: - Experiment 2: \(2Ag^+_{(aq)} + CrO^{2-}_{4(aq)} \rightarrow Ag_2CrO_{4(s)}\) - Experiment 3: \(Ca^{2+}_{(aq)} + CrO^{2-}_{4(aq)} \rightarrow CaCrO_{4(s)}\) - Experiment 4: \(2Ag^+_{(aq)} + C^{2-}_{2O^2-_4(aq)} \rightarrow Ag_2C_2O_{4(s)}\) - Experiment 5: \(Ca^{2+}_{(aq)} + C^{2-}_{2O^2-_4(aq)} \rightarrow CaC_2O_{4(s)}\) - Experiment 6: \(Ag^+_{(aq)} + Cl^-_{(aq)} \rightarrow AgCl_{(s)}\)

Step 4: (Step 4: Identify the Precipitates Formed)

Finally, let's identify the precipitates formed in each experiment: - Experiment 2: Red precipitate (Silver chromate, \(Ag_2CrO_4\)) - Experiment 3: Yellow precipitate (Calcium chromate, \(CaCrO_4\)) - Experiment 4: White precipitate (Silver oxalate, \(Ag_2C_2O_4\)) - Experiment 5: White precipitate (Calcium oxalate, \(CaC_2O_4\)) - Experiment 6: White precipitate (Silver chloride, \(AgCl\))

Key Concepts

Net Ionic Equations

Net ionic equations are crucial for simplifying chemical reactions by showing only the species that undergo change. They omit spectator ions that do not participate in the actual chemical reaction. Here’s how to construct a net ionic equation:

• Start with the balanced molecular equation of the reaction.
• Identify the compounds that dissociate into ions in solution.
• Write down all the ions separately for each species.
• Compare the ions on both sides and cancel out those that are unchanged.

Let’s take Experiment 2, where a red precipitate forms between sodium chromate (\( Na_2CrO_4 \)) and silver nitrate (\( AgNO_3 \)). Silver ions (\( Ag^+ \)) react with chromate ions (\( CrO_4^{2-} \)) to form silver chromate (\( Ag_2CrO_4 \)):
\[ 2Ag^+_{(aq)} + CrO_4^{2-}_{(aq)} \rightarrow Ag_2CrO_4_{(s)} \]
Notice how only the ions involved in forming the precipitate are shown, reflecting the core transformation within the reaction.

Precipitate Formation

Precipitate formation occurs when two aqueous solutions mix and an insoluble compound forms. The appearance of the precipitate serves as an evidence of a chemical change. To predict whether precipitation will occur, one should refer to solubility rules, which are guidelines for the solubility of various ionic compounds in water.
A precipitate forms when the product of the concentration of ions exceeds the solubility product (\( K_{sp} \)) of the compound. In simpler terms, if two ions like \( Ag^+ \) and \( Cl^- \) are present in sufficient concentrations, they will react to form insoluble silver chloride (\( AgCl \)), which appears as a white precipitate:

• Experiment 6: \( Ag^+_{(aq)} + Cl^-_{(aq)} \rightarrow AgCl_{(s)} \)

Understanding these interactions helps in predicting possible outcomes when dealing with mixtures in a laboratory setting.

Chromate and Oxalate Ions

The chromate (\( CrO_4^{2-} \)) and oxalate (\( C_2O_4^{2-} \)) ions play significant roles in precipitation reactions. They provide unique insights into evaluating solubility and identifying particular substances.
Chromate ions are recognized by their vibrant yellow color in solutions such as sodium chromate. When chromate ions react with silver ions, silver chromate forms as a red precipitate:

• Experiment 2: \( 2Ag^+_{(aq)} + CrO_4^{2-}_{(aq)} \rightarrow Ag_2CrO_4_{(s)} \)

This red precipitate contrasts with the usual outcomes, showcasing the distinct characteristics of chromate-containing compounds.
Similarly, oxalate ions can combine with metal ions like calcium and silver to form precipitates, as seen in Experiments 4 and 5:

• Experiment 4: \( 2Ag^+_{(aq)} + C_2O_4^{2-}_{(aq)} \rightarrow Ag_2C_2O_4_{(s)} \) (white precipitate)
• Experiment 5: \( Ca^{2+}_{(aq)} + C_2O_4^{2-}_{(aq)} \rightarrow CaC_2O_4_{(s)} \) (white precipitate)

Such experiments underline the importance of knowing how different ions react, assisting similarly in the understanding of solubility rules and reaction predictions.