Problem 94 - Chemistry: The Central Science in SI Units, Global Edition

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 \((\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D})\) of four water- soluble salts: \begin{tabular}{lll} \hline Solution & Solute & Color of Solution \\ \hline \(\mathrm{A}\) & \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) & Yellow \\ \(\mathrm{B}\) & \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) & Colorless \\ \(\mathrm{C}\) & \(\mathrm{AgNO}_{3}\) & Colorless \\ \(\mathrm{D}\) & \(\mathrm{CaCl}_{2}\) & Colorless \\ \hline \end{tabular} When these solutions are mixed, the following observations are made: \begin{tabular}{lll} \hline Experiment Number & Solutions Mixed & Result \\ \hline 1 & \(\mathrm{~A}+\mathrm{B}\) & Noprecipitate, yellow solution \\ 2 & \(\mathrm{~A}+\mathrm{C}\) & Red precipitate forms \\ 3 & \(\mathrm{~A}+\mathrm{D}\) & Yellow precipitate forms \\ 4 & \(\mathrm{~B}+\mathrm{C}\) & White precipitate forms \\ 5 & \(\mathrm{~B}+\mathrm{D}\) & White precipitate forms \\ 6 & \(\mathrm{C}+\mathrm{D}\) & White precipitate forms \\ \hline \end{tabular} (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

(a) Net ionic equations for each experiment: 1. No reaction 2. \(2Ag^+(aq) + CrO_4^{2-}(aq) \rightarrow Ag_2CrO_4(s)\) 3. \(Ca^{2+}(aq) + CrO_4^{2-}(aq) \rightarrow CaCrO_4(s)\) 4. \(2Ag^+(aq) + C_2O_4^{2-}(aq) \rightarrow Ag_2C_2O_4(s)\) 5. \(Ca^{2+}(aq) + C_2O_4^{2-}(aq) \rightarrow CaC_2O_4(s)\) 6. \(Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)\) (b) Precipitates formed in each experiment: 1. No precipitate 2. Red precipitate: \(Ag_2CrO_4\) 3. Yellow precipitate: \(CaCrO_4\) 4. White precipitate: \(Ag_2C_2O_4\) 5. White precipitate: \(CaC_2O_4\) 6. White precipitate: \(AgCl\)

Step 1: Experiment 1: A + B

Mixing Sodium chromate (\(Na_2CrO_4\)) with Ammonium oxalate (\((NH_4)_2C_2O_4\)): No precipitate is formed. So, there is no reaction and thus no net ionic equation.

Step 2: Experiment 2: A + C

Mixing Sodium chromate (\(Na_2CrO_4\)) with Silver nitrate (\(AgNO_3\)): There is a red precipitate observed which means a reaction occurs. The possible reaction and products can be represented as follows: \(Na_2CrO_4(aq) + 2AgNO_3(aq) \rightarrow 2NaNO_3(aq) + Ag_2CrO_4(s)\) Net ionic equation: \(2Ag^+(aq) + CrO_4^{2-}(aq) \rightarrow Ag_2CrO_4(s)\)

Step 3: Experiment 3: A + D

Mixing Sodium chromate (\(Na_2CrO_4\)) with Calcium chloride (\(CaCl_2\)): There is a yellow precipitate which means a reaction occurs. The possible reaction and products can be represented as follows: \(Na_2CrO_4(aq) + CaCl_2(aq) \rightarrow 2NaCl(aq) + CaCrO_4(s)\) Net ionic equation: \(Ca^{2+}(aq) + CrO_4^{2-}(aq) \rightarrow CaCrO_4(s)\)

Step 4: Experiment 4: B + C

Mixing Ammonium oxalate (\((NH_4)_2C_2O_4\)) with Silver nitrate (\(AgNO_3\)): There is a white precipitate which means a reaction occurs. The possible reaction and products can be represented as follows: \(2AgNO_3(aq) + (NH_4)_2C_2O_4(aq) \rightarrow 2NH_4NO_3(aq) + Ag_2C_2O_4(s)\) Net ionic equation: \(2Ag^+(aq) + C_2O_4^{2-}(aq) \rightarrow Ag_2C_2O_4(s)\)

Step 5: Experiment 5: B + D

Mixing Ammonium oxalate (\((NH_4)_2C_2O_4\)) with Calcium chloride (\(CaCl_2\)): There is a white precipitate which means a reaction occurs. The possible reaction and products can be represented as follows: \((NH_4)_2C_2O_4(aq) + CaCl_2(aq) \rightarrow 2NH_4Cl(aq) + CaC_2O_4(s)\) Net ionic equation: \(Ca^{2+}(aq) + C_2O_4^{2-}(aq) \rightarrow CaC_2O_4(s)\)

Step 6: Experiment 6: C + D

Mixing Silver nitrate (\(AgNO_3\)) with Calcium chloride (\(CaCl_2\)): There is a white precipitate which means a reaction occurs. The possible reaction and products can be represented as follows: \(AgNO_3(aq) + CaCl_2(aq) \rightarrow Ca(NO_3)_2(aq) + AgCl(s)\) Net ionic equation: \(Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)\) (b) Identify the precipitate formed, if any, in each of the experiments:

Step 7: Experiment 1: A + B

No precipitate

Step 8: Experiment 2: A + C

Red precipitate: \(Ag_2CrO_4\)

Step 9: Experiment 3: A + D

Yellow precipitate: \(CaCrO_4\)

Step 10: Experiment 4: B + C

White precipitate: \(Ag_2C_2O_4\)

Step 11: Experiment 5: B + D

White precipitate: \(CaC_2O_4\)

Step 12: Experiment 6: C + D

White precipitate: \(AgCl\)

Key Concepts

Net Ionic Equations

Net ionic equations are useful tools for studying chemical reactions in aqueous solutions. They focus on the ions that directly participate in forming new products. By omitting the spectator ions that do not change during the reaction, these equations offer a clearer look at what happens in the solution.

For instance, consider the reaction between sodium chromate and silver nitrate in Experiment 2. The full reaction can be written as:

• \( Na_2CrO_4(aq) + 2AgNO_3(aq) \rightarrow 2NaNO_3(aq) + Ag_2CrO_4(s) \)

The net ionic equation focuses on ions that form the precipitate:

• \( 2Ag^+(aq) + CrO_4^{2-}(aq) \rightarrow Ag_2CrO_4(s) \)

This equation illustrates the formation of silver chromate as a solid, while sodium and nitrate ions remain in solution.

Precipitation Reactions

Precipitation reactions occur when soluble ions in different solutions form an insoluble compound. This insoluble compound is called a precipitate, which we can visibly observe as it separates from the solution.

In Experiment 3, when sodium chromate is mixed with calcium chloride, a yellow precipitate is formed. This indicates the creation of calcium chromate. The equation for this reaction is:

• \( Ca^{2+}(aq) + CrO_4^{2-}(aq) \rightarrow CaCrO_4(s) \)

Precipitation reactions are essential for understanding the formation of solid products from aqueous solutions. They are driven by the principles of solubility rules, which help predict whether a precipitate will form.

Chromate Ion

The chromate ion, \(CrO_4^{2-}\), is distinctive for its yellow color in solutions. It is an essential part of some colorful chemical reactions, particularly in precipitation.

In Experiments 2 and 3, the chromate ion reacts with silver and calcium ions, respectively, forming precipitates. These reactions highlight chromate's role as an indicator of certain reactions due to its vivid coloration:

• Red precipitate: \( Ag_2CrO_4 \)
• Yellow precipitate: \( CaCrO_4 \)

These reactions not only demonstrate the chemical behavior of chromate but also underscore the application of solubility rules in predicting outcomes.

Oxalate Ion

The oxalate ion, \(C_2O_4^{2-}\), is an essential ion often involved in reactions forming white precipitates. It is especially known for its interaction with metal ions.

In Experiments 4 and 5, the oxalate ion reacts with silver and calcium ions to form white precipitates:

• White precipitate with silver ion: \( Ag_2C_2O_4 \)
• White precipitate with calcium ion: \( CaC_2O_4 \)

These reactions are not only key for understanding solubility rules but also illustrate how oxalate ions interact with different cations to form solid compounds.