Question

Explain the following phenomenon: You have a test tube with about \(20 \mathrm{~mL}\) silver nitrate solution. Upon adding a few drops of sodium chromate solution, you notice a red solid forming in a relatively clear solution. Upon adding a few drops of a sodium chloride solution to the same test tube, you notice \(a\) white solid and a pale yellow solution. Use the \(K_{s p}\) values in the book to support your explanation, and include the balanced equations.

Short answer

Upon adding sodium chromate (Na₂CrO₄) to the silver nitrate (AgNO₃) solution, the reaction forms a sparingly soluble red solid, silver chromate (Ag₂CrO₄), with a Ksp value of \(1.1 \times 10^{-12}\). The balanced equation for this reaction is: \(2 AgNO₃ (aq) + Na₂CrO₄ (aq) \rightarrow Ag₂CrO₄ (s) + 2 NaNO₃ (aq)\). When sodium chloride (NaCl) is added, a white solid, silver chloride (AgCl), forms with a Ksp value of \(1.8 \times 10^{-10}\), indicating its low solubility as well. The balanced equation for this reaction is: \(AgNO₃ (aq) + NaCl (aq) \rightarrow AgCl (s) + NaNO₃ (aq)\). The pale yellow solution is possibly due to trace amounts of chromate ions remaining from the previous reaction.

Step-by-step solution

Identify the reactants

In the test tube, we have silver nitrate (AgNO₃) solution. When we add a few drops of sodium chromate (Na₂CrO₄) solution, a reaction occurs that results in the formation of a red solid. In the second case, adding a few drops of sodium chloride (NaCl) solution results in a white solid and a pale yellow solution being formed. **Step 2: Write the balanced chemical equations for the reactions**

Write balanced chemical equations

For the reaction between silver nitrate and sodium chromate: \(2 AgNO₃ (aq) + Na₂CrO₄ (aq) \rightarrow Ag₂CrO₄ (s) + 2 NaNO₃ (aq)\) For the reaction between silver nitrate and sodium chloride: \(AgNO₃ (aq) + NaCl (aq) \rightarrow AgCl (s) + NaNO₃ (aq)\) **Step 3: Use the Ksp values to explain the phenomenon**

Explain the phenomenon using Ksp values

We know that the solubility product constant (Ksp) gives us an idea of the solubility of a salt in water. A lower Ksp value indicates lower solubility in water, making it more likely to form a solid precipitate when mixed with another solution. For the Ksp values, we need to refer to the book and find the values for Ag₂CrO₄ and AgCl: Ksp(Ag₂CrO₄) = \(1.1 \times 10^{-12}\) Ksp(AgCl) = \(1.8 \times 10^{-10}\) Based on the Ksp values, we can infer that Ag₂CrO₄ is less soluble in water compared to AgCl, as its Ksp value is lower. When sodium chromate is added to a silver nitrate solution, Ag₂CrO₄ forms a red solid due to its low solubility in water. The remaining solution appears relatively clear as most of the Ag₂CrO₄ has come out of the solution as a solid. In the second case, when sodium chloride is added to the silver nitrate solution, AgCl is formed, which is also sparingly soluble in water and forms a white solid. The accompanying yellow color in the solution could be due to trace amounts of chromate ions from the previous reaction still present in the solution.

Key concepts

Chemical Equilibrium

The concept of chemical equilibrium is central to understanding why certain reactions in a test tube might result in formation of solids or clear solutions. This state is reached when the rates of the forward and reverse chemical reactions are equal, leading to a static system where the concentrations of reactants and products remain constant over time. But, how does this relate to our test tube scenario?

Applying this to the observable changes after adding sodium chromate or sodium chloride to the silver nitrate solution, we can deduce that the reaction has shifted towards the formation of a particular product, which in this case is a solid precipitate. The equilibrium is disrupted momentarily as the reactants combine to form a less soluble compound, and then reestablishes at a new point where the solid and its ions coexist in a dynamic balance.

Precipitation Reactions

Precipitation reactions are fascinating chemical processes where ions in solution combine to form an insoluble compound, which precipitates out of the solution as a solid. The formation of the red and white solids in our test tube is a direct result of such reactions.

When silver nitrate interacts with sodium chromate, a red solid of silver chromate precipitates, whereas the reaction with sodium chloride leads to the formation of white solid silver chloride. These changes are indicative of the low solubility of the newly formed compounds, influenced by the existing ions in solutions and their tendencies to come together to form a solid that is visibly distinct from the liquid phase.

Chemical Solubility

Chemical solubility is the property of a substance to dissolve in a solvent, which in the context of our exercise is water. Solubility can be influenced by various factors including temperature, pH, and presence of other chemicals. Particularly, the solubility product constant (Ksp) is a numerical value representing the equilibrium between a solid and its ions dissolved in a solution.

It's noteworthy that the Ksp for silver chromate (Ag₂CrO₄) is substantially lower than that for silver chloride (AgCl), indicating that silver chromate has a lesser tendency to dissolve in water and is thus more prone to form a precipitate. This aligns with the observation in the test tube that upon mixing, silver chromate precipitates out as a red solid much more readily than silver chloride does as a white solid.

Balanced Chemical Equations

Balanced chemical equations are the mathematicians of the chemistry world, ensuring that atoms are conserved in a chemical reaction. For all reactions, the principle of conservation of mass dictates that the number of atoms for each element must be the same on both sides of the equation.

In our test tube reactions, the balanced chemical equation for the precipitation reaction between silver nitrate and sodium chromate is written to ensure that there are equal numbers of silver, nitrate, sodium, and chromate ions on both reactant and product sides. Similarly, the reaction between silver nitrate and sodium chloride follows this rule, demonstrating that one mole of silver nitrate reacts with one mole of sodium chloride to produce one mole of silver chloride and one mole of sodium nitrate. These balanced equations serve as a blueprint for understanding the stoichiometry of the reactions in the test tube, allowing us to predict the proportions of reactants needed and products formed.