Problem 23 - Chemistry The Central Science In Si Units

Question

Will precipitation occur when the following solutions are mixed? If so, write a balanced chemical equation for the reac- tion. (a) \(\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\) and \(\mathrm{NaOH},(\mathbf{b}) \mathrm{K}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), (c) \(\mathrm{Na}_{2} \mathrm{~S}\) and \(\mathrm{FeCl}_{2}\)

Step-by-Step Solution

Answer

Precipitation occurs in (a) and (c) forming \( \text{Ca(OH)}_2 \) and \( \text{FeS} \), no precipitation in (b).

Step 1: Identify the Ions Present

When two solutions are mixed, determine the ions present in each solution. For (a) \( \text{Ca(CH}_3\text{COO)}_2 \) dissociates into \( \text{Ca}^{2+} \) and \( \text{CH}_3\text{COO}^- \), and \( \text{NaOH} \) dissociates into \( \text{Na}^+ \) and \( \text{OH}^- \). For (b) \( \text{K}_2\text{CO}_3 \) gives \( \text{K}^+ \) and \( \text{CO}_3^{2-} \), and \( \text{NH}_4\text{NO}_3 \) gives \( \text{NH}_4^+ \) and \( \text{NO}_3^- \). For (c) \( \text{Na}_2\text{S} \) yields \( \text{Na}^+ \) and \( \text{S}^{2-} \), and \( \text{FeCl}_2 \) yields \( \text{Fe}^{2+} \) and \( \text{Cl}^- \).

Step 2: Predict Possible Products

Identify the possible products by swapping the ions of the reactants to see if any combination forms an insoluble compound. For (a), possible products are \( \text{Ca(OH)}_2 \) (insoluble) and \( \text{NaCH}_3\text{COO} \) (soluble). For (b), products are \( \text{KNO}_3 \) and \( \text{NH}_4\text{CO}_3 \), both soluble. For (c), possible products include \( \text{NaCl} \) (soluble) and \( \text{FeS} \) (insoluble).

Step 3: Apply Solubility Rules

Use solubility rules to determine if any of the products from Step 2 are insoluble in water. Recall that most hydroxides and sulfides are insoluble, except those of alkali metals and \( \text{NH}_4^+ \). Thus, \( \text{Ca(OH)}_2 \) and \( \text{FeS} \) are insoluble, but the rest are soluble.

Step 4: Write Balanced Chemical Equations

Write balanced equations for any reactions where precipitation occurs. In (a), \( \text{Ca}^{2+} + 2\text{OH}^- \rightarrow \text{Ca(OH)}_2(s) \). In (c), \( \text{Fe}^{2+} + \text{S}^{2-} \rightarrow \text{FeS}(s) \). No precipitation occurs in (b).

Step 5: Conclude Which Mixes Precipitate

Summarize that precipitation will occur in mixes (a) and (c) because insoluble \( \text{Ca(OH)}_2 \) and \( \text{FeS} \) are formed respectively, while in (b), all products remain dissolved in solution.

Key Concepts

Balanced Chemical Equations

When solutions are mixed in a chemistry experiment, predicting whether a reaction will occur involves writing out balanced chemical equations. A balanced equation shows the chemical reaction with equal numbers of each type of atom on both sides of the equation. This is crucial because it reflects the law of conservation of mass, which states that matter cannot be created or destroyed in a closed system.
To write a balanced chemical equation, you first need to know the reactants, which are the starting chemicals, and the products, which are the chemicals formed as a result of the reaction.

• For example, in reaction (a) from the exercise, the reactants are calcium acetate \(\text{Ca(CH}_3\text{COO)}_2\) and sodium hydroxide \(\text{NaOH}\), and a new compound, calcium hydroxide \(\text{Ca(OH)}_2\), is formed, alongside sodium acetate \(\text{NaCH}_3\text{COO}\).

Next, you will break apart each compound into its constituent ions, then recombine them into new products, ensuring that the number of atoms of each element is the same on both sides of the equation. Remember, the equation must be balanced for both mass and charge.

Solubility Rules

Solubility rules are guidelines that help predict whether or not a compound will dissolve in water. They are essential when considering precipitation reactions because they help identify if any of the products formed are insoluble and will precipitate out of the solution.
Some general rules include:

• All sodium, potassium, and ammonium salts are soluble.
• Nitrates, acetates, and most perchlorates are soluble.
• Most chlorides, bromides, and iodides are soluble, except those of silver, lead, and mercury.
• Sulfates are generally soluble, with exceptions like barium sulfate and lead sulfate.
• Most hydroxides and sulfides are insoluble except those of alkali metals and calcium, barium, and strontium.

For example, in the original exercise, using these rules can help determine that \(\text{Ca(OH)}_2\) is insoluble and will precipitate, while \(\text{NaCH}_3\text{COO}\) remains soluble in water.

Ion Dissociation

Ion dissociation is a process where ionic compounds split into their individual ions when dissolved in water. This is crucial for understanding how ions interact in a solution, especially during precipitation reactions.
When an ionic compound such as \(\text{Ca(CH}_3\text{COO)}_2\) (calcium acetate) is dissolved in water, it dissociates into calcium ions \(\text{Ca}^{2+}\) and acetate ions \(\text{CH}_3\text{COO}^-\).

• Similarly, \(\text{NaOH}\) dissociates into sodium ions \(\text{Na}^+\) and hydroxide ions \(\text{OH}^-\).

During ion dissociation, each ion contributes to the overall reaction and interacts with other ions to potentially form new compounds. In cases where ions combine to form an insoluble compound, a precipitate will form, which is what we observe in some of these exercises.

Insoluble Compounds

Insoluble compounds are those that do not dissolve much in water. When two solutions containing soluble ionic compounds are mixed, an insoluble compound may form if certain ions combine and exceed the solubility threshold. The result is a solid substance appearing in the liquid, known as a precipitate.
For instance, in reaction (c) from the exercise, \(\text{FeS}\) forms when iron ions \(\text{Fe}^{2+}\) from \(\text{FeCl}_2\) interact with sulfide ions \(\text{S}^{2-}\) from \(\text{Na}_2\text{S}\). This reaction results in an insoluble compound that separates out as a solid from the aqueous solution.

• Recognizing the formation of insoluble compounds is key to understanding precipitation reactions.
• These products are typically removed from solutions in various chemical processes because they do not dissolve in water.

Overall, identifying when and where insoluble compounds will form is an essential part of predicting precipitation reactions in chemistry.