Problem 23 - Chemistry: The Central Science in SI Units, Global Edition

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 \(\mathrm{FeCl}_{3}\)

Step-by-Step Solution

Answer

(a) Precipitation occurs, the balanced chemical equation is: \[\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}+2\mathrm{NaOH}\longrightarrow\mathrm{Ca(OH)_2}+\mathrm{2Na}\left(\mathrm{CH}_{3} \mathrm{COO}\right)\] (b) No precipitation occurs (c) Precipitation occurs, the balanced chemical equation is: \[\mathrm{2Na}_{2} \mathrm{S}+3\mathrm{FeCl}_{3}\longrightarrow6\mathrm{NaCl}+\mathrm{2FeS}\]

Step 1: Identifying the potential products

We will begin by predicting the products formed from the reaction of the given solutions in part (a) \(\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\) and \(\mathrm{NaOH}\), part (b) \(\mathrm{K}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), and part (c) \(\mathrm{Na}_{2} \mathrm{~S}\) and \(\mathrm{FeCl}_{3}\). Note that these are double displacement reactions.

Step 2: Check the solubility rules

Now, we will apply the solubility rules to determine if any of the potential products formed are insoluble (forming a precipitate). (a) Starting with \(\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\) and \(\mathrm{NaOH}\): The potential products are: \(\mathrm{Ca(OH)_2}\) and \(\mathrm{Na}\left(\mathrm{CH}_{3} \mathrm{COO}\right)\) Solubility rules: \(\mathrm{Ca(OH)_2}\): calcium hydroxide is considered slightly soluble in water, thus forming a precipitate \(\mathrm{Na}\left(\mathrm{CH}_{3} \mathrm{COO}\right)\): sodium salts are soluble in water The balanced chemical equation is: \[\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}+2\mathrm{NaOH}\longrightarrow\mathrm{Ca(OH)_2}+\mathrm{2Na}\left(\mathrm{CH}_{3} \mathrm{COO}\right)\] (b) Moving on to \(\mathrm{K}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NH}_{4} \mathrm{NO}_{3}\): The potential products are: \(\mathrm{KNO_3}\) and \(\mathrm{NH_4}\mathrm{(CO_{3})}\) Solubility rules: \(\mathrm{KNO_3}\): potassium salts are soluble in water \(\mathrm{NH_4}\mathrm{(CO_{3})}\): ammonium salts are soluble in water In this case, no precipitation occurs. (c) Finally, \(\mathrm{Na}_{2} \mathrm{S}\) and \(\mathrm{FeCl}_{3}\): The potential products are: \(\mathrm{NaCl}\) and \(\mathrm{FeS}\) Solubility rules: \(\mathrm{NaCl}\): sodium salts are soluble in water \(\mathrm{FeS}\): Metal sulfides, in general are insoluble, including \(\mathrm{FeS}\) which forms a precipitate The balanced chemical equation is: \[\mathrm{2Na}_{2} \mathrm{S}+3\mathrm{FeCl}_{3}\longrightarrow6\mathrm{NaCl}+\mathrm{2FeS}\]

Step 3: Summarize the results

Now we can summarize the results for the given parts: (a) Precipitation occurs, the balanced chemical equation is: \[\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}+2\mathrm{NaOH}\longrightarrow\mathrm{Ca(OH)_2}+\mathrm{2Na}\left(\mathrm{CH}_{3} \mathrm{COO}\right)\] (b) No precipitation occurs (c) Precipitation occurs, the balanced chemical equation is: \[\mathrm{2Na}_{2} \mathrm{S}+3\mathrm{FeCl}_{3}\longrightarrow6\mathrm{NaCl}+\mathrm{2FeS}\]

Key Concepts

Chemical Equations

When substances react chemically, a new set of compounds is formed from the initial reactants, and this transformation is expressed through chemical equations. These equations represent what happens during a chemical reaction. Each side of the equation has chemical formulas that denote substances involved in the reaction.
On the left are the reactants, and on the right, the products are displayed.

• Reactants: Substances that start the reaction.
• Products: Compounds that are formed as a result of the reaction.

To comply with the law of conservation of mass, chemical equations need to be balanced. This means that the number of atoms for each element should be equal on both sides of the equation.
In the original exercise, we see this in action with balanced equations for reactions, such as:\[\text{Ca(CH}_3\text{COO)}_2 + 2\text{NaOH} \rightarrow \text{Ca(OH)}_2 + 2\text{Na(CH}_3\text{COO)}\]
Understanding chemical equations helps us visualize and predict the outcome of chemical reactions.

Solubility Rules

Solubility rules are like guidelines that help predict whether a compound will dissolve in water or form a precipitate. They’re crucial when determining outcomes of reactions involving dissolved salts and are essential for forecasting precipitation reactions.
With solubility rules, we can generally assess if the new products in a reaction remain dissolved or form a solid. Helpful solubility guidelines include:

• Most nitrate (\( ext{NO}_3^-\)) salts are soluble.
• Most sodium (\( ext{Na}^+\)), potassium (\( ext{K}^+\)), and ammonium (\( ext{NH}_4^+\)) salts are soluble.
• Salts containing carbonate (\( ext{CO}_3^{2-}\)) are generally insoluble, except with alkaline metals (like potassium and sodium) and ammonium.
• Hydroxides, such as calcium hydroxide (\( ext{Ca(OH)}_2\)), are generally considered slightly soluble.

In the step-by-step solution, solubility rules were used effectively to understand that in reactions (b), the products potassium nitrate (\( ext{KNO}_3\)) and ammonium carbonate (\( ext{NH}_4 ext{CO}_3\)) are both soluble, leading to no precipitate.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions are mixed, and an insoluble solid, called a precipitate, forms and separates from the solution. This type of reaction is a subset of double displacement reactions and is typical in situations involving salts.
The driving force of these reactions is the formation of a product that does not dissolve in water. In the textbook exercise, we saw:

• The combination of \( ext{Na}_2 ext{S}\) and \( ext{FeCl}_3\) forming \( ext{FeS}\), a classic precipitate, follows this rule.
• Another example is \( ext{Ca(OH)}_2\), which is slightly soluble, formed from \( ext{Ca(CH}_3 ext{COO)}_2\) and \( ext{NaOH}\).

To predict the occurrence of a precipitation reaction, it is important to refer to solubility rules: a precipitate is likely to form if any of the possible products of your aqueous reactants are insoluble in water. This understanding allows chemists to plan reactions thoughtfully and utilize separation techniques based on solubility.