Problem 52 - Chemistry An Atoms First Approach

Question

Give an example how each of the following insoluble ionic compounds could be produced using a precipitation reaction. Write the balanced formula equation for each reaction. a. \(\mathrm{Fe}(\mathrm{OH})_{3}(s)\) c. \(\mathrm{PbSO}_{4}(s)\) b. \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}(s)\) d. \(\mathrm{BaCrO}_{4}(s)\)

Step-by-Step Solution

Answer

a. \(\mathrm{Fe(NO_3)_3 (aq) + 3NaOH (aq) \rightarrow Fe(OH)_3 (s) + 3NaNO_3(aq)}\) b. \(\mathrm{Hg}_2(NO_3)_2(aq) + 2NaCl(aq) \rightarrow Hg_2Cl_2(s) + 2NaNO_3(aq)\) c. \(\mathrm{Pb(NO_3)_2 (aq) + Na_2SO_4 (aq) \rightarrow PbSO_4 (s) + 2NaNO_3(aq)}\) d. \(\mathrm{Ba(NO_3)_2 (aq) + K_2CrO_4 (aq) \rightarrow BaCrO_4 (s) + 2KNO_3(aq)}\)

Step 1: a. Precipitation Reaction for Fe(OH)₃

To produce \(\mathrm{Fe}(\mathrm{OH})_{3}(s)\) as a precipitate, we need two soluble reactants that form this compound. One option is choosing a soluble iron compound (such as \(\mathrm{Fe}^{3+}\) salt) and a source of the hydroxide ion, \(\mathrm{OH}^{-}\) (like \(\mathrm{NaOH}\)). The balanced formula equation for this reaction is: \[\mathrm{Fe(NO_3)_3 (aq) + 3NaOH (aq) \rightarrow Fe(OH)_3 (s) + 3NaNO_3(aq)}\]

Step 2: b. Precipitation Reaction for Hg₂Cl₂

To form \(\mathrm{Hg}_{2}\mathrm{Cl}_{2}(s)\) as the precipitate, we need a soluble mercury(I) compound and a soluble source of chloride ions. For instance, we can use \(\mathrm{Hg}_{2}(\mathrm{NO}_{3})_{2}\) as the mercury(I) compound and \(\mathrm{NaCl}\) as the chloride source. The balanced formula equation for this reaction is: \[\mathrm{Hg}_2(NO_3)_2(aq) + 2NaCl(aq) \rightarrow Hg_2Cl_2(s) + 2NaNO_3(aq)\]

Step 3: c. Precipitation Reaction for PbSO₄

In order to form \(\mathrm{PbSO}_4(s)\) as a precipitate, we can choose a soluble lead(II) compound such as \(\mathrm{Pb(NO_3)_2}\) and a soluble sulfate source like \(\mathrm{Na_2SO_4}\). The balanced formula equation for this reaction is: \[\mathrm{Pb(NO_3)_2 (aq) + Na_2SO_4 (aq) \rightarrow PbSO_4 (s) + 2NaNO_3(aq)}\]

Step 4: d. Precipitation Reaction for BaCrO₄

To produce \(\mathrm{BaCrO}_4(s)\) precipitate, we must select a soluble barium compound, for example, \(\mathrm{Ba(NO_3)_2}\), and a soluble source of chromate ions like \(\mathrm{K_2CrO_4}\). The balanced formula equation for this reaction is: \[\mathrm{Ba(NO_3)_2 (aq) + K_2CrO_4 (aq) \rightarrow BaCrO_4 (s) + 2KNO_3(aq)}\]

Key Concepts

Insoluble Ionic Compounds

Insoluble ionic compounds are those that do not dissolve significantly in water. They tend to form precipitates when mixed in aqueous solutions. A precipitate is a solid substance that appears out of a liquid solution due to a chemical reaction. Known for their low solubility, these compounds are crucial in predicting precipitation reactions.
For example,

• Fe(OH)_3 is insoluble and forms when iron(III) ions meet hydroxide ions in solution.
• PbSO_4 precipitates when lead(II) ions bond with sulfate ions.

Recognizing insoluble ionic compounds helps in conducting practical experiments that require precise outcomes, like in laboratory synthesis or wastewater treatment.

Balanced Formula Equations

Balanced formula equations ensure that the number of atoms for each element is the same on the reactant and product sides of a chemical equation. This law, known as the Law of Conservation of Mass, states that mass cannot be created or destroyed in a chemical reaction. Therefore, equations need accurate balancing to reflect this principle.
For instance, to produce Fe(OH)_3, the equation is balanced as:

• Fe(NO_3)_3(aq) + 3NaOH(aq) → Fe(OH)_3(s) + 3NaNO_3(aq)

Here, the balanced equation means every atom present in the reactants is accounted for in the products. This ensures precision in lab settings and accurate predictions in chemical processes.

Solubility Rules

Solubility rules are guidelines used to predict the solubility of ionic compounds in water. These rules are not absolute but provide a good basis for anticipating whether a compound will dissolve or form a precipitate. Understanding these rules is pivotal in determining the outcomes of mixing solutions.
Here are some basics:

• Most nitrates (NO_3) are soluble.
• Chlorides (Cl), bromides (Br), and iodides (I) are generally soluble, except those of silver (Ag), lead (Pb), and mercury (Hg).
• Sulfates (SO_4) are mostly soluble, with exceptions like barium (Ba), lead (Pb), and calcium (Ca) sulfates.
• Hydroxides (OH) are typically insoluble except for those of strong base cations like sodium (Na) and potassium (K).

Learning these guidelines helps predict possible reactions and informs laboratory practices necessary for creating desired chemical substances.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds, resulting in different substances called products. These reactions can produce gases, changes in color, temperature shifts, or precipitates. Precipitation reactions, in particular, involve the formation of an insoluble solid from aqueous reactants.
For example, the formation of PbSO_4 from Pb(NO_3)_2 and Na_2SO_4 happens when lead and sulfate ions react to form an insoluble precipitate. This process is part of many natural and industrial procedures, including water purification and metal recovery.

• Understanding chemical reactions helps in designing processes, synthesizing new compounds, and analyzing chemical changes.