Problem 44 - Chemistry The Molecular Science

Question

Classify each of these exchange reactions as an acid-base reaction, a precipitation reaction, or a gas-forming reaction. Predict the products of the reaction and then balance the completed equation. (a) \(\mathrm{MnCl}_{2}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{~S}(\mathrm{aq}) \longrightarrow\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}(\mathrm{aq})+\mathrm{ZnCl}_{2}(\mathrm{aq}) \longrightarrow\) (c) \(\mathrm{K}_{2} \mathrm{CO}_{3}(\mathrm{aq})+\mathrm{HClO}_{4}(\mathrm{aq}) \longrightarrow\)

Step-by-Step Solution

Answer

(a) Precipitation: \(\mathrm{MnS}(s) + 2\mathrm{NaCl}(aq)\). (b) Precipitation: \(\mathrm{ZnCO}_3(s) + 2\mathrm{NaCl}(aq)\). (c) Gas-forming: \(2\mathrm{KClO}_4(aq) + \mathrm{CO}_2(g) + \mathrm{H}_2\mathrm{O}(l)\).

Step 1: Identify the Type of Reaction (a)

For the reaction \( \mathrm{MnCl}_2(\mathrm{aq}) + \mathrm{Na}_2 \mathrm{~S}(\mathrm{aq}) \rightarrow \), the exchanging ions are \(\mathrm{Mn}^{2+}\) and \(\mathrm{S}^{2-}\). Since sulfide ions can form a solid with certain metal ions, this reaction likely results in a precipitation (solid formation). To confirm, \(\mathrm{MnS}(\mathrm{s})\) is known to be an insoluble compound, predicting its formation. Thus, this is a precipitation reaction.

Step 2: Predict the Products and Write the Equation (a)

From the reactants \( \mathrm{MnCl}_2 \) and \( \mathrm{Na}_2 \mathrm{S} \), the products \( \mathrm{MnS} \) and \( \mathrm{NaCl} \) are formed: \[ \mathrm{MnCl}_2(\mathrm{aq}) + \mathrm{Na}_2 \mathrm{S}(\mathrm{aq}) \rightarrow \mathrm{MnS}(\mathrm{s}) + 2\mathrm{NaCl}(\mathrm{aq}) \]This reaction has balanced molecules: 1 \( \mathrm{Mn} \), 2 \( \mathrm{Cl} \), 2 \( \mathrm{Na} \), and 1 \( \mathrm{S} \) ion on both sides.

Step 3: Identify the Type of Reaction (b)

In the reaction \( \mathrm{Na}_2 \mathrm{CO}_3(\mathrm{aq}) + \mathrm{ZnCl}_2(\mathrm{aq}) \rightarrow \), the exchanging ions are \(\mathrm{Na}^{+}, \mathrm{CO}_3^{2-}, \mathrm{Zn}^{2+},\) and \(\mathrm{Cl}^{-}\). Carbonate forms a precipitate with metal ions such as zinc, suggesting precipitation. Therefore, \( \mathrm{ZnCO}_3 \) solid is a predicted product.

Step 4: Predict the Products and Write the Equation (b)

For \( \mathrm{Na}_2 \mathrm{CO}_3 \) and \( \mathrm{ZnCl}_2 \), \( \mathrm{ZnCO}_3 \) and \( \mathrm{NaCl} \) are the products. The balanced equation is:\[ \mathrm{Na}_2 \mathrm{CO}_3(\mathrm{aq}) + \mathrm{ZnCl}_2(\mathrm{aq}) \rightarrow \mathrm{ZnCO}_3(\mathrm{s}) + 2\mathrm{NaCl}(\mathrm{aq}) \]This equation has molecules balanced on both sides: 2 Na, 1 CO3, 1 Zn, 2 Cl ions.

Step 5: Identify the Type of Reaction (c)

For \( \mathrm{K}_2 \mathrm{CO}_3(\mathrm{aq}) + \mathrm{HClO}_4(\mathrm{aq}) \rightarrow \), the exchanging ions are \( \mathrm{K}^{+}, \mathrm{CO}_3^{2-}, \mathrm{H}^{+}, \) and \( \mathrm{ClO}_4^{-}\). Since carbonates react with acids to produce a gas \(\mathrm{CO}_2\), this is identified as a gas-forming reaction.

Step 6: Predict the Products and Write the Equation (c)

The expected products are \( \mathrm{KClO}_4\), \( \mathrm{CO}_2 \), and \( \mathrm{H}_2\mathrm{O} \). The balanced equation is:\[ \mathrm{K}_2 \mathrm{CO}_3(\mathrm{aq}) + 2\mathrm{HClO}_4(\mathrm{aq}) \rightarrow 2\mathrm{KClO}_4(\mathrm{aq}) + \mathrm{CO}_2(\mathrm{g}) + \mathrm{H}_2\mathrm{O}(\ell) \]This equation balances the molecules: 2 K, 1 CO3, 2 H, 2 ClO4 ions, 1 CO2, 1 H2O.

Key Concepts

Acid-Base Reactions

Acid-base reactions are a class of chemical reactions that involve the transfer of protons. These reactions typically occur when an acid reacts with a base, producing water and a salt as the main products. In an acid-base reaction, the acid donates a proton (H+ ion) to the base.

Common indicators of an acid-base reaction include the formation of water as one of the products and often a shift in pH of the solution. For example, when hydrochloric acid (\( ext{HCl} \)) reacts with sodium hydroxide (\( ext{NaOH} \)), the products formed are water (\( ext{H}_2 ext{O} \)) and sodium chloride (\( ext{NaCl} \)).

These reactions are crucial in various chemical processes, including digestion in our stomachs where hydrochloric acid helps break down food.

Precipitation Reactions

Precipitation reactions involve the formation of an insoluble solid, known as a precipitate, from two aqueous solutions. These reactions occur because certain combinations of ions in solution form compounds that don't dissolve in water. When these ions meet, they combine to form a solid.

This is observed in reaction (a) of the original exercise, where the mixing of \( ext{MnCl}_2 \) and \( ext{Na}_2 ext{S} \) results in the creation of \( ext{MnS} \), which precipitates out of the solution. Precipitation reactions are widely used in chemical analysis and wastewater treatment, helping to remove ions from solutions by forming solid compounds.

To predict such reactions, one can refer to solubility rules that determine which combinations of ions will lead to a precipitate. These rules can help ascertain whether a precipitation reaction will occur based on the components involved.

Gas-Forming Reactions

Gas-forming reactions produce a gas as one of the products and can often involve carbonates or bicarbonates reacting with acids. When these compounds meet, they undergo a chemical change that releases carbon dioxide (\( ext{CO}_2 \)) as a gas.

This behavior is illustrated in reaction (c) of the exercise, where \( ext{K}_2 ext{CO}_3 \) reacts with \( ext{HClO}_4 \) to form \( ext{CO}_2 \), along with other products. The production of gas bubbles can be observed as evidence of the reaction.

Gas-forming reactions are also used in baking, where baking soda (\( ext{NaHCO}_3 \)) produces \( ext{CO}_2 \) to leaven doughs and batters, creating air pockets and helping items rise. Given their hallmark gas release, these reactions are easily identifiable in both laboratory settings and everyday life.