Problem 45 - 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) \(\mathrm{Fe}(\mathrm{OH})_{3}(\mathrm{~s})+\mathrm{HNO}_{3}(\mathrm{aq}) \longrightarrow\) (b) \(\mathrm{FeCO}_{3}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \longrightarrow\) (c) \(\mathrm{FeCl}_{2}(\mathrm{aq})+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}(\mathrm{aq}) \longrightarrow\) (d) \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{CO}_{3}(\mathrm{aq}) \longrightarrow\)

Step-by-Step Solution

Answer

(a) Acid-base, (b) Gas-forming, (c) Precipitation, (d) Precipitation.

Step 1: Reaction Type and Products for (a)

For the reaction \(\mathrm{Fe(OH)_3(s) + HNO_3(aq) \rightarrow}\), we identify it as an acid-base reaction because it involves a base (\(\mathrm{Fe(OH)_3}\)) and an acid (\(\mathrm{HNO_3}\)). The products will be water \(\mathrm{H_2O}\) and \(\mathrm{Fe(NO_3)_3}\), as the hydrogen ions from \(\mathrm{HNO_3}\) will react with the hydroxide ions in \(\mathrm{Fe(OH)_3}\). The unbalanced equation is: \[ \mathrm{Fe(OH)_3(s) + 3 HNO_3(aq) \rightarrow Fe(NO_3)_3(aq) + 3 H_2O(l)} \]

Step 2: Balance Reaction (a)

The equation is already balanced: \(\mathrm{Fe(OH)_3 + 3 HNO_3 \rightarrow Fe(NO_3)_3 + 3 H_2O}\). Each element has the same number of atoms on both sides.

Step 3: Reaction Type and Products for (b)

For \(\mathrm{FeCO_3(s) + H_2SO_4(aq) \rightarrow}\), this is a gas-forming reaction because it involves a carbonate reacting with an acid to produce carbon dioxide gas. The products are \(\mathrm{FeSO_4(aq)}\), \(\mathrm{CO_2(g)}\), and \(\mathrm{H_2O(l)}\). The unbalanced equation is: \[ \mathrm{FeCO_3(s) + H_2SO_4(aq) \rightarrow FeSO_4(aq) + CO_2(g) + H_2O(l)} \]

Step 4: Balance Reaction (b)

The equation \(\mathrm{FeCO_3 + H_2SO_4 \rightarrow FeSO_4 + CO_2 + H_2O}\) is already balanced. The number of atoms for each element on both sides is equal.

Step 5: Reaction Type and Products for (c)

The reaction \(\mathrm{FeCl_2(aq) + (NH_4)_2S(aq) \rightarrow}\) is a precipitation reaction. The possible products include \(\mathrm{FeS(s)}\), a solid precipitate, and \(\mathrm{NH_4Cl(aq)}\). The unbalanced equation is: \[ \mathrm{FeCl_2(aq) + (NH_4)_2S(aq) \rightarrow FeS(s) + 2 NH_4Cl(aq)} \]

Step 6: Balance Reaction (c)

Check and balance: \(\mathrm{FeCl_2 + (NH_4)_2S \rightarrow FeS + 2 NH_4Cl}\). This equation is balanced.

Step 7: Reaction Type and Products for (d)

For \(\mathrm{Fe(NO_3)_2(aq) + Na_2CO_3(aq) \rightarrow}\), this is another precipitation reaction leading to the formation of \(\mathrm{FeCO_3(s)}\) as a precipitate and \(\mathrm{NaNO_3(aq)}\). The unbalanced equation is: \[ \mathrm{Fe(NO_3)_2(aq) + Na_2CO_3(aq) \rightarrow FeCO_3(s) + 2 NaNO_3(aq)} \]

Step 8: Balance Reaction (d)

Check and balance: \(\mathrm{Fe(NO_3)_2 + Na_2CO_3 \rightarrow FeCO_3 + 2 NaNO_3}\). This equation is balanced.

Key Concepts

Acid-Base Reactions

Acid-base reactions are fundamental processes where acids and bases react to neutralize each other and form water alongside a salt. In chemical terms, an acid donates hydrogen ions (\(H^+\)) while a base donates hydroxide ions (\(OH^-\)). These ions combine to create water (\(H_2O\)).

In the example reaction \(\mathrm{Fe(OH)_3 + 3 \ HNO_3 \rightarrow \ Fe(NO_3)_3 + 3 \ H_2O}\), iron(III) hydroxide (\(\mathrm{Fe(OH)_3}\)) acts as the base, and nitric acid (\(\mathrm{HNO_3}\)) as the acid. The acid's hydrogen ions react with the base's hydroxide ions to form water. The remaining components, iron ions and nitrate ions, create the salt iron(III) nitrate (\(\mathrm{Fe(NO_3)_3}\)).

• Water formation is a key indicator of an acid-base reaction.
• These reactions help maintain pH balance in many biological and chemical processes.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions react and form an insoluble solid, known as a precipitate. This usually happens when the product of a reaction is not soluble in water, leading to the formation of a solid that settles out of the solution.

For example, the reaction \(\mathrm{FeCl_2 + (NH_4)_2S \rightarrow FeS + 2 \ NH_4Cl}\), involves iron(II) chloride and ammonium sulfide. Iron(II) sulfide (\(\mathrm{FeS}\)), the precipitate, forms a solid while ammonium chloride remains dissolved.

• Common signs of precipitation reactions include cloudiness or solid formation in a solution.
• These reactions are crucial in processes like water purification and analytical chemistry.

Gas-Forming Reactions

Gas-forming reactions are chemical reactions where one of the products is a gas. These reactions often start with a solid reacting with an acid to produce a gaseous product.

The reaction \(\mathrm{FeCO_3 + H_2SO_4 \rightarrow FeSO_4 + CO_2 + H_2O}\) illustrates this process. Here, iron(II) carbonate reacts with sulfuric acid to form iron(II) sulfate, water, and carbon dioxide gas. The release of gas can be observed as bubbling or fizzing in the solution.

• Gas formation is often accompanied by characteristic effervescence.
• Such reactions are prominent in geological processes and in various industrial applications.