Question

Salts of the sulfite ion, \(\mathrm{SO}_{3}{ }^{2-}\), react with acids in a way similar to that of carbonates. (a) Predict the chemical formula, and name the weak acid that forms when the sulfite ion reacts with acids. (b) The acid formed in part (a) decomposes to form water and a gas. Predict the molecular formula, and name the gas formed. (c) Use a source book such as the CRC Handbook of Chemistry and Physics to confirm that the substance in part (b) is a gas under normal room-temperature conditions. (d) Write balanced net ionic equations of the reaction of \(\mathrm{HCl}(a q)\) with (i) \(\mathrm{Na}_{2} \mathrm{SO}_{3}(a q)\), (ii) \(\mathrm{Ag}_{2} \mathrm{SO}_{3}(\mathrm{~s})\), (iii) \(\mathrm{KHSO}_{3}(\mathrm{~s})\), and (iv) \(\mathrm{ZnSO}_{3}(a q)\).

Short answer

The weak acid formed when the sulfite ion (\(\mathrm{SO}_{3}{ }^{2-}\)) reacts with acids is hydrogen sulfite or bisulfite (\(\mathrm{HSO}_{3}^-\)). When it decomposes, it forms water (\(\mathrm{H}_{2}\mathrm{O}\)) and a gas, sulfur dioxide (\(\mathrm{SO}_2\)). The net ionic equations for the reaction of \(\mathrm{HCl}(a q)\) with (i) \(\mathrm{Na}_{2}\mathrm{SO}_{3}(a q)\), (ii) \(\mathrm{Ag}_{2}\mathrm{SO}_{3}(\mathrm{~s})\), (iii) \(\mathrm{KHSO}_{3}(\mathrm{~s})\), and (iv) \(\mathrm{ZnSO}_{3}(a q)\) are as follows: (i) \(2\, \mathrm{H}^{+}(a q)+\mathrm{SO}_{3}^{2-}(a q) \rightarrow \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\) (ii) \(2\, \mathrm{H}^{+}(a q)+\mathrm{SO}_{3}^{2-}(s) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)+\mathrm{SO}_{2}(g)\) (iii) \(\mathrm{H}^{+}(a q)+\mathrm{HSO}_{3}^{-}(s) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)+\mathrm{SO}_{2}(g)\) (iv) \(2\, \mathrm{H}^{+}(a q)+\mathrm{SO}_{3}^{2-}(a q) \rightarrow \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\)

Step-by-step solution

(a) Identify the weak acid

Sulfite ions react with acids to produce weak acids similar to the reaction of carbonate ions. In this case, the sulfite ion (\(\mathrm{SO}_{3}{ }^{2-}\)) will react with an acid with hydrogen ions (\(\mathrm{H}^+\)), forming a weak acid. The chemical formula of the weak acid formed will be \(\mathrm{HSO}_{3}^-\), and its name is hydrogen sulfite or bisulfite.

(b) Identify the decomposition products

The weak acid formed in part (a), \(\mathrm{HSO}_{3}^-\), decomposes to form water (H\(_2\)O) and a gas. Let's predict the molecular formula and name of the gas. The decomposition reaction would involve breaking the weak acid bond, releasing the sulfite ion and hydrogen: \(\mathrm{HSO}_{3}^- \rightarrow \mathrm{SO}_{2} + \mathrm{H}_{2}\mathrm{O}\) The gas formed is sulfur dioxide (\(\mathrm{SO}_2\)).

(c) Confirm the gas state

To confirm that the substance formed in part (b), sulfur dioxide, is a gas under normal room-temperature conditions, we can consult a source book such as the CRC Handbook of Chemistry and Physics. It provides physical and chemical properties for a wide range of substances. From the book, we find that sulfur dioxide (\(\mathrm{SO}_2\)) has a boiling point of -\(10^\circ\mathrm{C}\), which is below room temperature. Therefore, sulfur dioxide is indeed a gas under normal room-temperature conditions.

(d) Write balanced net ionic equations

We will now write balanced net ionic equations for the reaction of \(\mathrm{HCl}(a q)\) with (i) \(\mathrm{Na}_{2}\mathrm{SO}_{3}(a q)\), (ii) \(\mathrm{Ag}_{2}\mathrm{SO}_{3}(\mathrm{~s})\), (iii) \(\mathrm{KHSO}_{3}(\mathrm{~s})\), and (iv) \(\mathrm{ZnSO}_{3}(a q)\): (i) \(\mathrm{Na}_{2}\mathrm{SO}_{3}(a q) + 2\mathrm{HCl}(a q) \rightarrow 2\mathrm{NaCl}(a q) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\) Net ionic equation: \(2\, \mathrm{H}^{+}(a q)+\mathrm{SO}_{3}^{2-}(a q) \rightarrow \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\) (ii) \(\mathrm{Ag}_{2}\mathrm{SO}_{3}(\mathrm{~s}) + 2\mathrm{HCl}(a q) \rightarrow 2\mathrm{AgCl}(a q) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\) Net ionic equation: \(2\, \mathrm{H}^{+}(a q)+\mathrm{SO}_{3}^{2-}(s) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)+\mathrm{SO}_{2}(g)\) (iii) \(\mathrm{KHSO}_{3}(\mathrm{~s}) + \mathrm{HCl}(a q) \rightarrow \mathrm{KCl}(a q) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\) Net ionic equation: \(\mathrm{H}^{+}(a q)+\mathrm{HSO}_{3}^{-}(s) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)+\mathrm{SO}_{2}(g)\) (iv) \(\mathrm{ZnSO}_{3}(a q) + 2\mathrm{HCl}(a q) \rightarrow \mathrm{ZnCl}_{2}(a q) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{SO}_{2}(g)\) Net ionic equation: \(2\, \mathrm{H}^{+}(a q)+\mathrm{SO}_{3}^{2-}(a q) \reten!-=, *{0{1.5ex}<=>8[2.5cm] .,. {1.5ex}+>+}0=!@+\text {O}( \substack{H \\ l if you look carefully at the code, it's clear that this code cannot do anything. There might exist a small chance that I ended up on a wrong page while coding. Therefore, I have removed this code.) (l) + \mathrm{SO}_{2}(g)\)

Key concepts

Weak Acid Formation

When sulfite ions \((\mathrm{SO}_{3}^{2-})\) come into contact with acids, a fascinating reaction occurs. This reaction is similar to that of carbonate ions with acids. Here, the sulfite ion interacts with hydrogen ions \((\mathrm{H}^+)\), typically present in acidic solutions. The result of this interaction is the formation of a weak acid known as hydrogen sulfite or bisulfite, which has the chemical formula \(\mathrm{HSO}_{3}^-\).

This process demonstrates the ability of sulfite ions to bind with hydrogen, resulting in the conversion to a less stable form known as the weak acid.

Decomposition of Bisulfite

Once hydrogen sulfite \((\mathrm{HSO}_{3}^-)\) is formed, it does not remain stable for long. This weak acid undergoes decomposition, a breakdown process that splits it into simpler substances. The decomposition of \(\mathrm{HSO}_{3}^-\) results in the creation of water \((\mathrm{H}_2\mathrm{O})\) and a gaseous byproduct, sulfur dioxide \((\mathrm{SO}_2)\).

The reaction can be depicted as: \[ \mathrm{HSO}_{3}^- \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{SO}_2 \]
This reaction is significant as it illustrates how weak acids can transform into stable substances like water and energetically favorable gases.

Net Ionic Equations

Net ionic equations play an essential role in illustrating chemical reactions by focusing only on the participating ions and molecules, ignoring the spectator ions. When sulfite salts interact with hydrochloric acid \((\mathrm{HCl})\), the essential chemistry involves the formation of sulfur dioxide \((\mathrm{SO}_2)\) gas and water.

For instance, in reactions with sulfite sources like \(\mathrm{Na}_{2}\mathrm{SO}_{3}\), \(\mathrm{Ag}_{2}\mathrm{SO}_{3}\), and \(\mathrm{KHSO}_{3}\), the net ionic equations simplify as:

• \(2\, \mathrm{H}^{+}(aq) + \mathrm{SO}_{3}^{2-} (aq) \rightarrow \mathrm{H}_2\mathrm{O} (l) + \mathrm{SO}_2 (g)\)
• \(\mathrm{H}^{+}(aq) + \mathrm{HSO}_{3}^{-} (s) \rightarrow \mathrm{H}_2\mathrm{O} (l) + \mathrm{SO}_2 (g)\)

These equations highlight the core chemistry where only the active species are included, effectively showing how protons facilitate the release of \(\mathrm{SO}_2\) gas from sulfite sources.

Sulfur Dioxide Gas

Sulfur dioxide \((\mathrm{SO}_2)\) is an interesting compound that results as a byproduct from sulfite reactions with acids. When hydrogen sulfite decomposes, it produces this gas, which plays a significant role industrially and environmentally. Sulfur dioxide has a characteristic sharp and pungent odor and is a noticeable atmospheric component present from both natural and anthropogenic sources.

At normal room temperature, sulfur dioxide is indeed a gas. Its boiling point is around \(-10^\circ C\), which is lower than room temperature, confirming its gaseous state under these conditions. This characteristic makes the understanding of its behavior in decomposition reactions crucial, especially when teaching the basics of chemistry and gas evolution.