Question

Write molecular and net ionic equations for the successive neutralizations of each acidic hydrogen of sulfurous acid by aqueous calcium hydroxide. \(\mathrm{CaSO}_{3}\) is insoluble; the acid salt is soluble.

Short answer

The net ionic equations detail each neutralization step, forming soluble and insoluble sulfurous acid compounds.

Step-by-step solution

Write Balanced Molecular Equations

First, identify the balanced molecular equation for each stage of the reaction. Sulfurous acid, \( \text{H}_2\text{SO}_3 \), reacts with calcium hydroxide, \( \text{Ca(OH)}_2 \). In the first stage, one \( \text{H}^+ \) is neutralized: \( \text{H}_2\text{SO}_3 \text{(aq)} + \text{Ca(OH)}_2 \text{(aq)} \rightarrow \text{CaHSO}_3 \text{(aq)} + \text{H}_2\text{O} \text{(l)} \) Continue with the second stage where the second \( \text{H}^+ \) is neutralized, forming an insoluble compound: \( \text{CaHSO}_3 \text{(aq)} + \text{Ca(OH)}_2 \text{(aq)} \rightarrow \text{CaSO}_3 \text{(s)} + \text{H}_2\text{O} \text{(l)} \)

Determine the Total Ionic Equations

Convert the molecular equations to total ionic equations. The soluble compounds dissociate into ions, while the insoluble \( \text{CaSO}_3 \) remains as a solid.For the first step:\( \text{H}_2\text{SO}_3 \text{(aq)} + \text{Ca}^{2+} \text{(aq)} + 2\text{OH}^- \text{(aq)} \rightarrow \text{Ca}^{2+} \text{(aq)} + \text{HSO}_3^- \text{(aq)} + \text{H}_2\text{O} \text{(l)} \)For the second step:\( \text{Ca}^{2+} \text{(aq)} + \text{HSO}_3^- \text{(aq)} + \text{Ca}^{2+} \text{(aq)} + 2\text{OH}^- \text{(aq)} \rightarrow \text{CaSO}_3 \text{(s)} + \text{H}_2\text{O} \text{(l)} \)

Write the Net Ionic Equations

Remove the spectator ions from the total ionic equations to form the net ionic equations.For the first neutralization:\( \text{H}_2\text{SO}_3 \text{(aq)} + \text{OH}^- \text{(aq)} \rightarrow \text{HSO}_3^- \text{(aq)} + \text{H}_2\text{O} \text{(l)}\)For the second neutralization:\( \text{HSO}_3^- \text{(aq)} + \text{Ca}^{2+} \text{(aq)} + \text{OH}^- \text{(aq)} \rightarrow \text{CaSO}_3 \text{(s)} + \text{H}_2\text{O} \text{(l)} \)

Key concepts

Molecular Equations

When dealing with acid-base reactions, writing the molecular equations is your starting point. A molecular equation shows all the reactants and products in their undissociated forms. It helps us see the overall reaction clearly without getting into details about the ions present. For sulfurous acid reacting with calcium hydroxide, the first reaction involves the neutralization of one hydrogen ion. The equation is \( \text{H}_2\text{SO}_3 \text{(aq)} + \text{Ca(OH)}_2 \text{(aq)} \rightarrow \text{CaHSO}_3 \text{(aq)} + \text{H}_2\text{O} \text{(l)} \). This tells us that calcium bisulfite and water are produced. Then, in the second stage, another hydrogen ion is neutralized. Here, the equation is \( \text{CaHSO}_3 \text{(aq)} + \text{Ca(OH)}_2 \text{(aq)} \rightarrow \text{CaSO}_3 \text{(s)} + \text{H}_2\text{O} \text{(l)} \). The formation of insoluble calcium sulfite is key here. Remember, molecular equations provide a complete overview but don't show the ionic details.

Net Ionic Equations

Net ionic equations focus on the ions directly involved in the chemical change. They exclude any spectator ions, which do not participate in the actual reaction. To derive net ionic equations, we start from the total ionic equations. For the first reaction, the net ionic equation is \( \text{H}_2\text{SO}_3 \text{(aq)} + \text{OH}^- \text{(aq)} \rightarrow \text{HSO}_3^- \text{(aq)} + \text{H}_2\text{O} \text{(l)} \). This simplifies the molecular equation by showing only the essential participants. In the second reaction, the net ionic equation becomes \( \text{HSO}_3^- \text{(aq)} + \text{Ca}^{2+} \text{(aq)} + \text{OH}^- \text{(aq)} \rightarrow \text{CaSO}_3 \text{(s)} + \text{H}_2\text{O} \text{(l)} \). Highlighting the essential ions gives a clearer insight into what is happening. This is especially useful in identifying precipitate formation, like \( \text{CaSO}_3 \) here, indicating that calcium sulfite is insoluble in water.

Solubility Rules

Solubility rules help determine whether a compound will dissolve in water to form a solution or remain as a solid. They are crucial for predicting the outcome of chemical reactions. Let's apply them to the reactions we've discussed.

• Compounds containing \( \text{OH}^- \) are generally soluble, except for those of \( \text{Ca}^{2+} \), \( \text{Sr}^{2+} \), and \( \text{Ba}^{2+} \), which tend to be less soluble.
• Calcium hydroxide is slightly soluble, which allows it to dissolve enough to react with sulfurous acid.
• According to the rules, \( \text{CaSO}_3 \) is largely insoluble, resulting in a solid precipitate.

The behavior of \( \text{CaSO}_3 \) in the final reaction illustrates the importance of understanding solubility. Knowing which compounds will precipitate lets us predict and write accurate net ionic equations. Students should familiarize themselves with these rules to solve similar problems effectively.