Problem 102 - Chemistry Matter and Change

Question

Paper A reaction occurs when sulfurous acid \(\left(\mathrm{H}_{2} \mathrm{SO}_{3}\right)\) is mixed with an aqueous solution of sodium hydroxide. The reaction produces aqueous sodium sulfite, a chemical used in manufacturing paper. Write the chemical and net ionic equations for the reaction.

Step-by-Step Solution

Answer

The net ionic equation is: \(\text{H}_2\text{SO}_3(aq) + 2\text{OH}^-(aq) \rightarrow \text{SO}_3^{2-}(aq) + 2\text{H}_2\text{O}(l)\).

Step 1: Write the Balanced Chemical Equation

To find the balanced chemical equation, consider the compounds involved. Sulfurous acid (\(\text{H}_2\text{SO}_3\)) reacts with sodium hydroxide (\(\text{NaOH}\)) to produce aqueous sodium sulfite (\(\text{Na}_2\text{SO}_3\)) and water (\(\text{H}_2\text{O}\)). The balanced chemical equation is given by:\[\text{H}_2\text{SO}_3(aq) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{SO}_3(aq) + 2\text{H}_2\text{O}(l)\]This shows that each molecule of sulfurous acid reacts with two molecules of sodium hydroxide.

Step 2: Determine the Ionic Equation

In the ionic equation, show the ions involved separately except for the weak acid and the water. Sodium hydroxide fully dissociates in water to form sodium ions (\(\text{Na}^+\)) and hydroxide ions (\(\text{OH}^-\)), while sodium sulfite dissociates into sodium ions and sulfite ions (\(\text{SO}_3^{2-}\)). The ionic equation is:\[\text{H}_2\text{SO}_3(aq) + 2\text{Na}^+(aq) + 2\text{OH}^-(aq) \rightarrow 2\text{Na}^+(aq) + \text{SO}_3^{2-}(aq) + 2\text{H}_2\text{O}(l)\]

Step 3: Write the Net Ionic Equation

Identify and remove the spectator ions, which are ions that appear on both sides of the ionic equation. Here, \(\text{Na}^+\) is the only spectator ion. After removing these ions, the net ionic equation is:\[\text{H}_2\text{SO}_3(aq) + 2\text{OH}^-(aq) \rightarrow \text{SO}_3^{2-}(aq) + 2\text{H}_2\text{O}(l)\]This equation shows the actual chemical change occurring, excluding the spectator ions.

Key Concepts

Balanced Chemical Equation

A balanced chemical equation is a fundamental concept in understanding chemical reactions. It provides a clear representation of the substances involved in the reaction and how they interact with each other.
In a balanced chemical equation, both sides of the equation have the same number of atoms for each element, reflecting the law of conservation of mass. This means that the quantity of each element remains constant throughout the reaction.

• The first step in balancing a chemical equation is to write the unbalanced equation based on the reactants and products.
• Next, adjust the coefficients (the numbers in front of molecules) to ensure that there is an equal number of each type of atom on both sides.
• It is important not to change the subscripts within the chemical formulas, as this would alter the substances involved.

For example, in the case of sulfurous acid and sodium hydroxide, the balanced chemical equation is:\[\text{H}_2\text{SO}_3(aq) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{SO}_3(aq) + 2\text{H}_2\text{O}(l)\]In this balanced equation, each molecule of sulfurous acid reacts with two molecules of sodium hydroxide, resulting in the formation of sodium sulfite and water. This equation clearly demonstrates how the atoms are rearranged during the reaction.

Net Ionic Equation

A net ionic equation offers a focused view of a chemical reaction, displaying only the particles that are directly involved in the chemical change. This approach helps simplify complex reactions by removing spectator ions—ions that do not participate in the actual reaction and remain unchanged.

• To develop a net ionic equation, begin with a balanced chemical equation.
• Break down all the strong electrolytes into their ions, showing each clearly.
• Identify and eliminate spectator ions, which appear identically on both sides of the equation.

In the given reaction between sulfurous acid and sodium hydroxide, the ionic equation is written first:\[\text{H}_2\text{SO}_3(aq) + 2\text{Na}^+(aq) + 2\text{OH}^-(aq) \rightarrow 2\text{Na}^+(aq) + \text{SO}_3^{2-}(aq) + 2\text{H}_2\text{O}(l)\]Here, sodium ions are spectator ions because they exist in the same form on both sides. Removing them gives the net ionic equation:\[\text{H}_2\text{SO}_3(aq) + 2\text{OH}^-(aq) \rightarrow \text{SO}_3^{2-}(aq) + 2\text{H}_2\text{O}(l)\]This equation highlights the actual chemical transformation where the sulfurous acid reacts with hydroxide ions leading to sulfite ions and water.

Acid-Base Reaction

Acid-base reactions are a classic type of chemical change, where an acid reacts with a base. These reactions often result in the formation of water and a salt. Understanding this concept helps clarify why certain reactions occur and what products are formed.

• An acid is a substance that donates hydrogen ions (\(\text{H}^+\)) in a reaction.
• A base is a substance that accepts these hydrogen ions, often providing hydroxide ions (\(\text{OH}^-\)) in return.
• When acids and bases come together, they undergo neutralization—a process that generally results in water and another compound called a salt.

In the exercise we are examining, sulfurous acid (\(\text{H}_2\text{SO}_3\)) acts as the acid, and sodium hydroxide (\(\text{NaOH}\)) serves as the base. When mixed, they react to form water and sodium sulfite, a salt:\[\text{H}_2\text{SO}_3(aq) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{SO}_3(aq) + 2\text{H}_2\text{O}(l)\]This reaction exemplifies an acid-base reaction, demonstrating how two reactants can neutralize each other, resulting in new products that include water.