Question

Iron in drinking water is removed by precipitation of the \(\mathrm{Fe}^{3+}\) ion by reaction with \(\mathrm{NaOH}\) to produce iron(III) hydroxide. Write the balanced chemical equation and the net ionic equation for this reaction.

Short answer

\( \mathrm{Fe^{3+} + 3 \, NaOH \rightarrow Fe(OH)_3 \downarrow + 3 \, Na^+} \) and \( \mathrm{Fe^{3+} + 3 \, OH^- \rightarrow Fe(OH)_3 \downarrow} \).

Step-by-step solution

Write the Reactants

Identify the chemical species involved in the reaction. Here, we have \( \mathrm{Fe}^{3+} \) ions and \( \mathrm{NaOH} \) reacting together to form a precipitate.

Write the Products

The reaction between \( \mathrm{Fe}^{3+} \) ions and \( \mathrm{NaOH} \) will result in the formation of iron(III) hydroxide, \( \mathrm{Fe(OH)_3} \), and sodium ions, \( \mathrm{Na^+} \).

Balance the Molecular Equation

Start by writing the unbalanced equation: \[ \mathrm{Fe^{3+} + 3 \, NaOH \rightarrow Fe(OH)_3 \downarrow + 3 \, Na^+} \]Make sure the number of each type of atom is the same on both sides.

Write the Net Ionic Equation

The net ionic equation involves only the species that change during the reaction. Remove the spectator ion \( \mathrm{Na^+} \) to get: \[ \mathrm{Fe^{3+} + 3 \, OH^- \rightarrow Fe(OH)_3 \downarrow} \]

Verify the Equations

Check each equation to ensure it is balanced in terms of both mass and charge. Both equations should maintain the conservation of atoms and charge.

Key concepts

Ionic Equations

In chemical reactions, particularly those occurring in aqueous solutions, it is important to understand the role of ionic equations. They help us focus on the substances involved in the chemical change. An ionic equation represents the particles involved as ions and dissociates soluble ionic compounds into their respective ions. For instance, in the reaction between iron(III) ions and sodium hydroxide, the full ionic equation would include all ions present in the solution:

• Iron(III) ions: \( \mathrm{Fe}^{3+} \)
• Hydroxide ions from sodium hydroxide: \( \mathrm{OH}^- \)
• Sodium ions: \( \mathrm{Na}^+ \)

However, in the context of net ionic equations, we only focus on the ions that undergo a change during the reaction. The net ionic equation for this reaction is simplified by removing any spectator ions, like \( \mathrm{Na}^+ \) in this case, which do not participate in forming the precipitate. This simplification provides clarity about the substances reacting and formed, as shown by:\[ \mathrm{Fe}^{3+} + 3 \, \mathrm{OH}^- \rightarrow \mathrm{Fe(OH)_3} \downarrow \]This equation highlights the precipitation of iron(III) hydroxide from its ions.

Reaction Balancing

Balancing a chemical equation involves ensuring that all elements exhibit the same quantity on both the reactant and product sides. This process respects the Law of Conservation of Mass, which states that matter cannot be created or destroyed. To balance a reaction, start by writing the unbalanced equation with the known reactants and products.

• Initial unbalanced equation: \( \mathrm{Fe}^{3+} + \mathrm{NaOH} \rightarrow \mathrm{Fe(OH)_3} + \mathrm{Na}^+ \)
• Count the number of each type of atom—and the charge if applicable—on both sides.

Clearly, we need 3 hydroxide ions to match the three needed to form one molecule of iron(III) hydroxide. Thus, ensure each iron(III) ion is adequately paired with three hydroxide ions:\[ \mathrm{Fe}^{3+} + 3 \, \mathrm{NaOH} \rightarrow \mathrm{Fe(OH)_3 \downarrow} + 3 \, \mathrm{Na}^+ \]By adjusting coefficients as needed, the balanced equation reflects equal numbers of each type of atom and charges on both sides. This provides an accurate representation of the chemical reaction involved.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions are mixed, leading to the formation of an insoluble product. This product is known as a precipitate, which often appears as a solid settling out of the liquid mixture. In the given reaction, the mixing of iron(III) ions and sodium hydroxide in water leads to the formation of an insoluble compound, iron(III) hydroxide.The general process of precipitation begins as the cations and anions from two different solutions meet. If the combination of these ions is less soluble in water, they form a solid. Key points to remember about precipitation reactions include:

• They require the formation of an insoluble compound.
• Precipitate is denoted as a solid (e.g., \( \downarrow \) or \( \mathrm{(s)} \) in equations).
• Not all ionic combinations will precipitate; solubility rules help predict possible formations.

In our focus equation, iron(III) hydroxide \( \mathrm{Fe(OH)_3} \) precipitates out of solution, shown as \( \mathrm{Fe(OH)_3 \downarrow} \), providing an effective method for the removal of iron from drinking water through precipitation.