Problem 90 - General Chemistry: Principles and Modern Applications

Question

Explain why \(1.0 \mathrm{M} \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(\mathrm{aq})\) is acidic.

Step-by-Step Solution

Answer

The solution of \(1.0 \mathrm{M} \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(\mathrm{aq})\) is acidic because the Fe3+ ions react with water to produce Fe(OH)2+ and H+, thus increasing the concentration of H+ ions or decreasing the pH of the solution.

Step 1: Determining the ions in solution

Firstly, identify the ions that the solute \(1.0 \mathrm{M} \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(\mathrm{aq})\) breaks down into in an aqueous solution. These ions are Fe^3+ and NO3-. Notice that Fe^3+ ions are capable of producing H+ ions in solutions.

Step 2: Reaction of Fe3+ ions

In an aqueous solution, Fe^3+ ion reacts with water to form Fe(OH)^2+ and releases H+ ions in the process. The reaction follows as: \[ \mathrm{Fe^{3+}} + \mathrm{H_2O} \rightarrow \mathrm{Fe(OH)^{2+}} + \mathrm{H^{+}} \]

Step 3: Checking the acidity

Now, as the Fe3+ ions react with water to produce H+ ions, it will increase the number of H+ ions in the solution thus making the solution acidic.

Key Concepts

Chemical Equilibrium

When discussing chemical reactions, it's important to understand the concept of chemical equilibrium. This is a state in which the rate of the forward reaction equals the rate of the reverse reaction, meaning that the concentrations of the reactants and products remain constant over time, but not necessarily equal. It is a dynamic balance where chemical processes occur continuously without net change.

In the context of our problem about why a solution of \(1.0 \mathrm{M} \mathrm{Fe}(\mathrm{NO}_{3})_{3}(\mathrm{aq})\) is acidic, the reaction between \(\mathrm{Fe^{3+}}\) ions and water reaches chemical equilibrium. However, the formation of hydronium ions (\(\mathrm{H}_3\mathrm{O}^{+}\)) shifts the equilibrium towards the products, which are \(\mathrm{Fe(OH)^{2+}}\) and \(\mathrm{H}^{+}\) ions, contributing to the solution's acidity.

The ability to recognize how equilibria can be affected by different factors, such as concentration, pressure, and temperature, is crucial for interpreting chemical reactions and understanding their outcomes.

Hydronium Ion Formation

One of the key players in determining a solution's acidity is the hydronium ion, represented as \(\mathrm{H}_3\mathrm{O}^{+}\). Hydronium ions are formed when \(\mathrm{H}^{+}\) ions, which are simply protons, associate with water molecules. The proton, due to its very small size and high charge density, is highly reactive and doesn't exist freely; instead, it attaches to a water molecule to form the more stable hydronium ion.

In our exercise, as the \(\mathrm{Fe^{3+}}\) ion reacts with water, it releases \(\mathrm{H}^{+}\) ions, which immediately combine with water molecules to form \(\mathrm{H}_3\mathrm{O}^{+}\). The presence of these hydronium ions is what makes the solution acidic. Therefore, understanding hydronium ion formation is essential to comprehend the pH level of solutions.

Metal Ion Reactivity in Water

Metal ion reactivity in water is an important concept in chemistry, especially when discussing the acidity or basicity of solutions. Metal ions can react with water in a process called hydrolysis, leading to the formation of hydroxide ions or, in many cases, hydrogen ions, which affect the pH of the solution.

In our example, the metal ion involved is \(\mathrm{Fe^{3+}}\). This particular ion is highly charged and tends to polarize the nearby water molecules, leading to a shift in the balance of hydrogen and oxygen in those water molecules. It results in a release of \(\mathrm{H}^{+}\) ions, leaving behind \(\mathrm{OH}^{-}\) ions still bound to the metal. This behavior of metals in water is pivotal for predicting and explaining the pH of various aqueous metal solutions.

pH and Acidity

The pH scale is a measure of how acidic or basic a solution is. The scale ranges from 0 to 14, with a pH less than 7 indicating an acidic solution, pH equal to 7 indicating a neutral solution, and pH greater than 7 indicating a basic solution. The pH is mathematically defined as the negative logarithm to base 10 of the hydronium ion concentration, \( \mathrm{pH} = -\log[\mathrm{H}_3\mathrm{O}^{+}] \).

Acidity refers to the concentration of hydronium ions in a solution. Higher concentrations of \(\mathrm{H}_3\mathrm{O}^{+}\) indicate a lower pH and therefore a more acidic solution. In the case of \(1.0 \mathrm{M} \mathrm{Fe}(\mathrm{NO}_{3})_{3}(\mathrm{aq})\), the formation of \(\mathrm{H}^{+}\) ions from the hydrolysis of \(\mathrm{Fe^{3+}}\) ions, subsequently leading to \(\mathrm{H}_3\mathrm{O}^{+}\), results in an acidic solution as these ions increase the acid character of the solution.