Question

Identify each salt as neutral, acidic, or basic. a) \(\mathrm{NaBr}\) b) \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}\) c) \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}\)

Short answer

a) Neutral, b) Acidic, c) Acidic

Step-by-step solution

Understand the concept of salt hydrolysis

Salts can be classified as neutral, acidic, or basic based on how their ions interact with water during a process called hydrolysis. If the anion of the salt comes from a strong acid and the cation from a strong base, the salt is neutral. If the anion comes from a weak acid or the cation from a weak base, the salt can be basic or acidic, respectively.

Analyze NaBr

Sodium bromide (\(\mathrm{NaBr}\)) is a salt formed from a strong base, sodium hydroxide (\(\mathrm{NaOH}\)), and a strong acid, hydrobromic acid (\(\mathrm{HBr}\)). Neither ion hydrolyzes in water, making \(\mathrm{NaBr}\) a neutral salt.

Analyze \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}\)

Iron(II) nitrate (\(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}\)) is a salt formed from a weak base, iron(II) hydroxide (\(\mathrm{Fe(OH)}_2\)), and a strong acid, nitric acid (\(\mathrm{HNO}_3\)). The \(\mathrm{Fe}^{2+}\) ions will hydrolyze in water, making the solution acidic.

Analyze \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}\)

Iron(III) nitrate (\(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}\)) is similar to iron(II) nitrate but involves \(\mathrm{Fe}^{3+}\) ions. \(\mathrm{Fe}^{3+}\) is a stronger Lewis acid compared to \(\mathrm{Fe}^{2+}\). Thus, \(\mathrm{Fe}^{3+}\) will more readily hydrolyze, making the solution even more acidic.

Key concepts

neutral salts

Neutral salts are fascinating because they don't react with water to change the pH of a solution. They are formed when the anion comes from a strong acid and the cation comes from a strong base.

For example, sodium bromide (\( \mathrm{NaBr} \)) is the product of the strong acid hydrobromic acid (\( \mathrm{HBr} \)) and the strong base sodium hydroxide (\( \mathrm{NaOH} \)).

Neither sodium nor bromide ions undergo hydrolysis in water. Hydrolysis is a common process where ions react with water to potentially result in either a basic or acidic solution. In the case of neutral salts like \( \mathrm{NaBr} \), this doesn't occur, so the solution remains neutral with a pH of approximately 7. That means they are neither basic nor acidic when dissolved in water.

acidic salts

Acidic salts occur when a salt's cation has acidic properties, often because it derives from a weak base. These salts, when dissolved in water, tend to create an acidic solution.

For example, consider iron(II) nitrate (\( \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2} \)). It is the result of a reaction between iron(II) hydroxide (\( \mathrm{Fe(OH)}_2 \)), a weak base, and nitric acid (\( \mathrm{HNO}_3 \)), a strong acid. \( \mathrm{Fe}^{2+} \) ions in this salt hydrolyze in water due to being less stable, releasing \( \mathrm{H}^+ \) ions. This leads to a decrease in the pH of the solution, making it acidic.

If we look at iron(III) nitrate (\( \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3} \)), the \( \mathrm{Fe}^{3+} \) ions are even more prone to hydrolysis than \( \mathrm{Fe}^{2+} \) ions. This makes the solution of iron(III) nitrate more acidic than that of iron(II) nitrate.

basic salts

Basic salts are formed when an anion with basic characteristics combines with a cation from a strong base. In this process, when such salts are added to water, they can result in a basic solution.

When a weak acid combines with a strong base, the resulting salt can often hydrolyze to produce hydroxide ions (\( \mathrm{OH}^{-} \)). This release of hydroxide ions causes the solution to have a higher pH, which is an indicator of basicity.

While this explanation outlines how basic salts behave, it is important to note that specific examples were not part of the original exercise. Therefore, the understanding of basic salts involves focusing on the interactions that elevate pH when an anion from a weak acid interacts with water.