Question

Predict whether a solution of each of the following salts is acidic, basic, or pH neutral: (a) KCl; (b) KF; (c) \(\mathrm{NaNO}_{3} ;\) (d) \(\mathrm{Ca}(\mathrm{OCl})_{2} ;\) (e) \(\mathrm{NH}_{4} \mathrm{NO}_{2}\)

Short answer

The solutions of KCl, NaNO3, and Ca(OCl)2 are neutral. The solution of KF is basic. The solution of NH4NO2 is acidic.

Step-by-step solution

Determine behavior of each ion for every salt

In this step, each salt will be broken down into its constituent cation and anion. Then, using known acid-base properties of the ions or the salts they came from, predict how each ion will behave in water.

Predict solution behavior for (a) KCl

KCl breaks down into K+ and Cl-. Neither of these ions hydrolyzes with water. Hence, solution of KCl will be pH neutral.

Predict solution behavior for (b) KF

KF breaks down into K+ and F-. While K+ does not hydrolyze, F- will because it comes from HF, a weak acid. Hence it will react with water to produce OH- ions, making the solution basic.

Predict solution behavior for (c) NaNO3

NaNO3 breaks down into Na+ and NO3-. Neither ions hydrolyze with water which makes the solution pH neutral.

Predict solution behavior for (d) Ca(OCl)2

Ca(OCl)2 breaks down into Ca2+ and OCl-. Neither ions hydrolyze with water and make the solution pH neutral.

Predict solution behavior for (e) NH4NO2

NH4NO2 breaks down into NH4+ and NO2-. NH4+ comes from NH4OH, a weak base, so it will react with water to produce H+ ions, while NO2- comes from HNO2, a weak acid, it will also react with water to produce H+ ions. Thus, the solution of NH4NO2 will be acidic.

Key concepts

Salt Hydrolysis

Salt hydrolysis is a critical concept to understand when predicting the pH of a solution containing salts. In essence, hydrolysis involves the reaction of an ion from a dissolved salt with water to form an acid or a base. Ions that are the conjugates of weak acids or bases will typically undergo hydrolysis, altering the pH of the solution.

For instance, consider the salt KF. The F- ion is a conjugate base from the weak acid HF. When KF dissolves in water, the F- ion can attract protons (H+) from water molecules, forming HF and simultaneously releasing hydroxide ions (OH-) into the solution. As a result, the solution becomes basic. This is why knowledge of the parent acid or base of the salt ions is essential for predicting hydrolysis behavior and, consequently, the pH level of the solution.

Acid-Base Behavior

Understanding acid-base behavior is fundamental to many areas of chemistry, including the analysis of salt solutions. An acid is a substance that can donate a proton (H+ ion), while a base is one that can accept a proton.

The strength of an acid or base is determined by its tendency to lose or gain a proton. When it comes to salts, we need to evaluate the potential of the cation and anion to act as an acid or a base. Strong acids and bases fully dissociate in water and do not affect the pH upon hydrolysis since their conjugates are neutral. However, if a salt is formed from a weak acid or base, its dissociated ions can alter the pH of the solution due to incomplete dissociation and the capacity to hydrolyze.

Prediction of Solution pH

To predict whether a solution of a salt will be acidic, basic, or neutral, consider the origins of its ions. If the cation is a conjugate acid of a weak base and the anion is a conjugate base of a weak acid, both may react with water, potentially affecting pH.

For example, in NH4NO2, NH4+ (from NH4OH) will donate H+ to the water, and NO2- (from HNO2) has the potential to accept H+ from water. However, because NH4OH is a weaker base than HNO2 is an acid, its conjugate acid NH4+ will have a greater effect on pH, resulting in a net acidic solution. To improve understanding, considering the strength and the nature of conjugate acids or bases from which the ions are derived can be crucial for accurately predicting the pH of salt solutions.

Ion Behavior in Water

The behavior of ions in water is a complex interplay of chemistry that has significant implications for the pH of the solution. Ions can behave differently based on their charge, size, and the pH of the environment. Ions from strong acids and bases, such as Na+ from NaOH or Cl- from HCl, will not typically react with water; as such, they yield neutral solutions.

Conversely, ions from weak acids or bases do react with water, influencing the pH. For learners, it is essential to understand the origin of an ion to predict its behavior in water. In essence, ions' propensity to donate or accept protons determines their impact on the solutions' acidity or basicity, showcasing the nuanced relationship ion behavior has with water.