Problem 74 - Chemistry The Science in Context

Question

If \(0.100 M\) aqueous solutions of each of these compounds were prepared, which one would have the lowest pH? (a) \(\mathrm{BaCl}_{2} ;\) (b) \(\mathrm{LiCl} ;\) (c) \(\mathrm{KCl} ;\) (d) \(\mathrm{TiCl}_{4}\)

Step-by-Step Solution

Answer

a) \(0.100 M\) \(\mathrm{BaCl}_{2}\) b) \(0.100 M\) \(\mathrm{LiCl}\) c) \(0.100 M\) \(\mathrm{KCl}\) d) \(0.100 M\) \(\mathrm{TiCl}_{4}\) Answer: d) \(0.100 M\) \(\mathrm{TiCl}_{4}\)

Step 1: Identify the compound types

First, we need to identify the type of each compound (whether it's acidic, basic, or neutral) by seeing which ions are being dissolved. (a) \(\mathrm{BaCl}_{2}\): Barium chloride is a salt formed from a strong base (barium hydroxide) and a strong acid (hydrochloric acid). Therefore, it is a neutral salt. (b) \(\mathrm{LiCl}\): Lithium chloride is also a salt formed from a strong base (lithium hydroxide) and a strong acid (hydrochloric acid). Therefore, it is a neutral salt. (c) \(\mathrm{KCl}\): Potassium chloride is a salt formed from a strong base (potassium hydroxide) and a strong acid (hydrochloric acid). Therefore, it is a neutral salt. (d) \(\mathrm{TiCl}_{4}\): Titanium tetrachloride is a covalent compound that reacts with water to form a strong acid, as shown in the following chemical equation: \(\mathrm{TiCl}_{4} + 4 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ti(OH)}_{4} + 4 \mathrm{HCl}\).

Step 2: Compare acidic tendencies

The three neutral salts, namely BaCl2, LiCl, and KCl, will not release or absorb \(\mathrm{H}^{+}\) ions when dissolved in water, so their pH will be close to 7. On the other hand, Titanium tetrachloride \(\mathrm{(TiCl}_{4})\) reacts with water to form a strong acid (hydrochloric acid), which will release a high concentration of \(\mathrm{H}^{+}\) ions, resulting in a lower pH value.

Step 3: Determine the lowest pH

Based on our analysis, the aqueous solution of \(\mathrm{TiCl}_{4}\) will have the lowest pH because it forms a strong acid upon the addition of water, resulting in a higher concentration of \(\mathrm{H}^{+}\) ions. Thus, the answer is (d) \(\mathrm{TiCl}_{4}\).

Key Concepts

Neutral Salts

Neutral salts are formed when a strong acid reacts with a strong base. In this reaction, the hydrogen ions from the acid and the hydroxide ions from the base neutralize each other. This results in the formation of water and a salt that does not significantly alter the pH of the solution. Neutral salts, such as \(\mathrm{BaCl}_2\), \(\mathrm{LiCl}\), and \(\mathrm{KCl}\), typically remain around a pH of 7 when dissolved in water.

Neutral salts are interesting because they don't cause the solution to become acidic or basic. They simply dissociate into their respective ions. For example, \(\mathrm{BaCl}_2\) dissociates into \(\mathrm{Ba}^{2+}\) ions and \(\mathrm{Cl}^{-}\) ions in water.

• Because the components are derived from a strong acid and a strong base, they do not interact with water to generate additional \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) ions.
• This lack of interaction is why the pH remains neutral.

Understanding neutral salts is crucial for predicting the behavior of various compounds in aqueous solutions.

Acidic Solutions

An acidic solution is characterized by an excess of \(\mathrm{H}^+\) ions. These ions come from substances that can donate protons to water, increasing the solution's acidity. The more \(\mathrm{H}^+\) ions present, the lower the pH of the solution.

In the case of \(\mathrm{TiCl}_4\), the compound reacts with water to produce \(\mathrm{HCl}\), a strong acid. This reaction leads to a high concentration of \(\mathrm{H}^+\) ions, dramatically lowering the pH:

\[\mathrm{TiCl}_{4} + 4 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ti(OH)}_{4} + 4 \mathrm{HCl} \]

• Because \(\mathrm{HCl}\) is generated, the solution becomes highly acidic.
• Titanium chloride solutions will therefore have significantly lower pH values compared to neutral salts.

Recognizing how different substances affect acidity can help in predicting the pH of a solution, such as understanding why \(\mathrm{TiCl}_4\) solutions have lower pH than neutral salts like \(\mathrm{BaCl}_2\), \(\mathrm{LiCl}\), and \(\mathrm{KCl}\).

Aqueous Solutions

Aqueous solutions are mixtures where water acts as the solvent. This type of solution is extremely common in chemistry and is essential for many reactions to occur.

In such solutions, compounds dissolve in water, allowing them to dissociate into ions. For example, when dissolving \(\mathrm{KCl}\) in water, it splits into \(\mathrm{K}^+\) and \(\mathrm{Cl}^-\) ions, free to move throughout the solution.

• The behavior of these ions in the solution can affect properties such as electrical conductivity and reactivity.
• Additionally, the pH of an aqueous solution is influenced by the specific ions that form when a compound dissolves.

All the compounds mentioned, such as \(\mathrm{BaCl}_2\), \(\mathrm{LiCl}\), \(\mathrm{KCl}\), and \(\mathrm{TiCl}_4\), form aqueous solutions with distinct properties and pH levels. Understanding how compounds behave in aqueous solutions helps predict how their individual ions affect overall chemical behavior and pH.