Question

Arrange the following aqueous \(0.1 \mathrm{M}\) solutions in order of increasing \(\mathrm{pH}\) (lowest to highest). \(\begin{array}{llll}\mathrm{ZnBr}_{2} & \mathrm{NaOH} & \mathrm{LiF} & \mathrm{Sr}(\mathrm{OH})_{2} & \mathrm{KClO}_{4}\end{array}\)

Short answer

Arrange the given aqueous 0.1 M solutions in order of increasing pH: \(\mathrm{ZnBr}_{2}\), \(\mathrm{NaOH}\), \(\mathrm{LiF}\), \(\mathrm{Sr}(\mathrm{OH})_{2}\), and \(\mathrm{KClO}_{4}\). Answer: \(\mathrm{ZnBr}_{2} < \mathrm{KClO}_{4} < \mathrm{LiF} < \mathrm{NaOH} < \mathrm{Sr}(\mathrm{OH})_{2}\)

Step-by-step solution

Analyze the compounds individually

Determine the acidic or basic behavior of each solution: \(\mathrm{ZnBr}_{2}\): ZnBr₂ is a salt, and it dissolves in water to produce Zn²⁺ and ₂Br⁻ ions. Zn²⁺ ions have acidic properties due to the hydrolysis reaction: \(\mathrm{Zn}^{2+} + 2\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{Zn}(\mathrm{OH})_{2} + ₂\mathrm{H}^{+}\). Br⁻ ions doesn't impact the pH since they are spectator ions. Thus, \(\mathrm{ZnBr}_{2}\) forms an acidic solution. \(\mathrm{NaOH}\): Sodium hydroxide is a strong base that dissociates completely in water to form Na⁺ and OH⁻ ions. Since OH⁻ ions increase the pH, NaOH forms a basic solution. \(\mathrm{LiF}\): Lithium fluoride is a salt that will dissolve in water to form Li⁺ and F⁻ ions. Li⁺ does not influence the pH since it doesn't undergo hydrolysis. However, F⁻ ions react with water to produce HF and OH⁻ ions: \(\mathrm{F}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{HF} + \mathrm{OH}^{-}\). This reaction makes the solution slightly basic. \(\mathrm{Sr}(\mathrm{OH})_{2}\): Strontium hydroxide is a strong base that dissociates completely in water to form Sr²⁺ and ₂OH⁻ ions. Since OH⁻ ions raise the pH, \(\mathrm{Sr}(\mathrm{OH})_{2}\) forms a basic solution. \(\mathrm{KClO}_{4}\): Potassium perchlorate is a salt that dissociates completely in water to form K⁺ and ClO₄⁻ ions. Both ions do not undergo hydrolysis and are considered spectator ions. Therefore, the solution is neutral.

Arrange compounds in order of increasing pH

Now, we can arrange the solutions in order of increasing pH (lowest to highest) based on their acidic, neutral, or basic behavior: 1. Acidic solution: \(\mathrm{ZnBr}_{2}\) 2. Neutral solution: \(\mathrm{KClO}_{4}\) 3. Slightly basic solution: \(\mathrm{LiF}\) 4. Basic solution: \(\mathrm{NaOH}\) and \(\mathrm{Sr}(\mathrm{OH})_{2}\) (Sr(OH)₂ is slightly more basic due to the presence of two OH⁻ ions) Finally, the solutions in increasing order of pH are: \(\mathrm{ZnBr}_{2} < \mathrm{KClO}_{4} < \mathrm{LiF} < \mathrm{NaOH} < \mathrm{Sr}(\mathrm{OH})_{2}\).

Key concepts

Acid and Base Behavior

Understanding the behavior of acids and bases is crucial in chemistry, especially when dealing with solutions. Acids are substances that release hydrogen ions (H^+) when dissolved in water, while bases provide hydroxide ions (OH^-). The strength of an acid or a base is determined by its ability to donate or accept these ions. Strong acids and bases dissociate completely in water, contributing to a significant change in pH. Meanwhile, weak acids and bases only partially dissociate, causing a more moderate shift in pH.

When we look at the exercise solution, ZnBr_2 is an example of a solution that undergoes hydrolysis, a reaction with water that leads to the release of H^+ ions, thus exhibiting acidic behavior. On the other hand, NaOH and Sr(OH)_2 are strong bases, dissociating to yield OH^- ions and raising the pH of the solution. Recognizing these behaviors is key in predicting how different chemicals will impact the pH of a solution.

Solution Hydrolysis

Solution hydrolysis involves a chemical reaction where ions from salts react with water to produce either acidic or basic solutions. The process depends on whether the ion is capable of attracting or donating protons. For instance, in the provided exercise, the Zn^2+ ion from ZnBr_2 reacts with water, leading to the production of H^+ ions in a reversible reaction, indicating an acidic solution due to the formation of Zn(OH)_2 and release of hydrogen ions.

By contrast, the fluoride ion (F^-) from LiF reacts with water to form hydroxide ions (OH^-), creating a slightly basic environment. It is by understanding these hydrolysis reactions that one can deduce the resultant pH of a solution. These concepts are essential when predicting the outcome of salt solutions in water and their influence on the overall pH.

pH Scale

The pH scale is a numeric scale that is used to specify the acidity or basicity of an aqueous solution. It typically ranges from 0 to 14, where a pH less than 7 indicates an acidic solution, pH equal to 7 is a neutral solution, and pH greater than 7 represents a basic solution. The pH value is calculated using the formula pH = -log[H^+], where [H^+] is the concentration of hydrogen ions in moles per liter.

In the context of the textbook exercise, knowing the pH scale helps students understand why ZnBr_2, being acidic, would have a lower pH compared to the neutral solution of KClO_4 and the basic solutions of LiF, NaOH, and Sr(OH)_2. The pH scale serves as a vital tool in predicting the behavior of substances in solution and allows for the comparison of their relative acidity or basicity.

Strong and Weak Electrolytes

Electrolytes are compounds that, when dissolved in water, dissociate into ions and conduct electricity. They can be classified as strong or weak based on their ability to dissociate. Strong electrolytes dissociate completely into their constituent ions. Examples from our exercise include NaOH, Sr(OH)_2, and KClO_4, which all dissociate completely, yielding ions that do not undergo hydrolysis and therefore either increase the pH or leave it unchanged.

Weak electrolytes, conversely, do not completely ionize in solution. While not highlighted in the exercise, a typical example of a weak electrolyte is acetic acid, which only partially dissociates. The understanding of strong and weak electrolytes aids in anticipating the extent of the electrical conductivity of a solution and also helps predict resulting pH levels when these electrolytes are dissolved in water.