Problem 57 - Chemistry Matter and Change

Question

Classify each compound as an Arrhenius acid or an Arrhenius base. \(\begin{array}{ll}{\text { a. } \mathrm{Henius}} & {\text { c. } \mathrm{Mg}(\mathrm{OH})_{2}} \\ {\text { b. RboH }} & {\text { d. } \mathrm{Hg} \mathrm{PO}_{4}}\end{array}\)

Step-by-Step Solution

Answer

a. "Henius" is not a known chemical compound, therefore it cannot be classified. b. RbOH dissociates into Rb+ and OH- ions when dissolved in water, therefore it is an Arrhenius base. c. Mg(OH)2 dissociates into Mg2+ and 2 OH- ions when dissolved in water, therefore it is an Arrhenius base. d. HgPO4 dissociates into Hg2+ and PO4^3- ions when dissolved in water but does not produce H+ or OH- ions, therefore it cannot be classified as an Arrhenius acid or base.

Step 1: a. For Henius:

To classify this compound, we need to see if it's producing H+ or OH- ions when dissolved in water. Unfortunately, "Henius" is not a known chemical compound, so it cannot be classified as an Arrhenius acid or base.

Step 2: b. For RbOH:

Rubidium hydroxide (RbOH) is a compound containing a metal (rubidium) and a hydroxide group (OH-). When dissolved in water, it dissociates into Rb+ and OH- ions. Since it produces hydroxide ions (OH-) in the solution, RbOH is classified as an Arrhenius base.

Step 3: c. For Mg(OH)2:

Magnesium hydroxide (Mg(OH)2) is a compound containing a metal (magnesium) and two hydroxide groups (2 OH-). When dissolved in water, it dissociates into Mg2+ and 2 OH- ions. Since it produces hydroxide ions (OH-) in the solution, Mg(OH)2 is classified as an Arrhenius base.

Step 4: d. For HgPO4:

Mercury(II) phosphate (HgPO4) is an ionic compound containing mercury cations (Hg2+) and phosphate anions (PO4^3-). When dissolved in water, it dissociates into Hg2+ and PO4^3- ions. This compound neither produces hydrogen ions (H+) nor hydroxide ions (OH-) when dissolved in water. Thus, HgPO4 cannot be classified as an Arrhenius acid or base.

Key Concepts

Chemical Compounds

Chemical compounds are substances formed by the combination of two or more chemical elements. They are held together by chemical bonds and exist in fixed ratios. In the context of acids and bases, these compounds can dissociate into ions when placed in water. The type of ions they release helps classify them as either acids or bases according to the Arrhenius theory.

• For example, Rubidium hydroxide ( ext{RbOH}) is a compound consisting of rubidium (Rb) and hydroxide (OH-) groups. It releases hydroxide ions upon dissociation.
• Magnesium hydroxide ( ext{Mg(OH)}_2) is another compound, made up of magnesium (Mg) and hydroxide groups. Similarly, it releases hydroxide ions and thus classified as an Arrhenius base.
• On the other hand, mercury(II) phosphate ( ext{HgPO}_4) does not dissociate to give hydroxide or hydrogen ions, showing that not all compounds can be neatly classified as bases or acids using Arrhenius definitions.

Understanding these classifications is crucial because they help predict the behavior of substances in chemical reactions.

Dissociation in Water

When a chemical compound is placed in water, it can undergo a process called dissociation. Dissociation is the separation of the compound into smaller particles, usually ions. This process is vital in distinguishing acids and bases according to Arrhenius.

• Acids dissociate in water to release hydrogen ions ( ext{H}^+), contributing to the acidity of the solution.
• Bases dissociate to release hydroxide ions ( ext{OH}^-), increasing the solution's basicity.

This concept explains how ext{RbOH} and ext{Mg(OH)}_2 are classified as bases because they both release hydroxide ions upon dissociation. The ability of these compounds to dissociate and release specific ions is what aids in their classification. In contrast, compounds such as ext{HgPO}_4 do not release ext{H}^+ or ext{OH}^- ions; hence, they are not classified as Arrhenius acids or bases.

Hydroxide Ions

Hydroxide ions ( ext{OH}^-) play a crucial role in the classification of bases. They are composed of one oxygen atom and one hydrogen atom carrying a negative charge. The presence of hydroxide ions in a solution makes it basic in nature according to Arrhenius' definition of a base.

• In the dissociation of ext{RbOH}, the release of hydroxide ions directly contributes to its classification as an Arrhenius base.
• Similarly, ext{Mg(OH)}_2 does the same, but with two hydroxide ions for each molecule, reinforcing its base properties.

The importance of hydroxide ions lies in their ability to influence the pH of a solution. By increasing the concentration of hydroxide ions once dissolved, these compounds elevate the pH, making the solution more basic.

Hydrogen Ions

Hydrogen ions ( ext{H}^+) are central to the concept of acids in the Arrhenius definition. They consist of a nucleus with a positive charge from a proton. When compounds dissociate to produce hydrogen ions in water, they are termed as acids under this classification system.

• Acids such as hydrochloric acid ( ext{HCl}) dissociate in aqueous solutions to produce hydrogen ions, thus lowering the pH.
• Some compounds may not produce any ext{H}^+ ions, such as the mercury(II) phosphate ( ext{HgPO}_4) mentioned previously, and as a result cannot be classified as an Arrhenius acid.

Grasping the role of hydrogen ions helps in understanding why certain substances act as acids, and how these ions interact with their environment, typically increasing the solution’s acidity.