Problem 87 - Chemistry The Science in Context

Question

Consider the following compounds: \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) \(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{Ca}(\mathrm{OH})_{2},\) and \(\mathrm{HClO}_{4}.\) Identify the Arrhenius acid(s). b. Identify the Arrhenius base(s). c. Identify the Brensted-Lowry acid(s). d. Identify the Brensted-Lowry base(s).

Step-by-Step Solution

Answer

Question: Classify the following compounds as Arrhenius acids or bases, and Brønsted-Lowry acids or bases: CH3NH2, CH3COOH, Ca(OH)2, and HClO4. Answer: The compounds can be classified as follows: - Arrhenius acid(s): CH3COOH, HClO4 - Arrhenius base(s): Ca(OH)2 - Brønsted-Lowry acid(s): CH3COOH, HClO4 - Brønsted-Lowry base(s): CH3NH2

Step 1: Identify the compounds

The given compounds are CH3NH2 (methylamine), CH3COOH (acetic acid), Ca(OH)2 (calcium hydroxide), and HClO4 (perchloric acid).

Step 2: Determine the Arrhenius acids

To be an Arrhenius acid, the compound must increase the concentration of hydrogen ions (H+) when dissolved in water. In this case, CH3COOH and HClO4 release H+ ions when they dissociate in water: CH3COOH → CH3COO- + H+ HClO4 → ClO4- + H+ Therefore, CH3COOH and HClO4 are Arrhenius acids.

Step 3: Determine the Arrhenius bases

To be an Arrhenius base, the compound must increase the concentration of hydroxide ions (OH-) when dissolved in water. In this case, the only compound that does this is Ca(OH)2: Ca(OH)2 → Ca2+ + 2 OH- Therefore, Ca(OH)2 is an Arrhenius base.

Step 4: Determine the Brønsted-Lowry acids

To be a Brønsted-Lowry acid, the compound must be able to donate a proton (H+). CH3COOH and HClO4 can donate protons as shown in the dissociation reactions in Step 2. Therefore, CH3COOH and HClO4 are Brønsted-Lowry acids.

Step 5: Determine the Brønsted-Lowry bases

To be a Brønsted-Lowry base, the compound must be able to accept a proton (H+). CH3NH2 is a weak base and can accept a proton to form CH3NH3+ as shown below: CH3NH2 + H+ → CH3NH3+ Therefore, CH3NH2 is a Brønsted-Lowry base. In conclusion, we have identified the following compounds as each type of acid and base: - Arrhenius acid(s): CH3COOH, HClO4 - Arrhenius base(s): Ca(OH)2 - Brønsted-Lowry acid(s): CH3COOH, HClO4 - Brønsted-Lowry base(s): CH3NH2

Key Concepts

Arrhenius Acid

In the world of acid-base chemistry, the concept of an Arrhenius acid is quite foundational. It describes any substance that increases the concentration of hydrogen ions \( \text{H}^+ \) in aqueous solutions. Think of hydrogen ions as the hallmark of Arrhenius acids. When these substances dissolve in water, they either provide or increase available hydrogen ions.

• For example, acetic acid \( \text{CH}_3\text{COOH} \) dissociates in water to produce \( \text{H}^+ \), and the acetate ion \( \text{CH}_3\text{COO}^- \).
• Similarly, perchloric acid \( \text{HClO}_4 \) dissociates to give \( \text{H}^+ \) and the perchlorate ion \( \text{ClO}_4^- \).

These reactions depict how Arrhenius acids operate by contributing hydrogen ions to the mix, thus lowering the pH.

Brønsted-Lowry Acid

Moving beyond Arrhenius, the Brønsted-Lowry definition expands on what an acid can be. Here, an acid is not just a donor of hydrogen ions in water, but is defined more broadly as any substance that can donate a proton \( \text{H}^+ \) to another molecule. This concept is incredibly useful because it covers reactions outside of aqueous solutions.

• Like Arrhenius acids, acetic acid and perchloric acid are also Brønsted-Lowry acids because they can donate protons to other compounds.
• In essence, if a molecule is willing to "give away" a proton to another molecule, it's considered a Brønsted-Lowry acid, regardless of the solution it's in.

Arrhenius Base

Shifting gears, an Arrhenius base is defined as a compound that increases the concentration of hydroxide ions \( \text{OH}^- \) in aqueous solutions. Bases typically take on the role of neutralizing acids and can alter the pH levels of a solution, often making it more alkaline.

• For example, calcium hydroxide \( \text{Ca(OH)}_2 \) is an Arrhenius base that dissociates in water to release two hydroxide ions for each formula unit dissolved.
• This release of \( \text{OH}^- \) raises the solution's pH, distinguishing it as an Arrhenius base.

The key takeaway is that Arrhenius bases can be recognized by their increase of hydroxide ions in water.

Brønsted-Lowry Base

Lastly, the Brønsted-Lowry theory also provides a broader view of what bases can be. In this context, a base is any molecule capable of accepting a proton \( \text{H}^+ \). This definition doesn't restrict the base to aqueous solutions, making it applicable in various chemical environments.

• A classic example is methylamine \( \text{CH}_3\text{NH}_2 \), which accepts a proton to become \( \text{CH}_3\text{NH}_3^+ \).
• This acceptance of a proton qualifies methylamine as a Brønsted-Lowry base.

Thus, any time a compound plays the role of a hydrogen ion accepter, it earns the title of being a Brønsted-Lowry base.