Problem 3 - Chemistry

Question

Classify each of the following species as a Brönsted acid or base, or both: (a) \(\mathrm{H}_{2} \mathrm{O},(\mathrm{b}) \mathrm{OH}^{-},(\mathrm{c}) \mathrm{H}_{3} \mathrm{O}^{+},(\mathrm{d}) \mathrm{NH}_{3},\) (h) \(\mathrm{CO}_{3}^{2-}\) (e) \(\mathrm{NH}_{4}^{+}\) (f) \(\mathrm{NH}_{2},(\mathrm{~g}) \mathrm{NO}_{3}^{-}\) (i) \(\mathrm{HBr}\) (j) HCN.

Step-by-Step Solution

Answer

(a) Both, (b) Base, (c) Acid, (d) Base, (e) Acid, (f) Base, (g) Neither, (h) Base, (i) Acid, (j) Acid.

Step 1: Determine Brönsted Acid or Base

A Brönsted acid is a proton donor, while a Brönsted base is a proton acceptor. Evaluate each compound based on this definition.

Step 2: Analyze Each Species

- (a) \(\mathrm{H}_{2}\mathrm{O}\): Can donate a proton (acts as an acid) and accept a proton (acts as a base). It is amphoteric.- (b) \(\mathrm{OH}^{-}\): Accepts a proton to form \(\mathrm{H}_{2}\mathrm{O}\), acting as a base.- (c) \(\mathrm{H}_{3}\mathrm{O}^{+}\): Donates a proton to become \(\mathrm{H}_{2}\mathrm{O}\), acting as an acid.- (d) \(\mathrm{NH}_{3}\): Accepts a proton to form \(\mathrm{NH}_{4}^{+}\), acting as a base.- (e) \(\mathrm{NH}_{4}^{+}\): Donates a proton to become \(\mathrm{NH}_{3}\), acting as an acid.- (f) \(\mathrm{NH}_{2}\): Typically acts as a base thanks to its lone pair of electrons, accepting a proton.- (g) \(\mathrm{NO}_{3}^{-}\): Typically does not accept protons; conjugate base of a strong acid and a poor base.- (h) \(\mathrm{CO}_{3}^{2-}\): Can accept protons to form bicarbonate, acting as a base.- (i) \(\mathrm{HBr}\): Strongly donates a proton, acting as an acid.- (j) HCN: Can donate a proton, acting as an acid.

Step 3: Classify Each Species

- (a) \(\mathrm{H}_{2}\mathrm{O}\): Both (amphoteric)- (b) \(\mathrm{OH}^{-}\): Base- (c) \(\mathrm{H}_{3}\mathrm{O}^{+}\): Acid- (d) \(\mathrm{NH}_{3}\): Base- (e) \(\mathrm{NH}_{4}^{+}\): Acid- (f) \(\mathrm{NH}_{2}\): Base- (g) \(\mathrm{NO}_{3}^{-}\): Neither (poor base)- (h) \(\mathrm{CO}_{3}^{2-}\): Base- (i) \(\mathrm{HBr}\): Acid- (j) HCN: Acid.

Key Concepts

Proton Donor

In the Brönsted-Lowry theory, an acid is commonly defined as a proton donor. This means that an acid can release a proton (\( ext{H}^+ \)), which is essentially a hydrogen ion. This process occurs because the molecule wants to stabilize by giving away a positive charge.
A molecule like \( ext{H}_3 ext{O}^+ \) exemplifies a proton donor. It readily donates a proton to become \( ext{H}_2 ext{O} \). Acids such as \( ext{HBr} \) and \( ext{HCN} \) are strong proton donors, where they release their proton easily into a solution. Here are some key points about proton donors:

• Proton donors increase the concentration of \( ext{H}^+ \) ions in a solution.
• The ability to donate a proton is dependent upon the structure of the molecule.
• Acids can be strong or weak based on how readily they donate protons.

Understanding proton donation is crucial in predicting chemical reactions and their outcomes.

Proton Acceptor

A fundamental aspect of Brönsted-Lowry base theory is that bases are proton acceptors. This means that they have a tendency to accept protons into their structure. There are usually lone pairs of electrons present in bases that allow them to attract and hold onto positively charged protons.
The common hydroxide ion \( ext{OH}^- \) is a classic example of a proton acceptor. It accepts a proton to become water \( ext{H}_2 ext{O} \). Similarly, the ammonia molecule \( ext{NH}_3 \) acts as a base because it accepts a proton to form ammonium, \( ext{NH}_4^+ \).
Some properties of proton acceptors include:

• The presence of lone electrons which facilitate the acceptance of protons.
• They can often neutralize acids by accepting protons.
• Bases can be strong or weak, depending on their ability to accept protons.

Grasping the concept of proton acceptors is crucial for understanding various chemical processes, including neutralization reactions.

Amphoteric Species

Amphoteric substances are quite fascinating because they can act as either an acid or a base, depending on the circumstances. This dual nature means they can both donate and accept protons. Water (\( ext{H}_2 ext{O} \)) is the most well-known amphoteric species.
In one reaction, water can donate a proton to another compound, acting as an acid. In another situation, it might accept a protons and thus function as a base.
Consider these core features of amphoteric species:

• Amphoteric compounds are versatile in chemical reactions, balancing the pH of solutions.
• They are crucial in buffering systems due to their dual capabilities.
• Understanding their behavior helps in predicting reaction paths and balances.

Learning about amphoteric substances can broaden one's understanding of reactions in both natural and synthetic systems.

Acid-Base Classification

One significant application of the Brönsted-Lowry theory is classifying substances into acids and bases. This is crucial for predicting interactions between different molecules and understanding reaction mechanisms.
To classify a compound:

• Determine if it donates or accepts protons. An acid donates, and a base accepts.
• Consider whether a substance is an amphoteric, like water, which can do both.
• Examining the structure and function helps in categorizing compounds.

The step-by-step analysis involves assessing each molecule, such as \( ext{H}_{2} ext{O}, ext{OH}^-, ext{NH}_3, ext{NO}_3^- \), to understand their behavior based on proton transfers. While some molecules may act predominantly as acids or bases, amphoterics blur the lines, showcasing the diversity of chemical interactions.
Accurate acid-base classification leads to better predictions about how compounds will interact in their environments.