Question

Which of the following can behave both as a Bronsted-Lowry acid and as a Bronsted-Lowry base? (a) \(\mathrm{HCO}_{3}^{-}\) (b) CN (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Short answer

(a) \\(\text{HCO}_3^-\\) and (c) \\(\text{H}_2\text{O}\\) can act as both.

Step-by-step solution

Understanding Bronsted-Lowry Theory

The Bronsted-Lowry theory defines acids as substances that can donate a proton ( ext{H}^+), while bases are substances that can accept a proton. To identify a compound that can act as both, we look for compounds that contain both a proton to donate and a site to accept a proton.

Analyzing \\( ext{HCO}_3^-\\)

The bicarbonate ion \( ext{HCO}_3^-\) can donate a proton to form \( ext{CO}_3^{2-}\), acting as an acid. It can also accept a proton to form \( ext{H}_2 ext{CO}_3\), acting as a base. Therefore, \( ext{HCO}_3^-\) can behave as both an acid and a base.

Analyzing CN

The cyanide ion (CN^-) generally acts as a base because it can accept a proton to form HCN, but it lacks a proton to donate, so it cannot act as an acid.

Analyzing \\( ext{H}_2 ext{O}\\)

Water \( ext{H}_2 ext{O}\) can donate a proton to become \( ext{OH}^-\), acting as an acid, and can accept a proton to form \( ext{H}_3 ext{O}^+\), acting as a base. Thus, water can act as both an acid and a base.

Analyzing \\( ext{H}_2 ext{CO}_3\\)

Carbonic acid \( ext{H}_2 ext{CO}_3\) predominantly acts as an acid because it can donate a proton to become \( ext{HCO}_3^-\). It easily donates but does not have the same ability to accept a proton, so it's not typically considered a base under the Bronsted-Lowry theory.

Conclusion

Among the given options, \( ext{HCO}_3^-\) and \( ext{H}_2 ext{O}\) can both act as Bronsted-Lowry acids and bases.

Key concepts

Acid and Base Behavior

The Bronsted-Lowry theory is a fundamental concept in chemistry that defines acids and bases by their ability to donate or accept protons. An acid is a substance that can donate a proton \( (\text{H}^+) \), while a base is one that can accept a proton. This theory helps us understand how different substances interact in chemical reactions, highlighting their dual nature in certain cases where a compound can act as both an acid and a base, such as water. By analyzing these interactions, we identify compounds with both acidic and basic behaviors.

Bicarbonate Ion

The bicarbonate ion \( (\mathrm{HCO}_3^-) \) is an interesting example of a compound that can act as both a Bronsted-Lowry acid and base.

• As an acid: It donates a proton to become carbonate ion \( (\mathrm{CO}_3^{2-}) \).
• As a base: It accepts a proton to form carbonic acid \( (\mathrm{H}_2\mathrm{CO}_3) \).

This ability makes it amphoteric, playing a vital role in buffer systems like the blood, where it helps maintain pH balance.

Cyanide Ion

The cyanide ion \( (\mathrm{CN}^-) \), in contrast, mainly acts as a base. It can accept a proton to form hydrogen cyanide \( (\mathrm{HCN}) \),which is a simple acid-base reaction. However, because it lacks a proton to donate, it cannot function as an acid in the Bronsted-Lowry sense. Its behavior is a good illustration of substances that have a predominant single-role function in acid-base chemistry.

Carbonic Acid

Carbonic acid \( (\mathrm{H}_2\mathrm{CO}_3) \) predominantly acts as a Bronsted-Lowry acid. It has the capacity to donate protons readily, forming bicarbonate ion\( (\mathrm{HCO}_3^-) \) in the process. While it acts well as an acid, it generally doesn’t accept protons, so it isn’t recognized as a base under this theory. Its primary role involves balancing blood pH by donating protons when necessary to interact with bicarbonate ions.

Water as an Acid and Base

Water \( (\mathrm{H}_2\mathrm{O}) \) is a quintessential example of a compound that can behave both as a Bronsted-Lowry acid and base.

• As an acid: It donates a proton to become hydroxide ion\( (\mathrm{OH}^-) \).
• As a base: It accepts a proton to become hydronium ion\( (\mathrm{H}_3\mathrm{O}^+) \).

This property of water not only supports its role in acid-base chemistry but also in sustaining the dynamic equilibrium in various biological and chemical systems.