Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of these can not be a bronsted acid? (a) \(\mathrm{HCO}_{3}^{-}\) (b) \(\mathrm{HCOO}^{-}\) (c) \(\mathrm{H}_{2} \mathrm{PO}_{3}^{-}\) (d) \(\mathrm{H}_{3} \mathrm{O}^{+}\)

Step-by-Step Solution

Answer

\( \mathrm{HCOO}^{-} \) is not a Bronsted acid.

Step 1: Define a Bronsted Acid

A Bronsted acid is a substance that can donate a proton (H\(^+\)) to another substance. It typically contains at least one hydrogen atom that can be readily released as a proton.

Step 2: Identify Proton Donors

Look at each option to determine if the compound can donate a hydrogen ion (H\(^+\)). A Bronsted acid must have a hydrogen atom that can be removed.

Step 3: Evaluate \\ \( \mathrm{HCO}_{3}^{-} \\)

The bicarbonate ion (\( \mathrm{HCO}_{3}^{-} \)) can donate a proton to form carbonate (\( \mathrm{CO}_{3}^{2-} \)), acting as a Bronsted acid.

Step 4: Evaluate \\ \( \mathrm{HCOO}^{-} \\)

Formate ion (\( \mathrm{HCOO}^{-} \)) is already a deprotonated form of formic acid and lacks an additional hydrogen to donate, hence it cannot act as a Bronsted acid.

Step 5: Evaluate \\ \( \mathrm{H}_{2} \mathrm{PO}_{3}^{-} \\)

Dihydrogen phosphite ion (\( \mathrm{H}_{2} \mathrm{PO}_{3}^{-} \)) has hydrogen atoms that can be donated as protons, allowing it to function as a Bronsted acid.

Step 6: Evaluate \\ \( \mathrm{H}_{3} \mathrm{O}^{+} \\)

Hydronium ion (\( \mathrm{H}_{3} \mathrm{O}^{+} \)) can donate a proton to form water (\( \mathrm{H}_{2} \mathrm{O} \)), making it a Bronsted acid.

Step 7: Determine the Non-Bronsted Acid

Based on the evaluation, \( \mathrm{HCOO}^{-} \) is the compound that cannot donate a proton and hence, cannot be a Bronsted acid.

Key Concepts

Proton Donor

In chemistry, a fundamental concept is the idea of a proton donor. According to the Bronsted-Lowry theory, a proton donor is essentially a Bronsted acid. This is a compound or molecule that can release a proton \( \mathrm{H}^+ \)to another molecule in a chemical reaction. Understanding this concept is crucial to identifying acids in chemical reactions. For a substance to be a proton donor, it must have at least one hydrogen atom that is loosely bound to it, allowing it to be easily released.
This hydrogen atom becomes a proton when it is lost from the molecule.
When a compound donates a proton, it is converted into its conjugate base. The role of a proton donor can be found across numerous reactions, including acid-base chemistry, where the transfer of protons determines the behavior and strength of an acid compared to a base.

• A proton donor always has hydrogen as part of its structure.
• Often, the ability to donate a proton depends on the stability of the resulting conjugate base.
• This concept explains why some substances act as acids in specific reactions.

Bicarbonate Ion

The bicarbonate ion \( \mathrm{HCO}_3^- \)is a fascinating species in chemistry known for its ability to act as both an acid and a base, which makes it amphiprotic. Its role as a proton donor confirms its ability to function as a Bronsted acid.
Bicarbonate can release a proton to form the carbonate ion \( \mathrm{CO}_{3}^{2-} \), a process that identifies it as an acid in acid-base reactions. This property is crucial, as bicarbonate acts as a buffer in solutions, helping to maintain pH levels by neutralizing excess acids or bases.

• In water, bicarbonate helps balance pH by donating protons.
• As a conjugate base of carbonic acid, it plays a vital role in biological systems, helping transport carbon dioxide.
• Its amphiprotic nature allows it to convert to either carbonate or carbonic acid, depending on the conditions.

Hydronium Ion

The hydronium ion \( \mathrm{H_{3}O}^{+} \)is a pivotal player in acid-base chemistry, evident in its common representation as the 'solution' form of protons in water. Functionally, it acts as a Bronsted acid by donating a proton to form water \( \mathrm{H_{2}O} \).
This ability makes it a critical component in the behavior of acids in aqueous solutions, where the level of hydronium ions determines the solution's pH.

In any acidic solution, the hydronium ion is a significant contributor to the acidity, representing the abundance of protons that have been donated by acids:

• Hydronium ions are generated when an acid dissolves in water.
• This ion is significant for measuring the acidity (or pH) of a solution.
• It's crucial in various biological and chemical processes, influencing how reactions occur in aqueous environments.

Examining the behavior of hydronium ions helps students understand how acids function in daily life and laboratory settings.