Problem 138 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following substances has the highest proton affinity? [2003] (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{PH}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{~S}\)

Step-by-Step Solution

Answer

Ammonia ( NH_{3} ) has the highest proton affinity.

Step 1: Understanding Proton Affinity

Proton affinity refers to the tendency of a substance to accept a proton. It is related to the basicity of the substance. A molecule with a high proton affinity readily gains a proton in comparison to others.

Step 2: Comparing Nitrogen and Phosphorus

Ammonia ( NH_{3} ) and phosphine ( PH_{3} ) have nitrogen and phosphorus respectively. Nitrogen is more electronegative than phosphorus, which means NH_{3} has a stronger ability to attract and hold a proton than PH_{3} .

Step 3: Comparing Oxygen and Sulfur

Water ( H_{2}O ) and hydrogen sulfide ( H_{2}S ) have oxygen and sulfur respectively. Oxygen is more electronegative than sulfur, giving H_{2}O a higher proton affinity than H_{2}S .

Step 4: Final Comparison Across Options

Among NH_{3} , H_{2}O , PH_{3} , and H_{2}S , ammonia ( NH_{3} ) is known to have the highest proton affinity because nitrogen's lone pair is less shielded and more available for protonation compared to oxygen in H_{2}O and the other options.

Key Concepts

Basicity

Basicity refers to the ability of a substance to accept protons. It's a fundamental concept in chemistry that links to how a substance behaves as a base. A higher basicity indicates a greater tendency to gain protons, leading to a higher proton affinity as well.
Proton affinity is a way to measure basicity, where substances with high proton affinity are considered strong bases.

• Basicity is determined by the availability of a lone pair of electrons to bond with a proton.
• The strength of the conjugate acid formed after a proton is accepted also influences basicity.

Molecules like ammonia ( NH_3 ) showcase high basicity because their lone pairs are more accessible, making them efficient proton acceptors in chemical reactions.

Ammonia

Ammonia, chemically denoted as NH_3 , is a classic example of a basic substance. Its molecular structure includes a nitrogen atom bound to three hydrogen atoms with a lone pair of electrons.
Here are some key features of ammonia that contribute to its basicity and proton affinity:

• The lone pair of electrons on nitrogen is readily available to pair with an incoming proton.
• This molecule showcases a trigonal pyramidal shape, which means the lone pair is less hindered and more accessible.

Ammonia's high electronegativity and its receptive lone pair make it a prime candidate for high proton affinity compared to other molecules like water or phosphine. This explains why NH_3 often holds a key role in reactions involving proton transfer, acting as a proton acceptor.

Electronegativity

Electronegativity is a chemical property that describes the ability of an atom to attract and bind electrons. It plays a significant role in determining the basicity and proton affinity of a molecule.

• A high electronegativity means that an atom strongly attracts electrons towards itself.
• In terms of proton affinity, an electronegative atom can hold its lone pair tightly, making them available for bonding with protons.

Nitrogen, with an electronegativity of 3.0 on the Pauling scale, is more electronegative than phosphorus, influencing the proton affinity in NH_3 over PH_3 . This greater electronegativity allows nitrogen to grip its lone pair closer to its nucleus, thereby enhancing its ability to accept a proton.

Nitrogen vs Phosphorus

When comparing nitrogen and phosphorus, it's important to consider their positions in the periodic table and their chemical properties. Nitrogen is in the second period, whereas phosphorus resides in the third.
Here's how these differences affect proton affinity:

• Nitrogen, being more electronegative, attracts and holds electrons more effectively than phosphorus.
• The lone pair in nitrogen is less shielded, increasing its availability for reaction with protons.

While PH_3 (phosphine) has a similar shape to NH_3 , its phosphorus atom doesn't attract electrons with the same strength as nitrogen, which results in a lower proton affinity. This difference explains why ammonia is generally more basic and has a stronger proton affinity compared to phosphine, making nitrogen a more effective acceptor of protons.