Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

The strongest base is (a) \(\mathrm{NH}_{2}\) (b) \(\mathrm{PH}_{3}\) (c) \(\mathrm{AsH}_{3}\) (d) \(\mathrm{SbH}_{3}\)

Step-by-Step Solution

Answer

The strongest base is (a) \(\mathrm{NH}_2^-\).

Step 1: Understanding the Question

The question asks us to identify the strongest base among the given chemical species: \(\mathrm{NH}_2^-\), \(\mathrm{PH}_3\), \(\mathrm{AsH}_3\), and \(\mathrm{SbH}_3\). A base is a substance that can accept protons or donate a pair of valence electrons to form a bond. In this context, we are also considering their ability to donate a pair of electrons.

Step 2: Analyzing Electron Donating Ability

Among the given options, the electron donating ability typically decreases down the group in the periodic table. Therefore, we should consider the position of these elements in the periodic table. \(\mathrm{N}\), \(\mathrm{P}\), \(\mathrm{As}\), \(\mathrm{Sb}\) belong to the same group.

Step 3: Comparing Basicity

\(\mathrm{NH}_2^-\) is derived from ammonia and is an anion, meaning it has an extra electron, making it much stronger at donating electrons compared to the other neutral molecules \(\mathrm{PH}_3\), \(\mathrm{AsH}_3\), and \(\mathrm{SbH}_3\) which do not have this extra electron characteristic.

Step 4: Evaluating the Options

Since \(\mathrm{NH}_2^-\) has an additional pair of electrons (compared to \(\mathrm{PH}_3\), \(\mathrm{AsH}_3\), and \(\mathrm{SbH}_3\)), it has a higher tendency to donate those electrons, making it the strongest base among the options.

Key Concepts

Electron Donating Ability

In understanding the concept of a base, it's crucial to grasp how electron donating ability plays a significant role. A base is fundamentally about electron donation—providing a pair of electrons to accept a proton. This ability is strongly influenced by the electronic properties of the substance in question.

When a chemical species has a high electron donating ability, it becomes a strong base. This is because the ease with which it can share electrons to bond with a proton is greater. Among the options given in the exercise, \(\text{NH}_2^-\) stands out because it possesses an extra electron pair ready for donation.

• Electron donating ability is key in determining basic strength.
• Anions, such as \(\text{NH}_2^-\), typically donate electrons more readily due to additional electrons.
• Neutral molecules like \(\text{PH}_3\), \(\text{AsH}_3\), and \(\text{SbH}_3\) lack the extra electrons available in anions, hence reduced ability to donate.

Periodic Table Groups

The periodic table is organized into groups that reflect the properties of elements. Elements in the same group often exhibit similar chemical characteristics, yet these characteristics can vary in intensity depending on their position within the group.

In the context of the exercise, elements \(N\), \(P\), \(As\), and \(Sb\) are all in Group 15 of the periodic table. As we move down this group from nitrogen to antimony, a noticeable trend occurs—electron donating ability generally decreases because:

• The atomic radius increases, causing electrons to be held less tightly.
• The energy required to donate electrons rises as atoms become larger.
• Electron shielding effects increase, further reducing effective nuclear charge.

These factors combine to make \(\text{NH}_2^-\) a considerably stronger base compared to \(\text{PH}_3\), \(\text{AsH}_3\), and \(\text{SbH}_3\), given that nitrogen's smaller size enables it to effectively donate electron pairs.

Anion Characteristics

Anions are negatively charged ions, meaning they have more electrons than protons. This excess of electrons is crucial when discussing bases and their strength, as these extra electrons enable anions to act as strong electron pair donors. In the problem and its solution, \(\text{NH}_2^-\) is highlighted as an anion.

Characteristics of anions that affect their behavior as bases include:

• Additional electrons that contribute to higher electron density.
• Increased ability to share electrons due to their negative charge.
• Potentially higher basicity compared to neutral molecules.

Thus, the anionic nature of \(\text{NH}_2^-\) significantly contributes to its high basic strength, surpassing the other molecules listed in the exercise. Its ability to donate electrons readily makes it the strongest base in the given set of options.