Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the order of basic nature of hydrides of group V elements? (a) \(\mathrm{AsH}_{3}>\mathrm{SbH}_{3}>\mathrm{PH}_{3}>\mathrm{NH}_{3}\) (b) \(\mathrm{NH}_{3}^{3}>\mathrm{SbH}_{3}^{3}>\mathrm{PH}_{3}^{3}>\mathrm{AsH}_{3}^{3}\) (c) \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}\) (d) \(\mathrm{PH}_{3}>\mathrm{NH}_{3}>\mathrm{SbH}_{3}>\mathrm{AsH}_{3}\)

Step-by-Step Solution

Answer

The correct order is (c) 7{NH}{3} > 7{PH}{3} > 7{AsH}{3} > 7{SbH}{3}.

Step 1: Understanding the Problem

The question asks about the order of basicity for hydrides of group V elements. These elements include nitrogen (N), phosphorus (P), arsenic (As), and antimony (Sb) with their hydrides being ammonia (7{NH}{3}), phosphine (7{PH}{3}), arsine (7{AsH}{3}), and stibine (7{SbH}{3}) respectively. The basic nature refers to the ability of these hydrides to donate a pair of electrons.

Step 2: Key Concept: Electronegativity and Basicity

The basicity of these compounds is influenced by the electronegativity of the central element. Higher electronegativity means a greater ability to hold on to the lone pair of electrons, hence more basic nature. As you move down the group, electronegativity decreases (N > P > As > Sb). This typically means that the higher the electronegativity, the higher the basicity.

Step 3: Reviewing the Available Options

Now, compare the theoretical knowledge with the given options. We expect the basicity to decrease as we move down the group from NH73 to SbH73, as nitrogen is the most electronegative, making ammonia (7{NH}{3}) the most basic. Therefore, the correct order is usually NH73 > PH73 > AsH73 > SbH73.

Step 4: Matching the Correct Option

From the options given, (c) 7{NH}{3} > 7{PH}{3} > 7{AsH}{3} > 7{SbH}{3} correctly reflects the order of decreasing basicity according to the trend in electronegativity and corresponds with our theoretical expectations.

Key Concepts

Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. In a straightforward sense, this property describes how "greedy" an atom is for electrons. When we talk about hydrides, which are compounds formed between hydrogen and another element, the central atom's electronegativity plays a significant role in defining the compound's characteristics.
- Higher electronegativity often means that the atom can tightly hold onto its electrons.
- For group V elements, as you move down the group, electronegativity decreases.
- Hence, nitrogen (N), being at the top, has the highest electronegativity among nitrogen, phosphorus (P), arsenic (As), and antimony (Sb).
This higher electronegativity affects the basicity of the hydrides. Ammonia (NH₃), with nitrogen at its center, can better hold onto and donate its electron pairs compared to the other, less electronegative elements.

Ammonia Basicity

Ammonia ( NH₃) serves as a classic example of a basic hydride, largely due to its ability to donate a lone pair of electrons. In chemistry, a basic substance is one that can accept protons (+) or donate electrons to form a new chemical bond.
- The nitrogen atom in ammonia possesses a lone pair that is ready to interact, making it highly effective at forming covalent bonds.
- This bond formation is particularly possible because nitrogen's high electronegativity allows it to maintain a strong hold on its electrons.
As a comparison, if we look at phosphine ( PH₃) or arsine ( AsH₃), the central atoms in these molecules are not as capable of holding onto their electron pairs due to lower electronegativity. Thus, ammonia remains the most basic among the group V hydrides, illustrating the importance of electronegativity in influencing basicity.

Hydride Chemistry

Hydrides are compounds formed when elements combine with hydrogen. For group V hydrides like ammonia ( NH₃), phosphine ( PH₃), arsine ( AsH₃), and stibine ( SbH₃), the variation in chemical behavior primarily depends on the central element.
- The basicity of these hydrides—essentially how readily they can donate a lone pair of electrons—decreases as one moves down the group.
- This is because the atoms become larger and less electronegative as you descend the group, making their hold on the electron pair weaker.
- Consequently, while ammonia is extremely effective as a base, emitting a strong 'fishy' odor thanks to its high basicity, arsenic and antimony's hydrides are considerably less basic.
The progression illustrates fundamental chemical principles where atomic properties such as size and electronegativity come into play, impacting how these hydrides behave in various chemical reactions.