Reactivity with Hydrogen: The Hydrides
Master the critical trends of $EH_3$ compounds: Basicity, Stability, and Boiling Points.
All elements of Group 15 (Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth) react with hydrogen to form volatile, toxic, and foul-smelling gases known as Hydrides. They all share the general formula $EH_3$. The properties of these hydrides change drastically down the group, and these trends are absolute favorites for JEE and NEET examiners.
1. Structure and Bonding
The central atom in all these hydrides ($NH_3, PH_3, AsH_3, SbH_3, BiH_3$) undergoes $sp^3$ hybridization.
- Out of the four $sp^3$ hybrid orbitals, three are involved in forming covalent $\sigma$-bonds with hydrogen atoms.
- The fourth hybrid orbital contains a lone pair of electrons.
- Because of the lone pair-bond pair repulsion, the perfectly tetrahedral angle ($109.5^\circ$) is compressed, resulting in a Trigonal Pyramidal geometry.
The Bond Angle Trend
As we move down the group, the bond angle rapidly decreases from $107.8^\circ$ in $NH_3$ to almost $90^\circ$ in $BiH_3$.
Down the group, the electronegativity of the central atom decreases, and atomic size increases. The shared electron pairs in the E-H bonds are pulled further away from the central atom. As the bond pairs move further apart from each other, the repulsion between them decreases. This allows the dominant lone pair at the top to push the bonds closer together, significantly decreasing the bond angle.
2. Thermal Stability and Reducing Power
These two properties are inversely related. They both depend entirely on the strength of the $E-H$ bond.
Thermal stability decreases down the group.
Reason: As the size of the central atom increases down the group, the $E-H$ bond length increases. A longer bond is a weaker bond (bond dissociation enthalpy decreases). Therefore, it takes less heat to break $BiH_3$ apart than $NH_3$.
Reducing character increases down the group.
Reason: A good reducing agent easily gives away hydrogen. Because the $Bi-H$ bond is the weakest, it is broken most easily, releasing hydrogen rapidly. Thus, Bismuthine ($BiH_3$) is the strongest reducing agent.
3. Basic Character (Lewis Basicity)
All hydrides of Group 15 act as Lewis bases because they have a lone pair of electrons on the central atom that they can donate. However, their basic strength decreases down the group.
In Ammonia ($NH_3$), the Nitrogen atom is very small. The lone pair of electrons is concentrated in a tiny volume, making the electron density very high. This highly concentrated lone pair is easily available for donation to a proton ($H^+$).
As we move down the group to larger atoms like Antimony or Bismuth, the lone pair occupies a much larger orbital. The electron density is diffused over a vast volume. It is "spread too thin" and is therefore much less available for donation, making them very weak bases.
4. Boiling Point: The $NH_3$ Anomaly
The boiling point of covalent hydrides generally depends on van der Waals forces, which increase with molecular mass and size. Therefore, you would expect the boiling point to increase steadily down the group. But Ammonia breaks the rule.
Figure 1: Ammonia's boiling point is anomalously high. The correct order is $PH_3 < AsH_3 < NH_3 < SbH_3$.
Because Nitrogen is highly electronegative and very small, it is capable of forming strong intermolecular Hydrogen Bonds ($\cdots H-NH_2 \cdots H-NH_2 \cdots$).
Phosphorus and the heavier elements are neither electronegative enough nor small enough to form hydrogen bonds. They rely purely on van der Waals forces. Therefore, the boiling point drops sharply from $NH_3$ to $PH_3$, and then increases steadily to $SbH_3$ as molecular mass dictates.
Mastery Check: Pnictogen Hydrides
15 High-Yield Questions to test your JEE/NEET Preparation
🚀 Complete Guide to the p-Block
Master Inorganic Chemistry. Dive deep into the anomalies, trends, and specific compounds of Groups 15, 16, 17, and 18 for JEE Advanced, JEE Main and NEET.
Explore the p-Block Master Hub
No comments:
Post a Comment