Hydrides — Revision Notes

⚡ 30-Second Revision

Hydrides: — Binary compounds of hydrogen with other elements.
Types: — Ionic (saline), Covalent (molecular), Metallic (interstitial).
Ionic Hydrides: — Gr 1 & heavier Gr 2 metals. $H^-$ ion. Solid, high MP, non-conductor (solid), conductor (molten). Reacts vigorously with $H_2O$ ($MH + H_2O \rightarrow MOH + H_2$). Strong reducing agents. E.g., $NaH$, $CaH_2$.
Covalent Hydrides: — p-block elements. Covalent bonds. Volatile (gas/liquid), low MP/BP, non-conductor.

- Electron-deficient: Incomplete octet. Lewis acids. E.g., $B_2H_6$ (dimer of $BH_3$). - Electron-precise: Complete octet, no lone pairs. E.g., $CH_4$. - Electron-rich: Lone pairs on central atom. Lewis bases. Hydrogen bonding. E.g., $NH_3$, $H_2O$, $HF$.

Metallic Hydrides: — d-block & f-block elements. H in interstitial sites. Often non-stoichiometric (e.g., $TiH_{1.7}$). Retain metallic conductivity. Hydrogen storage. Hydride gap (Gr 7, 8, 9).
Hydrogen Bonding: — Strong intermolecular force in $NH_3$, $H_2O$, $HF$ leads to anomalously high boiling points.

2-Minute Revision

Hydrides are binary compounds of hydrogen, classified into three main types based on bonding and the nature of the other element. Ionic (Saline) Hydrides are formed by highly electropositive Group 1 and heavier Group 2 metals.

They contain the hydride ion ($H^-$), are solid, non-volatile, and conduct electricity only in the molten state. Their characteristic reaction is vigorous interaction with water to produce hydrogen gas ($H_2$).

Covalent (Molecular) Hydrides are formed by p-block elements and some s-block elements like Be and Mg. They involve covalent bonding and are typically volatile gases or liquids. They are further categorized: electron-deficient (e.

g., $B_2H_6$) with incomplete octets, electron-precise (e.g., $CH_4$) with complete octets and no lone pairs, and electron-rich (e.g., $NH_3$, $H_2O$, $HF$) with lone pairs on the central atom.

Electron-rich hydrides of N, O, F exhibit strong hydrogen bonding, leading to unusually high boiling points. Metallic (Interstitial) Hydrides are formed by many d-block and f-block elements, where hydrogen atoms occupy interstitial sites in the metal lattice.

They are often non-stoichiometric, retain metallic conductivity, and are explored for hydrogen storage. Notably, Group 7, 8, and 9 elements do not form hydrides, a phenomenon known as the 'hydride gap'.

5-Minute Revision

Let's quickly review the essential aspects of hydrides for NEET. Hydrides are binary compounds of hydrogen. Their classification is crucial:

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Ionic (Saline) Hydrides: — These are 'salt-like' compounds formed by Group 1 (alkali metals) and heavier Group 2 (alkaline earth metals like Ca, Sr, Ba). Here, hydrogen accepts an electron to form the hydride ion ($H^-$). They are crystalline solids with high melting points and are electrical insulators in the solid state but conductors when molten. A key reaction is their vigorous and exothermic reaction with water: $NaH(s) + H_2O(l) \rightarrow NaOH(aq) + H_2(g)$. This highlights their strong reducing nature.

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Covalent (Molecular) Hydrides: — Formed by p-block elements (Groups 13-17) and some s-block elements (Be, Mg). Bonding is covalent. They are generally volatile (gases or liquids) and non-conductors of electricity. They are sub-classified based on electron count around the central atom:

* Electron-deficient: Central atom has an incomplete octet (e.g., $BH_3$, which dimerizes to $B_2H_6$ with 3-center-2-electron bonds). They act as Lewis acids. * Electron-precise: Central atom has a complete octet with no lone pairs (e.

g., $CH_4$). * Electron-rich: Central atom has lone pairs (e.g., $NH_3$, $H_2O$, $HF$). These act as Lewis bases and, importantly, exhibit hydrogen bonding due to the high electronegativity of N, O, F.

This leads to anomalously high boiling points for $H_2O$, $HF$, and $NH_3$ compared to their heavier congeners. For example, $H_2O$ has a much higher boiling point than $H_2S$ due to extensive hydrogen bonding.

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Metallic (Interstitial) Hydrides: — Formed by many d-block and f-block elements. Hydrogen atoms occupy the interstitial sites (voids) within the metal lattice. They are often non-stoichiometric (e.g., $TiH_{1.7}$), meaning the hydrogen-to-metal ratio is not a simple integer. They retain metallic luster and electrical conductivity. These are important for hydrogen storage. Remember the 'hydride gap': Group 7, 8, and 9 elements (Mn, Fe, Co, Ni) generally do not form hydrides.

For NEET, focus on identifying the type of hydride, its key properties, the role of hydrogen bonding, the reactivity of ionic hydrides, and the concept of the hydride gap.

Prelims Revision Notes

Hydrides: Quick Recall for NEET

1. Definition: Binary compounds of hydrogen with other elements.

2. Classification:

* Ionic (Saline) Hydrides: * Elements: Group 1 (Li, Na, K, Rb, Cs) and heavier Group 2 (Ca, Sr, Ba) metals. * Bonding: Ionic, $M^+$ and $H^-$ ions. * Physical State: Crystalline solids.

* Conductivity: Insulators in solid state; conductors in molten state (due to mobile ions). * Reactivity with Water: Highly reactive, vigorous, exothermic. $MH(s) + H_2O(l) \rightarrow MOH(aq) + H_2(g)$.

* Reducing Nature: Strong reducing agents due to $H^-$. * Examples: $NaH$, $CaH_2$.

* Covalent (Molecular) Hydrides: * Elements: p-block elements (Gr 13-17) and some s-block (Be, Mg). * Bonding: Covalent (electron sharing). * Physical State: Generally volatile (gases/liquids), some solids.

* Conductivity: Non-conductors. * Sub-classification (based on electron count around central atom): * Electron-deficient: Incomplete octet. Lewis acids. E.g., $BH_3$ (dimerizes to $B_2H_6$ with 3c-2e bonds).

* Electron-precise: Complete octet, no lone pairs. E.g., $CH_4$, $SiH_4$. * Electron-rich: Lone pairs on central atom. Lewis bases. Exhibit hydrogen bonding. * Hydrogen Bonding: Occurs when H is bonded to highly electronegative N, O, F.

Leads to anomalously high boiling points for $NH_3$, $H_2O$, $HF$. Order of BP: $H_2O > HF > NH_3$ (due to extent of H-bonding and number of H-bond donor/acceptor sites). * Examples: $NH_3$, $H_2O$, $HF$.

* Metallic (Interstitial) Hydrides: * Elements: Many d-block and f-block elements (Gr 3, 4, 5, 6, 10, 11, 12, Lanthanides, Actinides). * Bonding: Hydrogen atoms occupy interstitial sites in metal lattice.

* Stoichiometry: Often non-stoichiometric (e.g., $TiH_{1.7}$, $PdH_{0.6-0.8}$). Variable composition. * Properties: Retain metallic luster and electrical conductivity (though sometimes reduced).

Lower density than parent metal. * Applications: Hydrogen storage materials. * Hydride Gap: Group 7, 8, 9 elements (Mn, Fe, Co, Ni) generally do not form hydrides under normal conditions.

3. Key Reactions:

* $NaH + H_2O \rightarrow NaOH + H_2$ * $CaH_2 + 2H_2O \rightarrow Ca(OH)_2 + 2H_2$

4. Important Trends/Anomalies:

* Boiling points of Group 15, 16, 17 hydrides: $NH_3$, $H_2O$, $HF$ are exceptionally high due to H-bonding. For other hydrides in these groups, BP increases with molecular mass.

Vyyuha Quick Recall

To remember the types of hydrides and their key features:

In Class My Hydrides Interact Coolly Metallically.

Ionic: Ions ($H^-$), Insulators (solid), Interacts (with water).
Covalent: Covalent bonds, Conductivity (none), Classified (electron-deficient/precise/rich).
Metallic: Metallic lattice, Many (d/f-block), Many non-stoichiometric.