Chapter 9: Problem 5

Give reasons for the following: (a) \(\mathrm{H}^{-}\) is larger than He. (b) NaH is strongly hydrophillic but \(\mathrm{GeH}_{4}\) is weakly hydrophillic. (c) Acetylene reacts with active metals to liberate hydrogen. (d) Ionic hydrides are more dense than the parent metal while interstitial hydrides are less dense than the parent metal. (e) Palladium is used as a catalyst for hydrogenation.

Short Answer

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In short, the hydrogen ion has a larger radius than helium due to its extra electron. NaH is strongly hydrophilic due to its ionic nature, while GeH4 is weakly hydrophilic due to its covalent nature. Acetylene reacts with active metals to liberate hydrogen due to its triple bond. The density of hydrides depends on whether they are ionic or interstitial. Palladium is catalytic for hydrogenation due to its ability to absorb hydrogen.

Step by step solution

Answer (a)

Helium (He) has a filled shell, meaning it is stable and does not require any more electrons. Thus, the electron cloud does not extend far from the nucleus, making He smaller. On the other hand, hydride ion, \(\mathrm{H}^{-}\), has an extra electron, which extends its electron cloud, making \(\mathrm{H}^{-}\) larger than He.

Answer (b)

NaH, being an ionic compound, can interact with polar water molecules effectively and it can dissociate in water due to the electrostatic attraction between polar water molecules and ions of NaH. This explains why it is strongly hydrophilic. But \(\mathrm{GeH}_{4}\), being a covalent compound, cannot form strong electrostatic interactions with water molecules thus making it weakly hydrophilic.

Answer (c)

Acetylene has a triple bond between the two carbon atoms. Active metals, such as potassium or sodium, can break these triple bonds by donating electrons, which results in the liberation of hydrogen.

Answer (d)

Ionic hydrides are formed with more electropositive elements, leading to the incorporation of hydrogen ions into the lattice structure of the metal, thus increasing the overall density. However, interstitial hydrides involve the introduction of hydrogen into the interstices of a metal lattice that tend to expand the metal lattice, leading to a decrease in density.

Answer (e)

Palladium has the unique ability to absorb a high volume of hydrogen, thus making it an excellent catalyst for reactions involving hydrogen, like hydrogenation. The absorption of hydrogen by Palladium weakens the H-H bonds, allowing them to readily react with other compounds.

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