Q: 7. Discuss the consequences of high enthalpy of H-H bond in terms of chemical reactivity of dihydrogen.
The ionization enthalpy of H-H bond is very high . This indicates that hydrogen has a low tendency to form ions. Its ionization enthalpy value is comparable to that of halogens. Hence, it forms diatomic molecules (), hydrides with elements, and a large number of covalent bonds.
Since ionization enthalpy is very high, hydrogen does not possess metallic characteristics (lustre, ductility, etc.) like metals.
Q: 8. What do you understand by
(ii) Electron-precise, and
(iii) Electron- rich compounds of hydrogen? Provide justification with suitable examples.
Molecular hydrides are classified on the basis of the presence of the total number of
electrons and bonds in their Lewis structures as:
1. Electron-deficient hydrides
2. Electron-precise hydrides
3. Electron-rich hydrides
An electron-deficient hydride has very few electrons, less than that required for
representing its conventional Lewis structure e.g. diborane In , there are six bonds in all, out of which only four bonds are regular two centered-two electron bonds. The remaining two bonds are three centered-two electron bonds i.e., two electrons are shared by three atoms. Hence, its conventional Lewis structure cannot be drawn. An electron-precise hydride has a sufficient number of electrons to be represented by its conventional Lewis structure e.g. . The Lewis structure can be written as:
Four regular bonds are formed where two electrons are shared by two atoms. An electron- rich hydride contains excess electrons are lone pairs e.g. .
There are three regular bonds in all with a lone pair of electrons on the nitrogen atom.
Q: 9. What characteristics do you expect from an electron- deficient hydride with respect to its structure and chemical reactions?
An electron – deficient hydride does not have sufficient electrons to form a regular bond in which two electrons are shared by two atoms e.g., etc.
These hydrides cannot be represented by conventional Lewis structures. for example, contains four regular bonds and two three centered-two electron bond. Its structure can be represented as:
Since these hydrides are electron – deficient, they have a tendency to accept electrons. Hence, they act as Lewis acids.