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CHMB31H3 (8)
Chapter 10

CHMB31 Chapter 10

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Alen Hadzovic

Chapter 10: Hydrogen The Essentials • Hydrogen ranges in character from being a strong Lewis base to being a strong Lewis acid. • Hydrogen atoms can form bonds to more than one other atom simultaneously. • The proton, H+, is always found in combination with a Lewis base and is highly polarizing; the hydride ion, H-, is highly polarizable. o 3 Isotopes of Hydrogen: hydrogen (1H), deuterium (D, 2H) and tritium (T, 3H). o The hydride ion (H-) is highly polarizable because two electrons are bound by just one proton. • Hydrogen has unique atomic properties that place it in a special position in the periodic table; dihydrogen is an inert molecule and its reactions require a catalyst or initiation by radicals. o Hydrogen is placed at the head of Group 1 because it has only one electron in its valence shell, like the alkali metals. o Hydrogen requires only one electron to complete its valence shell, like the halogens. o The H2 molecule has a high bond enthalpy or strength (inert molecule) and a short bond length. o Hydrogen has a high ionization energy and a low but positive electron affinity. o It is normally assigned the oxidation number -1 when combined with metals and +1 when combined with nonmetals). • Compounds formed between hydrogen and other elements vary in their nature and stability. • In combination with metals, hydrogen is often regarded as a hydride; hydrogen compounds with elements of similar electronegativity has low polarity. o Molecular Hydrides: exist as individual, discrete molecules; they are usually formed with p-block elements of similar or higher electronegativity than H (covalent E-H bonds) like CH4, NH3 and H2O. o Saline Hydrides: known as ionic hydrides are formed with the most electropositive elements like LiH and CaH2. o Metallic Hydrides: non-stoichiometric, electrically conducting solids with a metallic luster; it is formed with many d- and f-block elements. • In the s and p blocks, strengths of E-H bonds decrease down each group but in the d block, strengths of E-H bonds increase down each group. o All s-block hydrides are exergonic (Δ Gf° < 0) except for BeH2, so they are thermodynamically stable with respect to their elements at room temperature. o For Group 13, AlH3 is the only exergonic at room temperature. o For the rest of the p block, the simple hydrogen compounds of the first members of the groups are exergonic but become progressively unstable down the group. • The reactions of binary compounds of hydrogen fall into three classes, depending on the polarity of the E-H bond. o If E and H have similar electronegativities, cleavage of the E-H bond tends to be hemolytic, producing, intitially, an H atom and a radical, each of which can go on to combine with other available radicals. o If E is more electronegative than H, heterolytic cleavage occurs, releasing a proton (H atom is protonic) = E-H  E- + H+. o If E is less electronegative than H, heterolytic bond cleavage also occurs but an H- ion is transferred to a Lewis acid (H atom is hydridic) = E-H  E+ + H-. The Detail • Three hydrogen isotopes H, D and T have large differences in their atomic masses and different nuclear spins, which give rise to easily observed changes
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