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10.1: Hydrogen - The Simplest Atom

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    34043
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    • Hydrogen (not carbon) forms more compounds than any other element! The isotopes are:
      • 1H
      • 2D (0.0156%)
      • 3T (Formation: 14N(p,n)14O and then (using the neutron) 14N(n,3H)12C
        Abundance: 10-15 - 10-16 %, Decay: 3H(b-)3He t½ = 12.35 y.
    • The different name arise because isotope effects, which affect kinetics and equilibria, are especially marked for hydrogen.
    • Deuterium is obtained by fractional distillation of water (or hydrogen sulphide?) or electrolysis of water. D2O is used as a moderator in nuclear reacters and as a source of D for chemical studies of all sorts.
    • Made in the lab from the action of acid on Zn or Fe and industrially by the catalysed reaction of steam with hydrocarbons, coal and other organic materials to give "synthesis gas, "syngas":
      CH4 + H2O alt CO + H2
      H2 can also be made by the "water-gas reaction" and the catalysed "water-gas shift reaction":
      H2O + CO alt CO + H2

      CO + H2O alt CO2 + H2

      The hydrogen can be isolated by absorption of the carbon dioxide, and removal of residual CO and CO2. (The "syngas" is an important industrial raw material itself.)
    • It, and "syngas" are used in the production of organics via alcohols:
      2CO + 4H2 alt CH3CH2OH + H2O
      and the production of ammonia via the Haber process:
      N2 + 3H2 alt 2NH3
    • Hydrogen has a bond energy of 434.1 kJ mol-1. It will burn in air but reacts explosively with oxygen and some halogens via chain reactions:
      Br2 + light alt 2Br chain initiation

      Br + H2 alt HBr + H chain propagation

      H + Br2 alt HBr + Br

      H + HBr alt H2 + Br

      2Br alt Br2 chain termination

      Notice there is no direct H-H bond fission in this processs.

      The Bonding of Hydrogen

      As mentioned before, hydrogen is found bonded in two covalent situations:
      1. The complexed proton, e.g. H3O+ or NH4+, and "conventional" covalent situations, e.g. CH4. The distinction between these two is rather artificial, except that the cations would have a rather more polarized and labile X-H bond, and are prone to transferring the proton to another molecule, i.e. behaviour as an acid.
      2. Hydrides such as KH, which contain the H- ion.

      It has some special ways of bonding as well:

      1. The formation of metallic hydrides, that is, hydrides which have metallic properties as apposed to ionic H- containing materials, for example PdH0.4-0.7.

        Note that the density of Pd is = 11.99 g cm-3 or 11.99/106.42 = 0.113 mol cm-3

        so in PdH0.7 the density of hydrogen is 0.113 x 0.7 = 0.0789 mol cm-3 or 0.0789 x 1.008 = 0.0790 g cm-3

        i.e. more than liquid hydrogen where the density (at -252.78 oC) is 0.07099 g cm-3!

        The form of the hydrogen in metals is ambiguous: it tends to migrate towards the negative end of a potential gradient in wires, so it seems proton-like. On the other hand, salt like properties and M-H bond distances in some metal hydrides are more suggestive of H- compounds. (Perhaps only a small part of the hydrogen is cationic in nature.)

      2. The formation of covalent hydrogen bridges for example in B2H6 and (CO)5CrHCr(CO)5. These are prototypes for 2e- - 3-centre bonding.
      3. A newish class of hydrogen bonds called "agostic" have been identified in certain transition metal compounds which seem to be a sort of frozen intermediate in the catalytic activation of C__H bonds. They come as "open" and "closed": alt
      4. So called "hydrogen bonding" described in the following section.

      The Hydrogen Bond

      • When hydrogen is bonded to an electronegative element, X, usually F, O, N or Cl, and there is another molecule around with a Lewis base donor atom,Y , a very strong largely electrostatic bond can form: Xd-__Hd+........Y The X__H bond is slightly longer than it would be without the proximity of the Y, and the H........Y "bond" is much longer than a H__Y bond would be. Usually, hydrogen bonding is identified if this distance appreciably less than the sum of the van der Waals' radii for H and Y (e.g. <(1.30 + 1.40) = 2.70 Å for H........O. In many cases the position of the hydrogen must be inferred: in this case O__H should be about 0.37 + 0.70 = 1.07 Å so an O to O distance less than about 2.70 + 1.07 = 3.75 Å would imply a hydrogen bond is present.)
      • The text refers to the case of crystalline NaHCO3 where there are 4 O to O distances: 3.12, 3.15 and 3.19 Å which are well over the van der Waals' contact distance, and 2.55 Å which is somewhat less, and tells us where the H is to be found.
      • The hydrogen bond can also be identified by a shift in the infra-red stretching frequency to lower wavelengths. It is also broadened and made more intense. For example, free O__H comes at 3500 cm-1 but can be lowered several hundred wave-numbers by H-bonding.
      • The table below gives some approximate hyhdrogen bond enthalpies:
        Bond Energy kJ mol-1
        F__H........F 30
        N__H........N 25
        O__H........O 25
        N__H........F 21
        O__H........N 20
        C__H........O 11
        N__H........O 10
      • Hydrogen bonding affects boiling points (and heats of vaporization) producing some anomalies. See text Figure 9-1.
      • There are specially stong hydrogen bonds that are 4e- - 3-centre bonds. Examples include [FHF]- with a centered proton and F to F distance of 2.26 Å.

      Ice and Water

      • There are 9 forms of ice which exist at high presssure, except the normal ice I.
      • The normal hexagonal ice structure is depicted in Fig 9-2.
      • In liquid water, an ice-like structure persists, but there are additional "interstitial" water molecules, and the whole system is "fluxional". The density of liquid water is greater than that of ice. The density is maximum at 4 oC.

      Hydrates and Water Clathrates

      Most hydrates are salts containing water in addition to the cations and anions. The water is sometimes in the first coodination sphere of the cations and is sometimes rather more loosely held:

      CuSO4.5H2O alt CuSO4.3H2O alt CuSO4.H2O alt CuSO4 anhydrous

      The water molecules are successively more difficult to remove, as the sulphate takes their place in the Cu2+ coordination sphere.

      Sometimes the water is so tightly bound, a decomposition occurs on heating:

      ScCl3.6H2O alt ScOCl + 2HCl + 5H2O

      "Gas hydrates" are an example of a class of "clathrate" compounds:

      1. There is a cubic symmetry form that features a 46-water molecule unit cell including six medium sized and two small cages. With the medium-sized cages trapping a "guest" molecule each the formula would be X.(7.67)H2O. All eight cages filled would give X.(5.76)H2O. Clathrates of the first type are known for Ar, Kr Xe, Cl2, SO2 and CH3Cl (among others).
      2. Another geometry, also cubic has 136 H2O's in the unit cell and features 8 larger cages and 16 smaller ones. Molcules such as CHCl3 and CH3CH2Cl can be trapped.
      3. One other class made with the salts of R4N+ or R3S+ have the anions acting as part of the "host" structure. Examples are: [(C4H9)4N][C6H5CO2].(39.5)H2O or [(C4H9)3S]F.(20)H2O.

      Hydrides

      Many hydrogen compounds are collectively called hydrides, including many cases where the hydrogen is actually less electronegative than the atom to which it is bound, and which are not individually named hydrides, in addition to those where it is more electronegative, upto the saline hydrides containing H-. In addition, there is the class of metallic hydrides, See Figure 9-4:

      Covalent

      1. Neutral - Group IVB (14) CH4 (methane), SiH4 (silane) etc
      2. Somewhat basic - Group VB (15) NH3, PH3 (phosphine) etc
      3. Weakly acidic - Group VIB (16) H2O, H2S (hydrogen sulphide) etc
      4. Strongly acidic - Group VIIB (17) HCl, HI (hydrogen halides) etc
      5. The boron hydrides (BnHm)
      6. Hydride anions eg LiAlH4 (alanate), LiBH4 (borohydride or boranate)

        Preparation:

        (8LiH + Al2Cl6 alt 2LiAlH4 + 6LiCl)

        (2NaH + B2H6 alt 2NaBh4)

        Typical Reactions:

        2LiAlH4 + 2SiCl4 alt 2SiH4 + 2LiCl + Al2Cl6

        I2 + LiBH4 alt B2H6 + 2NaI + H2

      Saline

      Some are made by direct reaction: M + H2 alt MH (Li - Cs)

      or MH2 (Mg - Ba)

      The hydrides of Li and Be have covalent character, especially BeH2 which is really a polymer. Most saline hydrides react violently with water to give H2

      They are also powerful hydride transfer reagents, sometimes useful for making other hydrides:

      B(OR)3 + NaH alt Na[BH(OR)3]

      4NaH + TiCl4 alt Tio + 4NaCl + 2H2

      NaH + ROH alt NaOR + H2

      Transition metal

      These include the stoichiometric hydrides, for example UH3 and HCo(CO)4 and hydride anions, for example [ReH9]2-.

      There are also the non-stoichiometric (interstitial) ones including PdH0.7 and ZrH1.9.

      Dihydrogen as a Ligand

      Dihydrogen complexes, where the hydrogen molecule is bonded sideways on to the metal, but the H-H bond is largely intact have been discovered only recently. The metal accepts electrons from the H-H s-orbital, and donates electrons back to the H-H s*-orbital. Both types of bonding should lead to weakening and ultimate cleavage of the H-H bond to give normal hydrides, and only very a few special cases short of this. An example is:

      altW(CO)3P(Pri3)2 + H2 alt W(CO)3P(Pri3)2(H2)

      H-H by neutron diffraction = 0.75 Å, by X-ray diffraction = 0.84 Å and in H2 = 0.74 Å

    Contributors and Attributions

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    10.1: Hydrogen - The Simplest Atom is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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