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Hydrogen

Hydrogen. Chemistry compounds. Hydrogen. Occurrence Most abundant element in the Universe 3rd most abundant element in the Earth's crust, found in minerals, oceans and all living things Hydrogen is a unique element, does not belong to any groups in the periodic table.

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Hydrogen

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  1. Hydrogen Chemistry compounds

  2. Hydrogen • Occurrence • Most abundant element in the Universe • 3rd most abundant element in the Earth's crust, found in minerals, oceans and all living things • Hydrogen is a unique element, does not belong to any groups in the periodic table

  3. In the Periodic Table: Positioned by resemblance to alkali metals “Compromise” Position Different Periodic Tables use different positions for H Positioned by resemblance to halogens

  4. Isotopes of Hydrogen • Hydrogen has three isotopes: • protium (H) • deuterium (D): about 0.0156 % natural abundance. • tritium (T): occurs in the upper atmosphere, radioactive, half-life 12.26 years, used in tracer studies

  5. Isotopes of Hydrogen • The three H isotopes have different nuclear spin which give rise to easily observed changes in IR, Raman and NMR spectra of molecules containing these isotopes.

  6. Isotopes of Hydrogen • Because the relative mass differences between H's isotopes are so large, there is a significant dissimilarity in physical properties • Heavy water D2O: can be separated from H2O by distillation.

  7. Kinetic Isotope Effect • Bond dissociation energy of D2 is about 7 kJ/mol greater than H2 • Therefore reaction rates are often measurably different for processes in which E-H and E-D bonds are broken, made or rearranged • For C-H vs C-D bonds, the kH/kD can be as high as about 7. This is usually referred as the kinetic isotope effect • supporting a proposed mechanism

  8. Properties of Dihydrogen • Dihydrogen is a colorless gas at room temperature • Boiling point -253 °C • Melting Point -259 °C • Bond Dissociation Energy 436 kJ/mol • The H-H bond is stronger than the bonds hydrogen has with most other non-metals (e.g., H-N: 390 kJ/mol)

  9. Properties of Dihydrogen • Bond Length 0.74 Å • Electronegativity 2.2 • Similar to B, C, Si • Therefore E-H bonds involving these elements are not expected to be polar

  10. Uses of Hydrogen • Manufacture NH3, CH3OH, HCN, HCl • Saturation of fats • Cutting/welding • Rocket fuel • Heating fuel • Reducing agent for producing metals • Tritium is used in the production of the hydrogen bomb

  11. Synthesis of H2 • Reduction of H2O over coke:H2O + C  H2 +COH2O + CO  H2 +CO2(How would H2 +CO2 be separated?) • Electrolysis:2NaCl + 2H2O  2NaOH + H2 + Cl2 • Thermal cracking of hydrocarbons:CnH2n+2 CnH2n + H2CnH2n+2 Cn-1H2n + H2 + C

  12. Lab Synthesis of Hydrogen • Electrolysis of 30% KOH (Ni electrodes) • Dissolve Zn in acid2H+ + Zn  H2 + Zn2+ • Purification: diffuse through heated Ni or Pd tubes

  13. Chemical reactions • Reaction of hydrogen with air • Mixtures of hydrogen gas and air do not react unless ignited with a flame or spark 2H2(g) + O2(g)  2H2O(l) • Reaction of hydrogen with water • Hydrogen does not react with water. It does, however, dissolve to the extent of about 0.00160 g kg-1 at 20°C (297 K) and 1 atmosphere pressure.

  14. Chemical reactions • Reaction of hydrogen with the halogens • Hydrogen gas, H2, reacts with fluorine, F2, in the dark to form hydrogen(I) fluoride. H2(g) + F2(g)  2HF(g)

  15. Elemental H • H2 2H by light: high dissociation energy • H2 2H by microwaves: >90% • H + H + M  H2 + M, where M is a dissociation energy-absorber • H is VERY reactive- reduce oxides to the element: SO2, CuO, PbO, Bi2O3, SnO2- lower oxidation states: NO2  NO

  16. H in Chain Reactions e.g., detonation of H2, O2 mix: • Initiation H2 2H H + O2  OH + O OH + H2 H2O + H • Termination OH + H + M  H2O + M O + H2 + M  H2O + M 2 OH + M  H2O2 + M

  17. Hydronium Ion • M(g)  M+(g) + e- • I = 1311 (H); 520 (Li); 490 (Na) kJ/mol • H+ (g) does not exist under ordinary conditions • R : H+ 10-5Å Li+ 0.9 Å Na+ 1.2 Å • Due to its large charge/size ratio, H+ has a high polarizing power:

  18. Hydronium Ion • In solutions, it is solvated by solvent molecules • In aqueous solution, exits as [H(H2O)n]+ (aq) • In liquid ammonia, form NH4+

  19. + H – O – H … O – H … O – H … O – H H H H H H3O+ + H2O H2O + H3O+ Rapid Exchange H3O+ t½  10-13 s  rapid exchange equilibrium Easy charge transport: Can OH- ions migrate? If so, how?

  20. Acids • Arrhenius: [H+]>[OH-] (after dissolving)H2SO4 + H2O  H3O+ + HSO4-Cl2O7 + 3 H2O  2 H3O+ + 2 ClO4- • Other examples: N2O5, P4O10, SO2, CO2, SeO2(nonmetal oxides tend to be acidic)

  21. Salts • Acids may form crystalline hydrates:HA . nH2O – really salts • Examples:HX . H2O [H3O]X X = F, Cl, Br, IHCl . 2H2O [H5O2]ClHCl . 3H2O [H5O2]Cl . H2OHBr . 4H2O [H7O3][H9O4]Br2. H2OHClO4. H2O [H3O]ClO4HClO4. 2H2O [H5O2]ClO4HClO4. 3H2O [H7O3]ClO4

  22. H O – H – O H H H H5O2+ 2.42 Å – small & symmetrical What might H9O4+ look like? How might we know that these substances exist as salts? Look in IR for characteristic absorptions These salts completely ionize in H2O

  23. Anhydrous H2SO4 • Usually a protonating agent, but can be protonated by superacids • E.g., HSO3F, HSO3CF3, H2S2O7HSO3F + H2SO4 H3SO4+ + SO3F-(note: H2SO4 acts as the base!)

  24. Hydrides • Covalent: e.g., H2O, B2H6, ReH92- • Salt-like: NaH, CaH2 (ionic) • Metallic: PdHn, UH3

  25. * figures are electronegativity

  26. Salt-like Hydrides • 2 M + H2  2 MH (exothermic)only LiH can be melted w/o decomposition • Ternary compounds possiblee.g., CaH2 + CaCl2  2 CaHClcolors deepen as polarizability of anions & cations increases: BaHI is black

  27. Hydride Ion • ½ X2(g) + e- -------> X- (g) • E = +145 (H2); -249 (Cl2); -228 (F2) kJ/mol • Only most electropositive metals (e.g. Group 1 and 2 metals) can stabilize H- in an ionic lattice

  28. Hydride Ion • Ionic Radii: • H- 1.30 Å (in LiH) ~ 1.54 Å (in CsH) F- 1.33 Å Cl- 1.67 Å • H-: low charge/size ratio; easily polarizable (i.e. it can easily distorted by any nearby cation) • Strong basic character, reacts violently with H+ source

  29. Ionic hydride preparation and structure Typically these compounds are prepared by direct interaction with the metals at 300-700oC: 2 M(l) + H2(g) 2MH(s) M(l) + H2(g) MH2(s) The rates of these reactions are Li> Cs> K> Na All produce pure white solids that appear grey when impure.

  30. Ionic hydride preparation and structure • Crystal Structure • Knowing that the ionic radius of H- is between that of Cl- and F- • What do you think about the crystal structure of alkali metal hydrides and LiH and CsH?

  31. CaH2 SrH2 BaH2 Crystal Structures of Metal hydrides Alkali metal hydrides and LiH and CsH take on the NaCl Structure. What about others? MgH2 adopts the rutile structure All take on a PbCl2-like distorted hcp array.

  32. Reactions of Ionic metal hydrides • All thermally decompose to give metal and hydrogen. Only LiH is stable to its melting point of 688 oC. • Note that LiH is unreactive at moderate temperatures toward oxygen and chlorine. • Generally ionic hydrides are highly reactive toward air and water. MH(s) + H2O H2(g) + MOH(s) MH2(s) + H2O H2(g) + MOH2(s)

  33. Reactions of Ionic metal hydrides • Ionic hydrides are powerful reducing agents and good hydrogen-transfer agents NaH + B(OCH3)3 Na[HB(OCH3)3] NaH + TiCl4 Ti0 + 4NaCl +2H2

  34. Covalent hydrides • Covalent hydrides: • Neutral binary XH4 compounds of Group 14, like methane • Slightly basic binary XH3 compounds of Group 15, NH3 and PH3 • Weakly acidic or amphoteric, binary XH2 of Group 16, H2O and H2S • Strongly acidic binary HX compounds of Group 17, HCl and HI

  35. Covalent hydrides • Covalent hydrides (continue): • Covalent hydrides of boron • Hydridic complex compounds of hydrogen. • Examples are LiAlH4 and NaBH4. • Notes: • ionic in nature • BUT possess tetrahedral anions containing covalent bonds to H • powerful reducing agents

  36. 8LiH + Al2Cl6 2LiAlH4 + 6 LiCl 2 NaH + B2H6 2NaBH4 2LiAlH4 + 2 SiCl4 2SiH4 + 2 LiCl + Al2Cl6 I2 + 2 NaBH4 B2H6 + 2NaI + H2 How are LiAlH4 and NaBH4 prepared? Both these reactions are carried out in ether. The anions are powerful hydrogen transfer agents.

  37. Some more details about covalent hydrides Covalent hydrides can be divided into three subcategories which are reliant on the nature of the H atom. • The H-atom is neutral. • The H-atom is positive. • The H-atom is negative. These are a generalization of the statements that we made before.

  38. Some more details about covalent hydrides • By far the majority of covalent hydrides fall into the first category • Given their low polarity these compounds are only held together by weak intermolecular forces … termed dispersion forces. • This results in low boiling points. SnH4 -52oC , PH3 -90oC • Carbon-based systems comprise the largest set of hydrides.

  39. If the H-atom is positive. H2O, HF, and NH3 These molecules have VERY high boiling points The H in water can bridge two molecules forming an intermolecular bond with an adjacent molecule. We can try to understand this using a molecular orbital picture for water.

  40. The H-Bond • When H bond to F, O, N, Cl, etc., the H bears partial positive charge while X bears partial negative charge • this results in a intermolecular interaction • They are the results of electrostatic forces • H-Bond X-H...Y parameters: • Bond length: d(X-Y); d(H-Y) • Bond angle: X-H...Y

  41. F F F H H H H H H F F F F – H … F H H F F H H F … H – F HF Hydrogen bonding produces system analogous to polymers: MP = -83oC 120o Gaseous HF as (HF)6 104o HF2- has esp. strong hydrogen bond: Dh [F – H – F]-

  42. MO Treatment of H-Bonding • X – H – X has orbital overlap e.g., HF2- H 1s & 2pz on both F’s: 3 (a.b.) ~ Pa + Pb- cs 2 (n.b.) ~ Pa - Pb 1 (b.) ~ Pa + Pb + cs Gives MO’s and diagram …

  43. + - - + +   MO Treatment of H-Bonding F H F 2pA 1s 2pB * 2F 2pz This gives potential energy curves … H 1s   

  44. Potential Energy Curve The H can shift between two energy minima: X – H … X E d(XH)

  45.     The molecular orbital picture of water     NB x2 2pz 2py 2px H  2s O

  46.         Bringing two water molecules together. NB  Water 1 Water 2

  47. The Hydrogen Storage Problem • Problems include difficult storage and transport if H2 is left as a liquid. • low boiling point and very low density • H2 forms explosive mixtures with air, hence there is an explosion risk on storage!

  48. The Hydrogen Storage Problem • One solution is to store hydrogen as a solid compound (hydride) from which it can be extracted. • two general types : 1. Reversibly but in small amounts (e.g. Pd, V, Nb, Ta) 2. Irreversibly in large quantities of hydrogen (e.g. rare earths, alkaline earths, Ti, Zr) • Intermetallic Alloys lie between the two classes and can be useful in hydrogen storage (e.g. LnNi5 class of alloy)

  49. Metallic Hydrides • Form with many transition metals • NonstoichiometricM + x/2 H2  MHx M = Ti, U, Pr, Pd, Pt, etc. • Stoichiometric formulas possible:TiH2, UH3, PrH2

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