Hydrogen

1
Chapter 10

• Hydrogen

2
History

• Discovered about 200 years ago (1766)

Antoine Lavoisier (1743-1794)
Henry Cavendish (1731-1810)
3
History

• In 1931, hydrogen was discovered to have isotopes

vs.
Harold C. Urey (1893-1981)
Frederick Soddy (1877-1956)
Nobel Laureate 1934
Nobel Laureate 1921
4
Isotopes of Hydrogen

• Three common isotopes
• Protium (H)
• common hydrogen
• 99.985 abundant
• Deuterium (D)
• one neutron
• 0.015 abundant
• Tritium (T)
• two neutrons
• 1x10-15 abundant

5
Physical Properties

• Vastly different among the three isotopes

6
Stability

• All are stable except for tritium
• synthesized in the universe and man-made by
  neutron bombardment
• undergoes radioactive ? decay

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Nuclear Magnetic Resonance

• Studies nuclear spin

8
Nuclear Magnetic Resonance

• Peaks depend upon the chemical environment

9
Nuclear Magnetic Resonance

• Intensity of the absorption depends upon the
  nucleus

10
Magnetic Resonance Imaging

• Measures the 1H nucleus in water

11
Properties of Hydrogen

• Periodic table placement
• reasons to place the element in both Group
  1(alkali metals) and Group 17 (halogens)

Argument for Placement Argument Against Placement
Alkali Metal Group forms monopositive ions is not a metal
Alkali Metal Group has a single s electron does not react with water
Halogen Group is a nonmetal rarely forms a mononegative ion
Halogen Group forms a diatomic molecule is comparatively unreactive
12
Properties of Hydrogen

• Colorless, odorless gas
• m.p. -259C
• b.p. -253C
• Relatively non-reactive
• diatomic bond energy of 436 kJ/mol

13
Reactions of Hydrogen with Diatomics

• 2H2(g) O2(g) ? 2H2O(g) (very fast)
• H2(g) F2(g) ? 2HF(g) (very fast)
• 3H2(g) N2(g) ? 2NH3(g) (very slow)

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Reduction Reactions of Hydrogen

• Acts to reduce many metallic elements
• CuO(s) H2(g) ? Cu(s) H2O(g)
• Can also reduce double and triple bonds with a
  catalyst
• H2CCH2(g) H2(g) ? H3CCH3(g)

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Preparation of Dihydrogen

• Reaction of dilute acids on metals
• Zn(s) 2HCl(aq) ? ZnCl2(aq) H2(g)

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Preparation of Dihydrogen

• Steam reformer process
• CH4(g) H2O(g) ? CO(g) 3H2(g)
• CO(g) H2O(g) ? CO2(g) H2(g)
• K2CO3(aq) CO2(g) H2O(l) ? 2KHCO3(aq)

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Hydrides

• Binary compounds of hydrogen
• has an intermediate electronegativity
• ionic hydrides
• LiH
• covalent hydrides
• HF
• metallic hydrides
• NiH2

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Ionic Hydrides

• white solids
• metal cation and hydride ion
• very reactive
• LiH(s) H2O(l) ? LiOH(aq) H2(g)
• reducing agents
• CaH2(s) H2O(l) ? Ca(OH)2(s) H2(g)

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Covalent Hydrides

• covalently bonds with all nonmetals and weakly
  electropositive metals
• gases at room temperature
• hydrogen can be
• nearly neutral
• substantially positive
• slightly negative

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Neutral Covalent Hydrides

• low polarity
• only dispersion forces
• Examples
• PH3
• CH4
• Hexene
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g)

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Positive Covalent Hydrides

• high melting and boiling points
• protonic bridging
• Examples
• ammonia
• water
• hydrogen fluoride

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Negative Covalent Hydrides

• Contains hydridic hydrogens
• Very reactive towards oxygen
• Examples
• B2H6
• SiH4
• GeH4
• GeH4(g) 2O2(g) ? GeO2(s) 2H2O(l)

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Borane Bonding

• possess bridging hydrogens
• hydridic bridges
• very reactive

B2H6
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Borane Bonding

• Three-center, two-electron bond
• banana bonds

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Borane Bonding

• MO Picture

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Borane Bonding

• Other borane structures

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Metallic Hydrides

• Hydrogen occupies the interstitial spaces
• non-stoichiometric
• TiH1.9
• less densities
• brittle
• lower conductivity

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Synthesis and Reactions of Metallic Hydrides

• Ti(s) H2(g) heat/pressure ? TiH1.9(s)
• TiH1.9(s) heat ? Ti(s) H2(g)
• applications in hydrogen storage

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Uses of Metallic Hydrides

• Batteries
• Cathode Ni(OH)2(s) OH-(aq) ? NiO(OH)(s)
  H2O(l) e-
• Anode Ni-alloy(s) H2O(l) e- ?
  Ni-alloyH(s) OH-(aq)

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Water and Hydrogen Bonding

• Without hydrogen bonding, water would melt at
  -100C and boil at -90C
• Liquid is denser than the solid

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Waters Phase Diagram

• Different from a normal phase diagram

Normal
Water
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Electrical Conductivity in Water

• highest for solutions of H3O and OH-

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Clathrates

• a substance which is trapped in the crystal
  lattice of another substance
• from clathratus
• enclosed behind bars
• gas hydrates
• methane
• noble gases

34
Biological Aspects of Hydrogen Bonding

• Hydrogens properties play two key roles to the
  existence of life
• closeness in electronegativity to carbon
• ability to form hydrogen bonds

35
Reaction Flowchart

• Shows the different types of reactions of a
  certain species