Chapter 20 Representative Elements:Groups 5A through 8A

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Chapter 20Representative ElementsGroups 5A
through 8A

• The last four columns of the periodic chart.
• Decrease in metallic character
• Increase in the atoms ability to accept electrons

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Groups 5A Elements

• N P As Sb Bi
• General electron configuration ns2np3
• Increase in metallic character going down the
  column
• N and P have a tendency to gain electrons
• Sb and Bi are metallic in character

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Groups 5A Elements

• N P As Sb Bi
• Tend to form molecules with 3, 5, or 6 covalent
  bonds
• Exception is N which can only form 3 covalent
  bonds
• With 3 covalent bond, Group 5A elements are Lewis
  bases due to the lone pair of electrons

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Groups 5A Elements

• N P As Sb Bi
• Tend to form molecules with 3, 5, or 6 covalent
  bonds
• With 5 covalent bond, Group 5A are expanding into
  the 3d orbitals and becoming dsp3 hybridized.
• With 6 covalent bonds d2sp3 hybridized. (Also
  will be ionic. Ex PF6-)

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Groups 5A Elements

• N P As Sb Bi
• Will form pyramidal, trigonal bipyramidal and
  octahedral geometries with 3, 5, and 6 ligands,
  respectively

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Nitrogen

• The great stability of the N?N bond means that
  most binary compounds containing nitrogen
  decompose exothermically to the elements
• NO2(g) ? 1/2N2(g) O2(g) ?H? ?34 kJ
• N2H4(g) ? N2(g) 2H2(g) ?H? ?95 kJ
• Virtually inert due to the high activation energy
  of the reverse reaction!!!!

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Nitrogen Fixation

• . . .the process of transforming N2 to other
  nitrogen-containing compounds.
• One thing to notice is the thermodynamic
  stability of NH3
• The formation of NH3 is one of the few reactions
  involving N2, that is an exothermic overall.
• The Haber Process
• N2(g) 3H2(g) ? 2NH3(g) ?H? ?92 kJ
• P 250 atm T 400?C

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Nitroglycerin

• What make Nitroglycerin so explosive?
• The formation of large amounts of gases!!!
• 4C3H5N3O9(g) ? 6N2(g) 12CO2(g) 10H2O(g)
  O2(g) energy
• Large amounts of gas are made in a very
  exothermic reaction!!!! BOOM!!!!
• Why is it so exothermic? N2 stability

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Nitrogen Hydrides

• Ammonia, NH3
• Hydrazine, N2H4
• Monomethylhydrazine, N2H3(CH3)

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Nitrogen Oxides
Nitrogen in its oxides has an oxidation state
from 1 to 5
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Nitrogen Oxides
The example of NOs molecular orbital diagram
gives us a clue as to the reactivity of NO
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Nitrogen Oxides
Note the unpaired electron, makes NO paramagnetic
and easier to ionize to NO
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Chemistry of Phosphorus

• Although directly below N, significantly
  different in chemical properties when compared to
  N.
• The differences arise from
• N forms stronger p bonds
• Greater electronegativity of N
• Larger size of P
• Availability of empty d orbitals

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Allotropes of Phosphorus - P4

• White Phosphorus (WP) tetrahedra - very
  reactive
• Black Phosphorus (BP) crystalline structure
  much less reactive
• Red Phosphorus (RP) amorphous with P4 chains.

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Phosphorus Oxides and Oxyacids
What do you get when you mix P4O10 with
water? Phosphoric acid
How about a condensation reaction between
individual Phosphoric acid molecules (biochemist
need to remember this reaction)
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Group 6A Elements
O S Se Te Po

• Typical trends of 6A elements
• No Group 6A element behaves as a typical metal
• All except O have a tendency to use d orbitals in
  bonding ( exapansion of octect)
• All form hydrides of formula H2X

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Oxygen

• The most abundant element in or near the earths
  crust.
• Present as O2, O3, in rocks, soil, water
• Remember that O2 is paramagnetic 2 unpaired
  electrons in the Molecular Orbital diagram.

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Ozone
3O2(g) ? 2O3(g) K ? 10?57
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Chemistry of Sulfur

• Produced from sulfur deposits with the Frasch
  process
• Involves injecting superheated water
• Then forcing the sulfur to the surface with air
  pressure.

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Elemental forms of Sulfur

• When comparing S to O one would think that
  possibly the elemental form is S2
• However, due to S stronger s bonds S exists in
  larger aggregates such as S6 and S8
• The rhombic crystal of sulfur is stacked rings of
  S8.

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Sulfur Oxide Reactions
The following reaction shows oxidation of S in
the presence of O2, Followed by the subsequent
oxidation to SO3 and hydrolysis into sulfurous
and sulfuric acids

• S8(s) 8O2(g) ? 8SO2(g)
• 2SO2(g) O2(g) ? 2SO3(g)
• SO2(g) H2O(l) ? H2SO3(aq)
• SO3(g) H2O(l) ? H2SO4(aq)

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Group 7A ElementsThe Halogens

• Because of Halides high reactivity, halogens are
  not found in nature as free elements.
• Usually found as the anion X-
• Halogens have a high electronegativity values.
• Tend to have polar covalent bonds with nonmetals
  and ionic bonds with metals

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Preparation of Hydrogen Halides

• H2(g) X2(g) ? 2HX(g)
• OR
• Treating halide salts with acid
• CaF2(s) H2SO4(aq) ? CaSO4(s) 2HF(g)
• 2NaCl(s) H2SO4(aq) ? Na2SO4(s) 2HCl(g)

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Strengths of halo acids

• H-I gt H-Br gt H-Cl gt H-F (trongest to weakest)
• Oxyacids HOCl, HO2Cl, HO3Cl, HO4Cl and the Br and
  I series all increase in acidity as the number of
  O atoms increases.

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Noble Gases

• He, Ne and Ar form no compounds.
• Kr and Xe have been observed to form chemical
  compounds
• Xe(g) 2F2(g) ? XeF4(s) 6 atm, 400?C
• XeF6(s) 3H2O(l) ? XeO3(aq) 6HF(aq)