CHEM 171 Lecture Notes - Lecture 1: Magnesium, Boranes, Lead

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25 Dec 2015
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1. Chapter 16 Summary
Bonding Trends (General Information) –
Period 2 elements generally obey the octet rule
Period 3 and later elements can expand their valance shell
Elements in periods 5 and 6 of the p-block show variable valance (inert
pair effect)
oInter pair effect: tendency of the electrons in the outermost
atomic s orbital to remain unionized or unshared in compounds
of post-transition metals.
oOxidation number of elements 2 less than period suggests
The smaller the atomic radius, the less number of atoms will bond to it
oPeriod 2 atoms from multiple bonds with elements in their
same period because they are the only ones small enough to
pack around them
Periodic Trends illustrated by hydrides –
# of valance = # of hydrogen molecules in a compound
Saline hydrides: when the H ion is the hydride ion [H-]
oFormed by all members of the s-block [except beryllium] and are
made by heating the metal in hydrogen
oWhite high melting point solids with crystal structures (rock-salt
structure)
Metallic hydrides: black, powdery, electrically conducing solids formed
by heating certain of the d-block metals in hydrogen
oHigh enthalpy densities
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Molecular hydrides: they consist of discrete molecules
oVolatile; many are Brønsted-Lowry acids
oGaseous: ammonia, the hydrogen halides and lighter
hydrocarbons
oLiquid: water and hydrocarbons such as octane and benzene
s-block d-block p-block
saline metallic molecular
Periodic trends illustrated by oxides –
Left of the table: soluble ionic oxides
Right of the p-block: insoluble, high melting-point oxides
Left of the p-block: low melting-point, gaseous molecules
Elements with low ionization energies: basic ionic oxides
Elements with intermediate ionization energies (beryllium, boron,
aluminum, metalloids): amphoteric oxides
Any gaseous oxides act as Lewis acids (they can accept an electron
pair)
oAcidic anhydrides: oxides of metals that react with water and
typically form acidic solutions
oFormal anhydrides: molecule obtained by removing the
elements of water (2 H’s and 1 O)
s-block d-block p-block
Ionic
Basic
Ionic
Basic to amphoteric
Covalent
Amphoteric to acidic
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Hydrogen –
Most hydrogen is present as water
oHydrogen is acquired from by electrolysis
Water splitting reaction:
** light is needed in order for the reaction to proceed
oHydrogen may be also obtained as a by product of petroleum
refining (in 2 steps)
Reforming Reaction:
** is known as synthesis gas
** A nickel catalyst is used
Splitting Reaction:
** An iron/copper catalyst is used
oOther sources of hydrogen include:
Dissolve a metal in acid
Electrolysis
Using sunlight to split water
** A titanium dioxide catalyst is needed
Biological production (photosynthesis by algae in low
sulfur environment)
Hydrogen is a colorless, odorless, tasteless gas
oIn liquid form, it has an extremely low density
Compounds of Hydrogen –
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