| Chem 110 |
General Chemistry |
Fall 2003 |
| Lecture Notes::Lec 38_8 December |
© R. Paselk 2003 |
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The Chemistry of the Elements
Fourth Period Transition Metals, cont.
- Vanadium (V): The +3 and +4 oxidation state are used
by some invertebrates (tunicates) as an oxygen transporter instead
of iron. These organisms concentrate V from sea water by more
than a million-fold, and their fossil remains have created important
vanadium deposits.
- very hard, strong, and corrosion resistant, but much denser
than Ti. Not used much as the pure metal, but an important alloying
metal for steels - makes them stronger and more ductile.
- VO, V2O3, VO2, and V2O5
are known.
- Chromium (Cr):
- silvery white, very hard, brittle and shiny metal.
- High corrosion resistance due to the formation of a thin,
impervious oxide coat.
- Used for alloying steels (e.g. 'chrome-vanadium' steel for
tools), and in particular, its a main component of many stainless
steels.
- Used as a protective and decorative plating for metal objects.
- Four important oxidation states
- Cr(II), chromous ion: aqueous solutions (blue hexaaquachromium(II)
ion) of Cr(II) are often used as reducing agents.
- Cr(III), chromic ion: Forms many octahedral complexes. The
hexaaquachromium(III) is violet. Chromium(III) hydroxide is a
gray-green gelatinous precipitate. Chromic oxide is used as a
green pigment.
- Cr(IV): CrO2 is the most important compound of
Cr(IV). It is a metallic conductor and can be magnetized. It
is an important coating in magnetic tapes etc.
- Cr(VI), chromate: This oxidation state is seen in the yellow,
tetrahedral, chromate ion, CrO42-. In acidic
aqueous solution chromate is converted to the orange dichromate
ion: 2CrO42- + 2H+ Æ
Cr2O72- + H2O.
- Chromium(VI) is highly toxic and carcinogenic.
- Manganese (Mn):
- A white, brittle metal.
- Used in steel alloys.
- We see three oxidation state of manganese in the lab:
- Mn(II), manganous ion: this is the very pale pink ion of
aqueous solutions (the d-d transition is forbidden).
- Mn(IV) is seen in manganese dioxide (manganese(IV) oxide)
a solid brown to black precipitate.
- Mn(VII), permanganate: the most important Mn(VII) species
is the permanganate ion. Permanganate is intensely purple (easily
seen in 10-4M solutions), and a powerful oxidizing
agent.
- Iron (Fe): Iron is very abundant (most abundant transition
element, 4.7 mass% of Earth's crust)with readily accessible ores
(hematite, Fe2O3, magnetite, Fe3O4,
siderite, FeCO3, and iron pyrite, FeS2).
Much of the iron ore (various iron oxides) used is the result
of biological deposition or bio-induced deposition by a variety
of bacteria.
- Has been the most important industrial metal since the beginning
of the industrial revolution, and though being displaced for
some uses (plastic bumpers and siding in cars etc., aluminum
in beverage cans etc.) it is still the predominant metal of our
civilization.
- Iron is ferromagnetic - it has a very strong attraction to
magnets. Results from many atoms in a crystal aligning so that
the paramagnetic spins line up. Thus its cause is paramagnetism,
but enhanced because normally atoms oriented randomly.
- Pure iron is a silvery-white, soft metal that corrodes readily
in moist air. It does not form a protective coat, and so can
readily be converted to oxides.
- Iron is prepared in blast furnaces - see text.
- Iron is further purified by burning off excess carbon to
give steels - see text.
- Iron is most important in its various alloys known as steels:
- Carbon steel is an alloy which contains essentially no metal
but iron. It is generally classified in three grades:
- Mild steel - contains <0.2% carbon. It is malleable and
ductile, and is used where strength is not paramount. It cannot
be tempered.
- Medium steels - contain 0.2-0.6% carbon. Used for structural
steel like beams and girders in construction etc.
- High-carbon steels - contain 0.8-1.5% carbon. Used to make
knives, tool bits, drills etc. where hardness is important. High-carbon
steels' hardness can be adjusted from relatively soft to very
hard and brittle by tempering. This characteristic is one of
steels most endearing properties since one can work it while
soft with files etc., then temper it to get a hard (but often
brittle) object. (see Heat Treatment of Steel in text)
- Alloy steels add other transition metals (and often carbon)
to give steels with special properties.
- Some of the metals added and properties they convey are:
- V - improves springiness, strength and ductility
- Cr - resistance to corrosion, increased hardness
- Mn - increased wear-resistance
- Ni - improves toughness, resistance to corrosion
- Co, W, and Mo are added to increase heat resistance for engine
parts and tool steels (e.g. cobalt steels can be used at red-heat
without losing their temper).
- Stainless steels generally have high concentrations of chromium
and nickel to give corrosion resistance. 18:8 stainless (18%Cr,
8%Ni) is one of the most common.
- Iron is also essential for life as we know it. It is used
by all oxygen using organisms in the various proteins of the
electron transport system, and in many animals as part of their
oxygen transport systems.
- Note Fe chemistry in lab book.
Cobalt, Nickel, Copper, and Zinc all have +2 as the
most common and important oxidation state.
- Cobalt (Co): is fairly rare.
- Hard, bluish-metal.
- Important for alloying.
- Note Co chemistry in lab book.
- Nickel (Ni): 22nd most abundant element in the Earth's
crust. Occurs in various sulfide ores.
- Silvery metal that polishes very well, which, along with
its corrosion resistance accounts for its wide use as a decorative
metals or plating.
- In fine-particulate form Ni is an important catalyst.
- As seen above it is an important alloying element, particularly
for stainless steels.
- Note Ni chemistry in lab book.
- Copper (Cu): copper is the only one of these elements
with an important oxidation state other than +2, +1. It is a
little less abundant than nickel, generally found in sulfide
ores, though other ores are also common, and it occurs rarely
in the native (metal) form.
- Soft, ductile, reddish metal. Resistant to corrosion.
- Most electrically conductive metal other than Ag.
- Important alloys as brasses and bronzes.
- Classically brass is an alloy of copper and zinc
- Classically bronze is an alloy of copper and tin. Other alloying
metals are used however and often referred to as bronzes.
- Note Cu chemistry in lab book.
- Zinc (Zn): Widely distributed and about as abundant
as copper. Most common ore is the sulfide, ZnS called sphalerite
(zinc blende).
- Note Zn chemistry in lab book.
Other transition metals:
- Ag & Au: (Note Ag chemistry in lab book.)
- Hg: (Note Hg chemistry in lab book.) Only metal which
is liquid at room temperature.
- Pt: The platinum metals (Ru, Rh, Pd, Os, Ir, Pt) are
all distinguished by exceptional resistance to corrosion.
- W: Highest melting temperature for any metal (second
only to carbon).
- Cd: Note Cd chemistry in lab book.
© R A Paselk
Last modified 8 December 2003
