METALS: MOLYBDENUM
Molybdenum, a rare element in the Earth's crust (0.002%), is usually
found as a sulfide (e.g., in the ore molybdenite). This transition
metal, which has a very high melting point (2617°C) and a high density
(10.28 g cm-³), forms coloured compounds in several oxidation states
[e.g., Mo(II), Mo(III), and Mo(VI)]. Molybdenum has been shown to be
essential to all living organisms, but its particular importance in
biological chemistry centres on nitrogenase; this molybdenum-containing
enzyme, which is present in nitrogen-fixing bacteria (e.g., Rhizobium),
is involved in the reduction of atmospheric nitrogen to ammonia.
[.. K > Ca > Mg > Al > Zn > Fe > Mo > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. The extraction of molybdenum from its ores is very complex, and so
only a simplified summary is presented here:
(a) Explain what the double-headed arrow means in the second equation.
The reaction is reversible; in a closed system, the forward and reverse
rates are equal at dynamic equilibrium.
[2]
(b) Suggest a different reductive method of obtaining molybdenum from
molybdenum(VI) oxide. By electrolytic reduction of the molten oxide
[1]
2. Many leguminous plants have evolved symbiotic relationships with
nitrogen-fixing bacteria living in their root nodules; a legume uses
the ammonium ions absorbed from these bacteria to biosynthesize amino
acids, ATP, chlorophylls, and nucleic acids.
(a) Name two legumes. Pea / Bean / Clover / Alfalfa / ...
(b) Suggest in what form chemical energy is absorbed by these bacteria
from the legume. As a soluble carbohydrate (... e.g., glucose)
(c) Cereal farmers will clearly reduce their dependence on artificial
nitrogenous fertilizers if they introduce legumes as 'break crops'.
Explain one disadvantage inherent in the use of such fertilizers.
Pollution of aquatic ecosystems, because fertilizers are leached from
soils. / Increased soil erosion, because crumb structure is destroyed.
[5]
3. Industrial nitrogen-fixation is commonly achieved by the Haber
process, which is usually summarized by the following equation:
Explain one reason why each of the following conditions speed up the
above reaction - so saving time (and, therefore, money).
Powdered iron Acts as a catalyst (large surface area), by reducing the
activation energy required for successful collisions of the particles.
High temperature Increases the kinetic energy of the particles, so
more have the required activation energy for successful collisions.
High pressure Increases the concentration of reactants, so there are
more collisions between the particles.
[6]
4. Molybdenum(VI) oxide is used as a co-catalyst in the industrial
oxidation of a propene and ammonia mixture to propenenitrile.
Propenenitrile, better known as acrylonitrile, is the monomer used to
manufacture which polymer? Poly(propenenitrile)
[1]
5. Molybdenum's position in the reactivity series can be determined
by measuring potential differences of electrochemical cells; the Table
shows some typical data, together with the electrical conductivities of
the selected metals at ambient temperatures.
Metal (M) |
Al |
Ti |
Cr |
Ga |
In |
Mo |
Rh |
Ir |
Au |
Cell p.d. (Au-M)/V * |
3.16 |
2.71 |
2.24 |
2.06 |
1.84 |
1.70 |
0.74 |
0.34 |
0.00 |
Conductivity/MW cm-¹ |
40.0 |
2.6 |
7.9 |
7.4 |
12.5 |
20.0 |
23.2 |
21.2 |
48.8 |
* The data, which have been obtained under standard conditions (25°C;
pH = 0), refer to the oxidation half-reaction M(s) ——® M3+(aq) + 3e- |
(a) Noting that each quantitative dependent variable is a linear scale
on the vertical axis, and that the qualitative independent variable is
shown as separate blocks on the horizontal axis, complete this double
bar-graph of these data.

[7]
(b) Molybdenum may prove attractive as a component of commercial cells,
particularly as it is at least ten times more abundant in the Earth's
crust than cadmium, mercury, or silver (which are used in Ni-Cd, Hg-Zn,
and Ag-Zn cells, respectively). For a specialized application which
required a p.d. of about 1 V, molybdenum could be paired with either of
which two metals? Titanium (... 1.10 V) or Rhodium (... 0.96 V)
[2]
Dr. R. Peters Next Contents' List & Teacher's Notes