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 (2617C) and a high density 
(10.28 g cm-), 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.
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(b) Suggest a different reductive method of obtaining molybdenum from
molybdenum(VI) 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. _________________________________________________
(b) Suggest in what form chemical energy is absorbed by these bacteria
from the legume. ______________________________________________________
(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. _____
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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 _________________________________________________________
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High temperature ______________________________________________________
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High pressure _________________________________________________________
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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? ____________________________________________
                                                                    [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
Cond. / MW cm-
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 (25C;
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.
 _       _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _       _
|_|     |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|     |/|
|_|     |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|     |/| 
        |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
C   3.0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_60   E
e       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      l
l       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      e
l       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      c
        |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      t
p   2.5_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_50   r
o       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      i
t       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      c
e       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      a
n       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      l
t   2.0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_40  
i       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      c   
a       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      o
l       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      n
        |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      d
d   1.5_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_30   u
i       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      c
f       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      t
f       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      i
e       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      v
r   1.0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_20   i
e       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      t
n       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      y
c       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|   
e       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|      /
    0.5_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_10   
(Au-M)  |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| MW cm-
        |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| 
/       |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
        |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
V   0.0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_0
          Al    Ti
                              Metal (M)
                                                                    [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?  ____________________________________________________
                                                                    [2]
Dr. R. Peters Next Contents' List