METALS: CADMIUM
Cadmium, a very rare element in the Earth's crust (0.00002%), occurs
mainly as the sulfide or carbonate in zinc ores. Compounds of cadmium,
in contrast to those of zinc, appear to have little or no biological
rτle; so, inevitably, cadmium(II) ions accumulate up the trophic levels
because very few living organisms have evolved methods for egesting or
excreting these ions. In addition, partly because of the observed and
predicted similarities between zinc and cadmium ions, and partly
because enzymes containing zinc(II) ions are ubiquitous, cadmium ions
could be expected to affect diverse biochemical processes.
[.. K > Ca > Na > Al > Zn > Cd > Fe > Ni > Pb > (H) > Cu > Hg > Ag ..]
1. The extraction of cadmium from zinc ores is fairly complex, but the
final process involves chemical reduction of aqueous cadmium sulfate
with zinc (DE = -70 kJ mol-Ή). For this redox reaction, construct the:
symbol equation; redox half equations; and net ionic equation.

[4]
Use the net ionic equation to complete and label this energy level
diagram.

[4]
2. Currently, the major use of cadmium is in rechargeable 'nicad'
batteries. Explain the direction of electron flow when a Ni-Cd cell is
in use (i.e., transducing chemical to electrical energy). Cadmium to
nickel, because cadmium is a better reducing agent.
[2]
3. A research chemist investigated the following hypothesis: 'The
speed (S) of the displacement reaction between cadmium and aqueous
sulfuric acid increases in linear proportion to the temperature (T) of
the acid; i.e., S = k Χ T + c'; the Table shows a summary of the chosen
conditions and raw data (no duplicate experiments were executed).
Constants: amount (0.050 mol) of cadmium granules (spheres with surface
areas of 4.2 ± 0.2 mm³); concentration (1.00 mol dm-³) and volume
(75 cm³) of aqueous sulfuric acid; absence of catalysts; thermostatted
water-bath; room temperature (295 K) and pressure (100 kPa); volume of
dihydrogen collected via a gas syringe (50 cm³).
Temperature (T) / K |
295 |
307 |
319 |
331 |
343 |
Reaction time (t) / s |
77 |
40 |
25 |
21 |
17 |
Reaction speed (S) / ms-Ή |
13 |
25 |
40 |
48 |
59 |
(a) State one reason why higher values of the independent variable were
not examined. Markedly decreased precision of reaction times / Safety
[1]
(b) Calculate the missing values for the dependent variable, and insert
these data into the above Table.
[2]
(c) Label both axes, with the independent variable on the horizontal
axis (cela va sans dire?), and then plot the data points.

[4]
Draw a best straight line through as many points as is sensible, and
then determine the gradient of the (plotted) graph; this value, 'k', is
the proportionality constant in the linearly proportional relationship
S = k Χ T + c.
(y2 - y1) (50 - 17.5) 32.5
k = = = = 0.96 ms-Ή K-Ή
(x2 - x1) (333 - 299) 34
[2]
Write a precisely worded conclusion based on the (plotted) graph. For
the reaction of 0.050 mol of spheroidal cadmium granules, with surface
areas of 4.2 mm³, and 1.00 mol dm-³ aqueous sulfuric acid (75 cm³),
in the absence of catalysts, the speed (S) at which 50 cm³ of hydrogen
gas evolved from the reaction mixture increased in linear proportion to
the temperature (T) of the acid (within the range 295-343 K); i.e.,
S = k Χ T + c, where k = ca. 1.0 ms-Ή K-Ή
[5]
(d) Sketch the graph that would be obtained if one of the constants was
changed separately, and explain why each of these graphs differs from
that obtained for the set of constants shown with the Table.
Aqueous sulfuric acid (1.00 mol dm-³) and copper (0.001 mol) The rate
would increase, because of the presence of a catalyst [Cu(0) or Cu(I)];
this would lower the activation energy, so more particles had the
required energy for successful collisions.
[3]
Aqueous ethanoic acid (2.00 mol dm-³) The rate would decrease, because
the concentration of hydrogen ions would be lower (as the position of
equilibrium lies far to the left in this partially dissociated acid):

[3]
Dr. R. Peters Next Contents' List & Teacher's Notes