Experiment 3b Kinetics of aquation of
pentaamminechlorocobalt(III) chloride
In this experiment you will study the aquation of
[CoCl(NH3)5]2+ under varying
concentration of acid and temperatures. In addition, the effect
of adding mercury (II) acetate on the rate of the reaction will
be examined.
Synthesis of
[CoCl(NH3)5]Cl2
In a fume hood, dissolve 1 g of ammonium chloride in 9 ml
concentrated aqueous ammonia in a 100-ml Erlenmeyer flask. (The
combination of NH4Cl and NH3 (aq.)
guarantees a large excess of the NH3 ligand.) Stir the
ammonium chloride solution vigorously while adding 2 g of finely
divided CoCl2.6H2O in small portions. A
yellow-pink precipitate of the hexaammine Co(II) salt forms on
slight warming as the reddish starting material dissolves. Any
air oxidation that occurs during this exothermic stage is ignored
since the solution is going to be fully oxidised by adding
hydrogen peroxide.
Caution: 30% hydrogen peroxide is a strong oxidizing agent that
will cause severe burns and bleaching of skin and clothing.
Slowly add 2 mL 30% hydrogen peroxide to the brown Co slurry,
using a burette. An addition rate of about 2 drops per second is
usually sufficient, but care should be taken to avoid excessive
effervescence. (If the reaction shows signs of excessive
effervescence, stopping the stirring momentarily will usually
prevent overflow of the solution.)
You should notice that all the Co(II) ammine dissolves to form a
deep red solution. (This corresponds to the formation of the
pentaammineaquacobalt(III) salt.)
When the effervescence has virtually ceased, cautiously add 6 mL
of concentrated HCl in small portions and with continuous
stirring. This operation needs to be carried out in a fume hood
since fumes of ammonium chloride will be produced during the
neutralisation. After this point the reaction may be removed from
the hood. A purple product should then precipitate from the hot
reaction mixture leaving a pale green-blue supernatant liquid.
While occasionally stirring, use a steam bath or hot plate to
heat the solution to 60°C. Hold the temperature between
55°C and 65°C for 15 min; this incubation period is
necessary to allow complete displacement of all aqua ligands.
Collect the purple product by filtration through a No. 3 sinter
glass crucible. The mother liquor may be discarded.
When the product has been drained well it is washed with 4 mL
ice-cold deionised water in small portions, followed by 5 mL
ice-cold 95% ethanol. (The solutions must be cold to prevent
undue loss of product by redissolving.) Transfer the product to a
crucible and dry in an oven at 100°C for one hour. This helps
complete the conversion of any remaining
pentaammineaquacobalt(III) salt. Submit your sample to the demonstrator.
Aquation
Kinetic runs for the aquation of the [CoCl(NH3)5]2+ complex are
to be done at 550 nm where the absorbance shows a maximum change
to occur between the reactant and product. Each student/group
will study the effect of one variation on the observed rate
constants from the following parameters.
- Hydrogen ion concentration (0.1 M to 0.6 M)
- Temperature (50-70°C)
- Addition of mercury(II) acetate
The complex concentration for all experiments should be 1.2 X
10-2 M and Ionic strength 1.0 M (adjusted with sodium
nitrate).
A typical run should be done as follows:
PROCEDURE
- Add the required volumes of stock HNO3 and NaNO3 solution to
a 100 mL volumetric flask and make it up to roughly 95% of the
mark with distilled water.
- Immerse the flask in a thermostatic water bath for
temperature equilibration for about fifteen minutes.
- Weigh, using an analytical balance, the required amount of
the complex required to give a 1.2 X 10-2 M complex concentration
and dissolve the complex in the equilibrated solution.
- Shake the flask until the complex completely dissolves, then
make the solution up to the mark with distilled water.
- Return the flask to the water bath.
- After fifteen minutes, withdraw samples using a pipette
filler (not your mouth).
- Record the time of withdrawal and transfer about 3 cm3 into a
cell/ cuvette.
- Place the cell in cell holder of the spectrophotometer and
measure the absorbance at 550 nm.
- Withdraw further aliquots at approximately fifteen minutes
intervals and record the absorbance at 550 nm.
Take about 10 sample readings.
CALCULATIONS
In order to calculate the rate constant you will need to
determine the A value. This can be calculated from the known
extinction coefficient of Co(NH3)5(OH2)3+,
which is 2.10 m2 mol-1 at 550 nm.
Plot a graph of ln(At-A ) versus time and determine the first
order rate constant as the slope of this plot.
Tabulate your results as follows:
| Time /min |
Absorbance (At) |
(At-A ) |
ln(At-A ) |
| |
|
|
|
| |
|
|
|
| |
|
|
|
Questions
- The aquation of
[CoCl(NH3)5]2+ is accelerated by
Ag+. Propose a mechanism for this acceleration.
- Suggest another method, other than the spectrophotometry, of
determining
- the rate of aquation of
[CoCl(NH3)5]2+.
- Why was 550 nm chosen as the wavelength at which the reaction
was followed?
- For the reaction
[CoCl(NH3)5]2+ + NH3
-> [Co(NH3)6]3+ + Cl-
The rate law for this reaction can be written in a general form
as:
Rate =
k[Co(NH3)5Cl2+]x[NH
3]y,
where the orders x and y ,are
to be determined. Rate studies were carried out for
[CoCl(NH3)52+] = 0.001 M and
various concentrations NH3 e.g 0.2, 0.3, 0.4 M etc.
Based on the results of these kinetic runs, suggest how you would
determine y.
- Explain why HNO3 and not HCl was used in the
aquation studies.
| Bench-Position |
[H+]/ M |
Ionic Strength/M |
[complex]/M |
[Hg(OAc)2]/M |
T/0C |
kobs/second |
| 1-I |
0.2 |
1 |
0.015 |
0 |
50° |
|
| 1-II |
0.4 |
1 |
0.015 |
0 |
|
|
| 1-III |
0.6 |
1 |
0.015 |
0 |
|
|
| 1-IV |
0.1 |
1 |
0.015 |
0.005 |
|
|
| 1-V |
0.1 |
1 |
0.015 |
0.010 |
|
|
| 1-VI |
0.1 |
1 |
0.015 |
0.015 |
|
|
| |
|
|
|
|
|
|
| 2-I |
0.1 |
1 |
0.015 |
0 |
55° |
|
| 2-II |
0.2 |
1 |
0.015 |
|
|
|
| 2-III |
0.3 |
1 |
0.015 |
|
|
|
| 2-IV |
0.4 |
1 |
0.015 |
|
|
|
| 2-V |
0.5 |
1 |
0.015 |
|
|
|
| 2-VI |
0.6 |
1 |
0.015 |
|
|
|
| |
|
|
|
|
|
|
| 3-I |
0.1 |
1 |
0.015 |
0 |
60° |
|
| 3-II |
0.2 |
1 |
0.015 |
|
|
|
| 3-III |
0.3 |
1 |
0.015 |
|
|
|
| 3-IV |
0.4 |
1 |
0.015 |
|
|
|
| 3-V |
0.5 |
1 |
0.015 |
|
|
|
| 3-VI |
0.6 |
1 |
0.015 |
|
|
|
| |
|
|
|
|
|
|
| 4-I |
0.1 |
1 |
0.010 |
0 |
65° |
|
| 4-II |
0.2 |
1 |
0.010 |
|
|
|
| 4-III |
0.3 |
1 |
0.010 |
|
|
|
| 4-IV |
0.4 |
1 |
0.010 |
|
|
|
| 4-V |
0.5 |
1 |
0.010 |
|
|
|
| 4-VI |
0.6 |
1 |
0.010 |
|
|
|
| |
|
|
|
|
|
|
| 5-I |
0.1 |
1 |
0.010 |
0 |
70° |
|
| 5-II |
0.2 |
1 |
0.010 |
|
|
|
| 5-III |
0.3 |
1 |
0.010 |
|
|
|
| 5-IV |
0.4 |
1 |
0.010 |
|
|
|
| 5-V |
0.5 |
1 |
0.010 |
|
|
|
| 5-VI |
0.6 |
1 |
0.010 |
|
|
|
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Copyright © 2006 by Robert John
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Lancashire
The Department of Chemistry, University of the West Indies,
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Created August 2004. Links checked and/or last
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