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.


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.

  1. Hydrogen ion concentration (0.1 M to 0.6 M)
  2. Temperature (50-70°C)
  3. 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:


Take about 10 sample readings.


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 )


  1. The aquation of [CoCl(NH3)5]2+ is accelerated by Ag+. Propose a mechanism for this acceleration.
  2. Suggest another method, other than the spectrophotometry, of determining
  3. the rate of aquation of [CoCl(NH3)5]2+.
  4. Why was 550 nm chosen as the wavelength at which the reaction was followed?
  5. 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.
  6. 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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The Department of Chemistry, University of the West Indies,
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Created August 2004. Links checked and/or last modified 7th October 2005.
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