Cobalt Catalysis

Summary:  The oxidation of tartarate ion by hydrogen peroxide is observed both in the presence and absence of catalyst cobalt chloride.  First, hydrogen peroxide is added to an 80°C sodium potassium tartarate solution.  No reaction occurs.  The catalyst cobalt chloride is then added to the mixture resulting in a vigorous reaction.

Hazards: 

The catalyzed reaction is a vigorous.  WEAR GOGGLES! 

Keep this reacton close to the recommended temperature, higher temperatures can react violently.

Although the solution is weak, hydrogen peroxide can bleach clothing.

Chemicals and Solutions: 

0.3M sodium potassium tartrate, NaOOC-CHOH-CHOH-COOK

0.3M cobalt chloride, CoCl2

6% hydrogen peroxide, H2O2

Materials: 

Two 250 mL beakers

One 50 mL graduated cylinder

Two 10 mL graduated cylinder

Hot plate

Thermometer, with stand and clamp

Procedure: 

  • Pour 30 mL of the tartrate solution into each of 250 mL beakers.  Heat the solution to ~80°C using a hot plate.
  • Remove both beakers from the hot plate.  Add 10 mL of 6% H2O2 solution to each beaker.

No reaction is apparent.

  • Return one beaker to the hotplate and keep it at about 80oC until Step 4.  To the other beaker add 4 mL of 0.3M CoCl2.  The solution will initially be pink but almost immediately the cobalt chloride catalyses a reaction. The solution then begins to foam up and turn green.  Eventually the foaming subsides and the solution will return to the pink color.
  • To prove that CoCl2 is a catalyst, pour the entire contents of the "reacted" beaker into the beaker containing no catalyst.  Once again a reaction occurs.

Hint:  Turn the heat off before returning the waiting beaker to the hotplate.  There’s enough residual heat to keep the beaker warm and it helps to minimize the potential for overheating (which is dangerous).


Discussion: 

In this experiment you observe the oxidation of tartrate ion by hydrogen peroxide in the absence and presence of a catalyst, cobalt chloride.  The catalyzed reaction is:

NaKC4H4O6 + 5H2O2 è reaction intermediate è 4CO2 + 6H2O + 2OH- (aq) + Na+(aq) + K+(aq)

A catalyst is a substance that acts to change the speed of a chemical reaction without itself undergoing a permanent chemical change in the process.  Many potentially useful chemical reactions normally have extremely slow rates of reaction but can nevertheless be successfully employed if a suitable catalyst can be found.  For example, the rate of the industrially important reaction of nitrogen gas with hydrogen gas to produce ammonia is immeasurably slow.  However, when iron is present the rate of reaction is greatly increased and the production of ammonia becomes economically feasible.  Relatively simple molecules or ions frequently function as catalysts, as noted in the case of ammonia production.  In biological systems, however, the catalysts are often very large, complex protein molecules called enzymes; these may have molecular weights from several thousand to a million or more.

The exact mechanism for the action of catalysts is not completely understood in all cases.  However, it is clear that catalysts provide an alternate path or series of steps by which the reaction can take place.  Carefully note that a catalyst does not shift the position of equilibrium in a chemical reaction - its action only increases the rate at which equilibrium is approached.

To schedule a demonstration, please send an e-mail to the demonstration lab.

 

Contact:

Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606

ericcamp@uw.edu

 

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