Chemistry Practice Problems Reaction Mechanism Practice Problems Solution: Homogeneous catalysis. Effect of catalyst on the s...

Solution: Homogeneous catalysis. Effect of catalyst on the speed of hydrogen peroxide decomposition to water and oxygen gas.Why does the solution in the middle cylinder have a brownish color?

Problem


At the initial time, a solid NaBr catalyst is about to be added to the reaction mixture in a graduated cylinder containing H2O2 (aqueous) in acidic solution. The solution is clear. At an intermediate time, the solution in the graduated cylinder has turned brown. The reaction is 2 Br- (aqueous) plus H2O2 (aqueous) plus 2 H+ (aqueous) goes to Br2 (aqueous, brown) plus 2 H2O (liquid). At the final time, the solution in the cylinder is again clear and there are bubbles near the surface. The reaction is Br2 (aqueous) plus H2O2 (aqueous) goes to 2 Br- (aqueous, colorless) 2 H+ (aqueous) plus O2 (gas, bubbles).


Homogeneous catalysis. Effect of catalyst on the speed of hydrogen peroxide decomposition to water and oxygen gas.


Why does the solution in the middle cylinder have a brownish color?

Solution

We’re asked to determine why the solution in the middle cylinder has a brownish color based on the given diagram.


The diagram shows the following:


  • Left cylinder: H2O2 in acidic solution before NaBr is added 
    • colorless solution
  • Middle cylinder: H2O2 solution after NaBr is added 
    • brown solution
  • Right cylinder: H2O2 decomposes completely into H2O and O2 (gas)
    • colorless solution with bubbles (slightly brownish)


We will determine the substance that caused the brown color in the middle cylinder by getting the overall reaction.


We can see that the decomposition of H2O2 occurs in two steps in the presence of a catalyst (NaBr:

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