5.7.2 Homogeneous Catalysts

1.		2.
	Experimental setup		The reaction begins
3.		4.
	Addition of phosphoric acid		Test for oxygen is negative

Photo1:	Experimental setup: Potassium iodide is added to a test tube containing 3% hydrogen peroxide solution.
Photo2:	Immediately following the addition of potassium iodide, the hydrogen peroxide decomposes into water and oxygen.
Photo3:	Addition of phosphoric acid.
Photo4:	A glowing splint test for oxygen is negative.

The reaction shown here is an example of a homogeneous catalyst. The reactants and the catalyst are in the same phase. The actual catalyst here is the iodide ions. The addition of phosphoric acid (inhibitor) causes the iodide ions to combine, and the reaction is brought to a halt:

2 H⁺ + H₂O₂ + 2 I^-® I₂ + 2 H₂O

The iodine produced is clearly identified by its violet color.

Catalysts can become inactivated or poisoned. In the example shown here, the catalyst is transported in a chemical reaction to a substance that can no longer influence the reaction speed. This effect is useful when we want to prevent unwanted reactions. For example, so-called "inhibitors" are added to consumer-brand hydrogen peroxide, preventing the hydrogen peroxide from decomposing into its constituent elements due to the presence of trace heavy metal catalysts.

Another example is when a vehicle with a catalytic converter is run using gas containing lead. The lead that is released as the fuel combusts adheres to the inside surface of the catalytic converter, thereby inhibiting future reactions from taking place. Such a case is referred to as "catalytic poisoning."

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