When a cube of manganese oxide was added to a solution of hydrogen peroxide, water and oxygen were produced. Manganese oxide was not used up in the reaction. Which of the following is likely to decrease the rate of the reaction?

The Manganese oxide would rather speed up the rate of reaction.

Explanation:

In this reaction, manganese oxide is the catalyst and catalyses the decomposition of hydrogen peroxide. Chemically this reaction can be written as:

2H_2O_2 - > O_2 + 2H_2O

MnO2 does not show itself in the equation because it is a catalyst.

A catalyst is a substance that speeds up a chemical reaction, but is not consumed by the reaction; hence a catalyst can be recovered chemically unchanged at the end of the reaction it has been used to speed up, or catalyze.