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? Using a larger amount of solution Increasing the mass of the cube Chilling the hydrogen peroxide solution Smashing the cube into pieces

Chilling the hydrogen peroxide solution  

Explanation:

The MnO₂ is acting as a catalyst.

Chilling the hydrogen peroxide solution reduces the average kinetic energy of the H₂O₂ molecules.

Fewer of them will have enough kinetic energy to overcome the activation energy of the reaction. The rate will decrease.

A is wrong. Using a larger amount of solution does not increase the number of H₂O₂ molecules that can reach the surface of the catalyst per second. There is no effect on the rate.

B is wrong. Increasing the mass of the cube increases its surface area. More molecules of H₂O₂ can reach the surface of the catalyst per second. The rate increases.

D is wrong. Smashing the cube into pieces vastly increases the surface area, so the rate increases