To monitor the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst

Theory

• Hydrogen peroxide (H₂O₂) decomposes slowly into water and oxygen

• The rate of reaction may be increased by adding a manganese dioxide (MnO₂) catalyst

H₂O₂ —MnO₂→ H₂O + ½ O₂

• The rate of this reaction is measured by measuring the volume of oxygen gas that is collected at regular time intervals

Procedure

➢ The apparatus in the diagram is set up

➢ The manganese dioxide is added to the hydrogen peroxide from a small test tube in the conical flask, the mixture is shaken, and the stopwatch is started at EXACTLY THE SAME TIME

➢ The oxygen gas produced is collected over water using a beehive shelf and graduated cylinder

➢ The volume of oxygen gas evolved is measured using the graduated cylinder at regular time intervals until the reaction is completed

➢ The results are entered in a table and graph of volume of oxygen (y axis) Vs time (x axis) is plotted.

➢ From the graph, the average rate of reaction and the instantaneous rate of reaction at any point can be found

Explain the general trend in the curved obtained when a graph of volume of oxygen vs time is plotted

A- Slope steepest – reaction occurring at its fastest rate, most volume of gas evolved per time

Concentration of reactants is greatest – more collisions means more effective collisions

B- Slope becomes less steep - reaction slows down, less volume of gas evolved per time

Concentration of reactants decreased – less collisions means less effective collisions

C- Slope is 0 – reaction is finished, no gas evolved

Reactant used up; no more product formed

Write a balanced equation for the catalysed decomposition of hydrogen peroxide using manganese dioxide

H₂O₂ —MnO₂→ H₂O + ½ O₂

What is the appearance of the manganese dioxide catalyst?

Black powder

How is it ensured the stopwatch is started at exactly the same time as the reaction starts?

• The manganese dioxide is added to the hydrogen peroxide from a small test tube in the conical flask

• The mixture is shaken, and the stopwatch is started at exactly the same time

What chemical hazard symbols are associated with i) hydrogen peroxide ii) manganese dioxide?

Explain their meaning and suggest how you could manage their risk

i) Hydrogen peroxide Oxidiser - Keep away from other substances

ii) Manganese dioxide Irritant - Wear gloves

What effect would increasing the mass of the catalyst have on the rate of reaction? Explain

• No effect

• Catalysts are only required in very small amounts

What type of catalysis is involved in this reaction? Explain

Heterogenous catalysis - the solid manganese dioxide catalyst is in a different phase to the reactants(liquid) and products

Name and describe the mechanism for the action of the manganese dioxide in speeding up the decomposition of hydrogen peroxide

Name: Surface adsorption theory - always used for a solid catalyst

Mechanism:

i. Adsorption - The hydrogen peroxide reactants are adsorbed onto the surface, forming temporary bonds

ii. Reaction - The hydrogen peroxide reactants are now in high concentration on the surface of the catalyst and are more likely to collide

The bonds within the hydrogen peroxide molecules are weakened and collisions of reactants are more likely to be effective i.e. the activation energy is lowered

iii. Desorption - The water and oxygen products are formed and leaves the surface of the catalyst