Effect of a Catalyst: Adding a catalyst generally increases the rate of reaction. Removing a catalyst generally decreases the rate of reaction.

Explanation: A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. Catalysts work by providing an alternative reaction pathway with a lower activation energy. This means that a larger proportion of the reactant particles will have sufficient energy to react, leading to a faster rate of reaction.

Enzymes as Catalysts: Enzymes are biological catalysts, typically proteins, that are highly specific for their substrates. They bind to the substrate at an active site, forming an enzyme-substrate complex. This complex lowers the activation energy of the reaction, accelerating the rate. Enzymes can be denatured by heat, pH changes, or other factors, which can remove their catalytic activity and slow down or stop the reaction. The effect of removing an enzyme is a decrease in the rate of reaction, as the reaction pathway with the lower activation energy is no longer available.

The catalyst lowers the activation energy of the reaction. Activation energy is the minimum amount of energy required for a reaction to occur. By providing an alternative reaction pathway with a lower activation energy, the catalyst allows a larger proportion of reactant molecules to have sufficient energy to react. This results in a faster reaction rate.

The catalyst is not consumed because it participates in the reaction mechanism but is regenerated at the end. It provides a surface for the reactants to interact on, facilitating the reaction. The amount of catalyst used does not change because it is only involved in the reaction mechanism and is not permanently altered or used up in the process. It can participate in multiple reaction cycles.

Heating the reaction mixture increases the rate of reaction between H₂O₂ and MnO₂ due to the following reasons, explained through collision theory:

Diagram:

The diagram illustrates the relationship between kinetic energy and the number of successful collisions. The key points are:

• (a) Number of particles per unit volume (Concentration): Heating the mixture does not directly change the concentration of H₂O₂ or MnO₂. Therefore, the number of collisions per unit time remains relatively constant.
• (b) Frequency of collisions between particles: Heating increases the speed of the reactant particles, leading to a higher frequency of collisions. More collisions mean a greater opportunity for successful reactions.
• (c) Kinetic energy of particles: Heating increases the average kinetic energy of the particles. This is the most important factor. A greater proportion of the collisions will now have kinetic energy equal to or greater than the activation energy (E_a). This significantly increases the number of successful collisions.
• (d) Activation energy (E_a): The activation energy remains constant. However, the increased kinetic energy at higher temperatures means that a larger fraction of the reactant particles possess the energy required to overcome this barrier. This results in a much higher rate of reaction.

In summary, heating provides the reactant particles with more kinetic energy, increasing the frequency of collisions with sufficient energy to overcome the activation energy barrier. This leads to a significantly faster rate of reaction.