Question

What is a negative catalyst? Which is the negative catalyst in the decomposition of ${{H}_{2}}{{O}_{2}}$?

Answer 1

Hint: Catalysts can be classified into different categories based on their mode of action on reaction. Catalysts can either increase or decrease the rate of a reaction. An inhibitor or a negative catalyst reduces the effectiveness of a catalyst in a catalyzed reaction.


Complete step by step solution:
 - Let's start with the idea of catalysis. It can be described as a process in which the rate or the outcome of the reaction is influenced by the presence of a substance called catalyst that is not consumed during the reaction. It provides a new way to a chemical reaction with the lowest activation energy.
   - As we mentioned, catalysts increase the rate of reaction by providing an alternative reaction pathway with lower activation energy so that more reactant molecules can cross the activation energy barrier at any given concentration and temperature. As evident from the definition, a catalyst may increase or decrease the rate of reaction.
 - The catalyst which increases the rate of reaction is known as positive catalyst and the process is known as positive catalysis.
 - The catalyst which decreases the rate of reaction is known as inhibitor or negative catalyst and the process is known as negative catalysis or inhibition. Negative catalysts are useful to slow down or stop any unwanted reactions.
 - In negative catalysis the rate of reaction is decreased by increasing the activation energy barrier. Thus, the number of reactant molecules that get transformed into products decreases and hence the rate of reaction decreases.
 - The decomposition of Hydrogen peroxide (${{H}_{2}}{{O}_{2}}$) into water and oxygen can be written as follows,
  $2{{H}_{2}}{{O}_{2}}\to 2{{H}_{2}}O+{{O}_{2}}$

This reaction can be retarded by using phosphoric acid(${{H}_{3}}P{{O}_{4}}$ ) ,Acetanilide(${{C}_{8}}{{H}_{9}}N{{O}_{{}}}$), or glycerol(${{C}_{3}}{{H}_{8}}{{O}_{3}}$).All of this acts as a negative catalyst to decrease the rate of decomposition of hydrogen peroxide.


Note: Catalyst poisoning should be distinguished from inhibition. Catalytic Poisons are substances that destroy the activity of catalysts even in the presence of trace amounts. The inhibitors are used to reduce the catalytic activity of the catalyst and this is desired and is usually added to the reacting system. Whereas poison is due to foreign undesired components.