Catalysts are those species that enter in any chemical reaction to change the rate of any chemical reaction. In other words, we can understand that catalysts are rate changer substances of any chemical reaction. Catalysts are also of two types positive catalysts and negative catalysts. So, negative catalysts are those catalysts that change the rate of reaction by decreasing the reaction rate. Positive catalysts change the rate of any chemical reaction by increasing the reaction rate.

Negative catalysts

The name ‘negative catalysts’ suggests that this catalyst’s presence in any chemical reaction will negatively affect the rate of any ongoing chemical response by increasing the activation energy level compared to the original level. In the reversible reaction, a negative catalyst will not alter the equilibrium position but will try to attain the equilibrium state of the reaction for a longer time.

Working of negative catalysts 

As we know, for the completion of any chemical reaction, a minimum amount of energy is required by the reactant molecules of that chemical reaction to attain the threshold energy point. 

• The threshold energy point is the point where an activated complex species is formed, and this state is known as the transition state because the formation of a product is initiated from this state.
•  In the activated complex species, partial bonds of reactant and partial bonds of the product are present or mixed bonds of reactant and product are present. 
• From the above description, it is clear that the required amount of energy is known as activation energy to reach the threshold energy point.

Suppose any negative catalysts are present in the chemical reaction. In that case, it hinders the participation of any reactant from reacting. It also poisons the given surface area to prevent the other reactant from responding. It decreases the rate of the given chemical reaction by providing high activation energy. 

• In the whole process of negatively catalysed chemical reaction, negative catalysts will not be purely occupied in the chemical reaction.
• They will provide different options for the reactant. 
•  After the completion of the reaction, we get the same product and pure catalyst as we put in the initial state of any chemical reaction.

Importance of Negative Catalysts

If any chemical reactions take place in the absence of any foreign material like catalyst then we do not get the expected result. Sometimes for the desired result we have to add a negative catalyst and we choose a wanted catalyst according to our desire. So, in the chemical world the importance of negative catalysts also exists and it will be used for different purposes with different composition and with differ quantities. And the importance of negative catalysts will be understood through the following given examples.

Examples of negative catalyst

• Vitamin -E is one of the good examples of negative catalysts as it is used in the biological oxidation of biochemicals, as they decrease the rate of degradation of anti -ageing chemical tissue. In this reaction vitamin – E will behave as an antioxidant.
• Hydrogen sulphide (H2S) molecule is also a negative catalyst in the hydrogenation process of unsaturated fatty acids. They poison the surface of the raney nickel molecule through the formation of nickel sulphide compounds.
• Phosphoric acid (H3PO4), a mineral inorganic acid, is also an example of negative catalysts as it decreases the decomposition rate of hydrogen peroxide molecule (H2O2).
• In the oxidation process of sodium sulfite (Na2SO3), alcohol (-ROH) is behave as a negative catalyst as it retards the rate of oxidation reaction.
• Tetraethyl lead (T.E.L.) is also an example of a negative catalyst as it decreases the ignition of the vapours of petroleum. 
• When we add ethanol (C2H5OH) to a chloroform (CHCl3), it behaves as a negative catalyst.
• Acetanilide is also a negative catalyst and it is used in the decomposition of hydrogen peroxide into the water and nascent oxygen.

Conclusion

The overview of the negative catalyst will be understood by the fact that it is a substance added in any chemical reaction to decrease the rate of that given chemical reaction. Negative catalysts engage in the participation of any one of the reactants with the other reactant and increase the level of activation energy for that given ongoing chemical reaction.