Question

A catalyst is used
(A). Only for increasing the velocity of reaction
(B). For altering the velocity of reaction
(C). Only for decreasing the velocity of reaction
(D). All A, B, C are correct

Answer 1

In a reaction, some outer species take part which enhance the rate of reaction. They are not consumed themselves. Such species are called catalysts and the process is called catalysis. A catalyst is not consumed in a reaction. It just affects the rate or the velocity of the reaction.

Complete step by step solution:
-The compounds which are present before the reaction occurs are called the reactants. Compounds formed after the reaction are called products. There are certain species that help the reaction to occur and therefore increase the rate of the reaction. Such species are called catalysts.
-Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as catalyst. One thing to note is that the catalyst is not consumed in the reaction process. It just enhances the rate of reaction. It is added in a small amount in the reaction chamber.
-As the catalysts alter the rate of reaction, it is very important for them to be able to interact properly with the reactants. So, the catalysts must be of the order of the reactants and in such a shape so as to ensure maximum surface area coverage.
-Catalysts are of two types – positive catalysts and negative catalysts. Positive catalysts increase the rate of reaction while negative catalysts decrease the rate of the reaction. Both of them have different roles in reactions.
- Positive catalysts decrease the time duration of the reaction. It increases the rate or the velocity of the reaction such that the reactions occur faster than before thereby decreasing the time duration to a large extent.
-Negative catalysts, also called inhibitors, are used mainly if the compound is too reactive and we need to make a certain product and stop it from reacting further. They decrease the rate or the velocity of the reaction thereby increasing the time duration of the reaction. It sometimes does not allow the reaction to move in forward direction.
-So a catalyst can increase as well as decrease the rate of reaction. The increment or decrement depends on the type of catalyst being used. The only thing to keep in mind is to increase the surface area for the catalyst to interact with the reactants. Only then can it act effectively.
-Some examples of catalytic reactions including the positive catalysts are
$CO\left( g \right)+3{{H}_{2}}\left( g \right)\xrightarrow{Co\left( s \right)}C{{H}_{4}}\left( g \right)+{{H}_{2}}O\left( g \right)$
Here cobalt acts as a positive catalyst.
$2KCl{{O}_{3}}\left( s \right)\xrightarrow{Mn{{O}_{2}}\left( s \right)}2KCl\left( s \right)+3{{O}_{2}}\left( g \right)$ where $Mn{{O}_{2}}$ acts as catalyst
-Some examples of catalytic reactions with negative catalysts are
Alcohol is a negative catalyst if used in the oxidation of $N{{a}_{2}}SO$
During the decomposition of water into hydrogen and oxygen, phosphoric acid works as a negative catalyst or as an inhibitor.

Therefore the correct option is (D).

Note: Catalysts are usually foreign particles only. But sometimes a product formed during a reaction can work as a catalyst. Such catalysts are called auto-catalysts. Also the term catalyst is used to indicate the positive catalysts only when nothing else is specified. Negative catalysts are specifically described when used in a reaction.