Question

Define the term- ‘catalyst’:
(A) increases the rate of reaction
(B) decreases the rate of reaction
(C) can increase or decrease the rate of reaction
(D) none of these

Answer 1

Hint: A substance that alters the rate of reaction by changing the path of reaction is known as a catalyst. The catalyst alters the rate of reaction by providing an alternative pathway or a mechanism with lower activation energy than the non-catalyzed mechanism.

Complete step by step solution:
-Catalysts are used to speed up or increase the rate of the reaction. They are also used to break or rebuild the chemical bonds between the atoms which are present in the molecules of different elements or compounds.
-Following are some of the important characteristic features of catalysts-
(i) A catalyst never initiates a chemical reaction.
(ii) A catalyst is never consumed in the reaction, it is obtained in the product the same as a reactant.
(iii) Catalysts tend to react with reactants to form intermediates and simultaneously facilitate the production of the final reaction-product. After the whole chemical reaction, a catalyst can be regenerated.
-Let us understand the mechanism of a catalyzed reaction-
Let X and Y be the reactants giving the final product Z and C to represent the catalyst.
$\begin{align}
& X+C\to XC...(1) \\
& Y+XC\to XYC...(2) \\
& XYC\to CZ...(3) \\
& CZ\to C+Z...(4) \\
\end{align}$
Although the catalyst C is consumed in reaction (1) but is also subsequently produced in reaction (4). As a catalyst is regenerated in a chemical reaction, often a very small amount of catalyst is needed to increase the rate of the reaction.

So, the correct answer is option C.

Note: The SI unit for measuring the catalytic activity of a catalyst is ‘katal’, which is moles per second.’ The productivity of a catalyst is described by the turnover number (or TON). The catalytic activity of a catalyst is given by the turnover frequency (TOF) which is TON per time.