Question

A catalyst is a substance which:
A-Increases the yield of the reaction
B-Increases the speed of the reaction
C-Decreases the speed of the reaction
D-Increases the speed of the reaction and remains unchanged.

Answer 1

Hint: The rate of a particular chemical reaction depends on a certain number of factors. Some reactions are very fast and some reactions are very slow. A catalyst is a substance that promotes a chemical reaction by speeding up the reaction and it does not get consumed in the entire reaction process.

Complete step by step answer:
A catalyst is typically defined as a substance that increases the rate of a particular chemical transformation and at the end of the reaction, it remains unchanged. The process by which a catalyst speeds up a chemical reaction is known as catalysis.
For every chemical reaction to take place, the reactant molecules have to gain a minimum amount of energy, called activation energy. If in a particular reaction, the activation energy barrier is very high, then the reactant molecules will require high amount of activation energy to cross the barrier. In that case, the reaction may take longer to complete. A catalyst generally reduces the activation energy barrier in a reaction so that the reactant molecules can easily achieve it and overall, increases the rate of the reaction. A catalyst may speed up a reaction by altering the reaction mechanism as well.
In every catalysis process, the catalyst does not get consumed and at the end of the reaction, the catalyst remains unaltered.

Example of catalyst:
A very common example of catalysis is the decomposition of hydrogen peroxide (${H_2}{O_2}$) in presence of platinum (Pt). The hydrogen peroxide decomposes into water and oxygen usually in a much slower rate. But when platinum is added, the decomposition rate is significantly increased and at the end of decomposition, the platinum remains unchanged.

$2{H_2}{O_2}\xrightarrow{{Pt(catalyst)}}2{H_2}O + {O_2}$

Enzymes are also a very common example of biological catalysts that increase the rate of many biochemical reactions.
Thus, we can say that a catalyst is a substance which increases the speed of the reaction and remains unchanged.
Now, let’s look at the answer options available:
-The primary function of a catalyst is to speed up a reaction. It does not get consumed in the main reaction process. Thus, a catalyst has no contribution towards the yield of a reaction. A catalyst cannot increase or decrease the yield of a reaction. Hence, option A is not correct.
-The substance that decreases the rate of a reaction is called an inhibitor. An inhibitor’s function is just opposite to that of a catalyst. Hence, option C is not correct.
-Both the options B and D are correct, but option D is providing the complete information about a catalyst whereas, option B is partially true. Hence, option B will not be the correct answer for this question.
So, the correct answer is “Option D”.

Note: Many students get confused between a catalyst and an inhibitor. A catalyst and an inhibitor are opposite in terms of functionality. Hence, a student should focus on what is given in question, catalyst or inhibitor. If two options are true for a given question (here B and D), you have to select the option which is providing more correct information (here D is providing more information about a catalyst than B).