Question

Select a reaction which requires a catalyst for proceeding the reaction:
A.${H_2} + {I_2} \to 2HI$
B.${H_2} + {F_2} \to 2HF$
C.${H_2} + C{l_2} \to 2Cl$
D.${H_2} + B{r_2} \to 2HBr$

Answer 1

To answer this question, you should recall the concept of catalysts. It can be defined as a substance that changes the reaction rate by changing the path of reaction. Use the knowledge of reactivity of a catalyst to answer the question. The compounds which are least reactive will require a catalyst.

Complete step by step answer:
Catalyst is a very usual word that comes up in the topic of chemical reactions. This is where a catalyst comes in. Generally, a catalyst speeds up and increases the reaction rate. Catalysts break or rebuild the chemical bonds between the atoms which are present in the molecules of different elements or compounds. Catalysts make the whole reaction process easier and efficient. Key points of a catalyst are that it does not initiate a chemical reaction. It cannot be consumed in the reaction. It forms intermediates with reactants and at the same time facilitates the production of the final reaction product. Halogens belong to the p block where the reactivity decreases from fluorine to astatine. In the periodic table fluorine is one of the most reactive elements.
The reaction ${H_2} + {I_2} \to 2HI\;$ is carried out in the presence of a catalyst as among all the halogens ${I_2}$ ​ is least reactive.

So, Option A is correct.

Note:
There are four types of catalysts:
Positive Catalysts: Catalysts which increase the rate of a chemical reaction are positive catalysts. It increases the rate of reaction by lowering the activation energy barriers such that a large number of reaction molecules are converted into products, thereby the percentage of yield of products increases.
Negative Catalysts: Catalysts which decrease the rate of reaction and negative catalyst.
Promoter or Accelerators: A substance which increases the catalyst activity are known as Promoters or accelerators.
Catalyst Poisons or Inhibitors: Substances which decrease the catalyst activity are known as catalyst poisons or inhibitors.