Exams › JEE Advanced › Chemistry

Even though the chlorination of methane releases energy, why is a high temperature necessary for the reaction to occur?

1. The activation energy is minimal.
2. The reaction's enthalpy change is negative.
3. The step that starts the chain reaction absorbs energy.
4. The step that ends the chain reaction absorbs energy.

Correct answer: The step that starts the chain reaction absorbs energy.

Solution

Although chlorination of methane is overall exothermic, it needs heat (or UV light) because the chain-initiation step, homolysis of Cl2 into two chlorine radicals, is endothermic and supplies the radicals that start the chain. The correct choice is that the initiation step absorbs energy, not the termination step.

Related JEE Advanced Chemistry questions

• Acetylene (HC≡CH) undergoes the following two-step transformation: Step 1: Passed over a red-hot iron tube at 873 K (cyclic trimerisation). Step 2: The product of Step 1 is treated with CO, HCl, and AlCl3 (Gattermann-Koch reaction). How many carbon atoms in the final product are sp2 hybridised?
• Which of the following species is non-aromatic (i.e., neither aromatic nor antiaromatic)?
• Among the following compounds, which one produces ONLY oxalic acid (HOOC-COOH) as the major product when treated with O3 followed by H2O2?
• Which of the following reactions proceeds in the forward direction as written?
• What reagent(s) can convert 1,3-dibromopropane (Br-CH2-CH2-CH2-Br) into acetylene (H-C≡C-H)?
• Consider the following reaction sequence: Step 1: C2H2 is passed over red hot iron tube to give compound A. Step 2: A reacts with Cl2 in the presence of anhydrous AlCl3 to give compound B. Step 3: B reacts first with CH3CH2COCl in the presence of anhydrous AlCl3 and then with Zn(Hg)/HCl to give the major product C. Identify the major product C.

⚔️ Practice JEE Advanced Chemistry free + battle 1v1 →