Solution:
Let's understand what an
endothermic reaction means first.
An
endothermic reaction is a chemical reaction that
absorbs heat energy from its surroundings. This means the reaction needs heat to happen. If you touch the container where an endothermic reaction is taking place, it might feel cold because it's taking heat away from your hand or the environment. Think of it like cooking: you need to supply heat for the food to cook.
Now let's look at each option:
Option A: Catalytic cracking
Cracking is a process where large, heavy hydrocarbon molecules (like those in heavy oil) are
broken down into smaller, lighter, and more valuable molecules (like gasoline). To break chemical bonds, you generally need to supply a lot of energy, usually in the form of heat. Therefore,
catalytic cracking is an endothermic process, as it absorbs heat.
Option B: Hydrocracking
Hydrocracking is similar to catalytic cracking, but it happens in the
presence of hydrogen. While breaking large molecules apart (cracking) is endothermic, the addition of hydrogen (hydrogenation) is typically a process that
releases heat (exothermic). In hydrocracking, the heat released by hydrogenation usually makes the overall reaction
exothermic or slightly exothermic, meaning it generates heat rather than absorbing it.
Option C: Dehydrogenation of naphthene to produce aromatic
Dehydrogenation means
removing hydrogen from a molecule. Naphthenes are ring-shaped saturated hydrocarbons. When you remove hydrogen from them, they can transform into
aromatic compounds (like benzene or toluene), which are special ring structures with double bonds. Removing hydrogen atoms and forming these aromatic rings requires a
significant input of energy (heat). This is a very characteristic example of an
endothermic reaction in petroleum refining. It
absorbs a lot of heat to proceed.
Option D: Catalytic polymerisation
Polymerisation is the process of joining many small molecules (called monomers) together to form a much larger molecule (a polymer). When new chemical bonds are formed to create larger molecules, energy is typically
released. Therefore, polymerisation reactions are generally
exothermic, meaning they release heat.
Conclusion:
Out of the given options, both catalytic cracking (A) and dehydrogenation (C) are endothermic reactions. However, the dehydrogenation of naphthenes to aromatics is a
very strongly endothermic process and is a classic example in petroleum refining that requires substantial heat input.
Therefore, the correct answer is:
Option C: Dehydrogenation of naphthene to produce aromatic
