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My class $12$ board exams are approaching and i'm having a doubt in this question: This is from my class $12$ NCERT exemplar book

Activation energy for the acid catalysed hydrolysis of sucrose is $\pu{6.22 kJmol^-1}$, while the activation is only $\pu{2.15kJmol^-1}$ when hydrolysis is catalysed by the enzyme sucrase. Explain

I've found nothing so good from the web. All I know is this:

Enzymes, the biocatalysts, work even under mild conditions, they do not require elevated temperatures or other physical conditions. While with ordinary chemical catalyst, this might be the case. Hence, activation energy for chemical catalyst reaction is little more than enzyme catalyst.

Is this reason valid? Or is there any other reason?

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I'll just quote the official answer given in the NCERT Exemplar book. It reads:

Enzymes, the biocatalysts, reduce the magnitude of activation energy by providing alternative path (my emphasis). In the hydrolysis of sucrose the enzyme sucrase reduces the activation energy from $\pu{6.22 kJ mol^–1}$ to $\pu{2.15 kJ mol^–1}$.

The key thing to note here is an alternate path is provided to the reaction. The values given in the question ($6.22$ and $2.15$) are only to distract you from the systematic answer. You are not supposed to rote learn them. Focus on the lowering of activation energy due to an alternate pathway. Here is a graph depicting such a situation:

enter image description here (source)

The mechanism how the catalyst exactly achieved this lowering of activation energy, that too specifically for sucrose, would be difficult to describe unless this is a research paper, and is surely not required from a 12th grader. All that you are required to answer is what I put in the yellow blockquote above.

I hope it helps!

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – user7951 Mar 4 '18 at 12:49
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Well, in general, no matter if biocatalyst or homogeneous or heterogeneous catalyst in organic chemistry or the industry the role of the catalyst itself is to lower the activation energy of the reaction. Often people have two mistakes when they think about that. One is that they think, that the catalyst is not changed in the reaction and the other one is that the catalyst can change any thermodynamic energy level or so.

About the first point, the catalyst takes part in the reaction. It is true that in general, it leaves the reaction as it entered (there is decomposition but we ignore that for a second) but inbetween the catalyst will take part in the reaction itself. And therefore we are speaking of a completely different reaction at all, which leads to the second point. The energy level of starting material, transition state (so activation energy or reaction barrier) and product is set. The catalyst does usually not influence this. Therefore you have to cross another reaction path.

For example, let's take the hydrogenation of ethylene, you add $H_2$ to ethylene to create ethane. If you do this with a metal catalyst on the metal's surface you will end up in an activation of the hydrogen itself fist. So the hydrogen molecule adsorbs on the surface of the metal and the bond is then cleaved. And those two separated hydrogen atoms can now hydrogenate the ethylene, which also adsorbs to the surface. So metal can help here and lower the overall energy of this reaction. The new path consists of several steps like coordination, cleavage, reaction and desorption from the catalyst. And if you choose this correctly then those energies are lower than for the uncatalyzed one. And in biocatalysis, it's basically the same. You can check for example the catalytic triad to see how many amino acids and the different sites in the catalytic center of the enzyme help to perform the reaction.

About your specific reaction if have no idea, I am no biochemist, this is the only thing I could find about that. enter image description here

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