GS-1 Biological Sciences Question 21

[samnpdd8521]

The answer is C and it seems right, but I feel like none of the answers are right.

The question ask for the role of acid catalyst,

C. says that the change in spontaineity (delta G) is maintained at the same value, lowers the activation evergy for the dehydration reaction.

I thought that delta G decreases, like it says in the explaination. How could C be right?? and all the other choices conflict with the facts.

[vijkrishna3659]

I thought a catalyst doesn't affect Enthalpy(delta H). I also thought that on a reaction coordinate, the Y axis could either be Enthalpy or Gibbs Free Energy (delta G). So I figured a catalyst only affects (lowers) the Activation Energy.
If you look at the G = H - TS, you can see that if a catalyst didn't affect H, how would it lower G. Furthermore, I believe a catalyst ONLY affects Kinetics, NOT Thermodynamics. G and H are thermodynamic quantities.

[admin]

Yes, that is correct. The original question was correct.

Let's review:

as mentioned, from G = H - TS where H does not change, T is the temp at which the reaction is carried out and S is a measure of entropy, indeed G would not change.

There is another formulation to calculate Gibbs Free Energy:

G = - RT Ln Keq

R is the gas constant, and clearly Keq is not affected by a catalyst thus G is unaffected.

Thus the catalyst affects speed (kinetics) by decreasing the activation energy but the spontaneity of the reaction (will it happen or not without taking into account time) is determined by delta G which therefore is not affected by the presence of a catalyst.

[admin]

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