What thermodynamic parameters are compatible with the spontaneity of a chemical

Question

What thermodynamic parameters are compatible with the spontaneity of a chemical reaction?

A. positive standard free energy change only.

B. positive standard free energy change and high activation energy.

C. negative standard free energy change and high activation energy.

D. positive standard free energy change and low activation energy.

E. negative standard free energy change.

A. positive standard free energy change only.

B. positive standard free energy change and high activation energy.

C. negative standard free energy change and high activation energy.

D. positive standard free energy change and low activation energy.

E. negative standard free energy change.

Explanation / Answer

Correct option. E.

Ans. A spontaneous reaction always has a negative standard free energy change.

That is, dG < 0          - for spontaneous process.

A reaction with positive standard free energy change is non-spontaneous.

            That is, dG > 0          - for non- spontaneous process.

So, all options with positive standard free enzyme change (A, B, D) are non-spontaneous.

Option. C. dG is negative, so the reaction is spontaneous once it’s started. But “high activation energy” of such reactions prevents it from starting by itself. For example, combustion is a spontaneous process because it has very large standard free energy change. However, combustion (say, of coal) does not starts by itself because it has higher activation energy. We first need to provide some energy input (to reach ignition point) to start reaction, which then becomes spontaneous. Therefore, a reaction with negative standard energy with high activation energy can’t occur by itself, so, in the terms of occurring by itself, it is non-spontaneous.

Option. E. Reaction has negative standard free energy change. The activation energy barrier is NOT specified. So, it such a reaction has very small activation energy, it can occur by itself under standard conditions.

Therefore, it is the most compatible parameter for spontaneity of a chemical reaction.      

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