free energy diagram question

[theonlytycrane]

The correct answer given is D. My initial interpretation was that since D is unfavorable in the forward direction, it wouldn't proceed at all. Based on the way the question is worded I guess D is still correct, except it would proceed the fastest in the reverse direction.

If the question specifically said forward, C would be correct. Yes?

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[Astra]

Fastest means kinetic product.

Kinetic product is determined by the Energy of Activation or the size of the hill to get to the product.

So looking at each hill, we can see that product D has the smallest amount of energy needed to get to the product.

Therefore, the answer is D.

http://mcat-review.org/kinetics-thermodynamics.gif

As for the forward reaction, the answer would still be D because it is asking the kinetic product.

https://upload.wikimedia.org/wikipe...ic_Considerations_for_a_Chemical_Reaction.png

 

[aldol16]

Fastest means kinetic product.

Kinetic product is determined by the Energy of Activation or the size of the hill to get to the product.

So looking at each hill, we can see that product D has the smallest amount of energy needed to get to the product.

You're confusing the concept of a kinetic product with the overall drive for a reaction. Assuming standard conditions, delta G0 for the "D" reaction is positive, meaning it is not spontaneous. That means that even if the kinetic barrier to reaction is low, it's not going to happen spontaneously because it's gonna be uphill. Now, this would also depend on the height of the kinetic barrier, which is not given in this diagram. If it's <20 kcals or so, it will equilibrate because in that case, there's enough thermal energy at RT to not only drive the reaction forward thermodynamically but also kinetically as well. Now, the problem is that they don't give you any values and as written, it's not a good question. The only logical conclusion is that the D would not react - you would observe no products since the only logical assumption is that the delta G0 is larger than allowed at RT.

The correct answer given is D. My initial interpretation was that since D is unfavorable in the forward direction, it wouldn't proceed at all. Based on the way the question is worded I guess D is still correct, except it would proceed the fastest in the reverse direction.

The answer should simply be C because the "reaction" D would not proceed at all. Thermodynamics tells us that a reaction with delta G0 > 0 is non-spontaneous under standard conditions, i.e. it does not react no matter how small the kinetic barrier.

 

[Astra]

You're confusing the concept of a kinetic product with the overall drive for a reaction. Assuming standard conditions, delta G0 for the "D" reaction is positive, meaning it is not spontaneous. That means that even if the kinetic barrier to reaction is low, it's not going to happen spontaneously because it's gonna be uphill. Now, this would also depend on the height of the kinetic barrier, which is not given in this diagram. If it's <20 kcals or so, it will equilibrate because in that case, there's enough thermal energy at RT to not only drive the reaction forward thermodynamically but also kinetically as well. Now, the problem is that they don't give you any values and as written, it's not a good question. The only logical conclusion is that the D would not react - you would observe no products since the only logical assumption is that the delta G0 is larger than allowed at RT.



The answer should simply be C because the "reaction" D would not proceed at all. Thermodynamics tells us that a reaction with delta G0 > 0 is non-spontaneous under standard conditions, i.e. it does not react no matter how small the kinetic barrier.

When they said identical conditions, I assumed a scenario where the temperature was high enough to drive the nonspontaneous reaction forward and the other spontaneous as well

If they said standard conditions, I definitely would have said what you said

 

[aldol16]

When they said identical conditions, I assumed a scenario where the temperature was high enough to drive the nonspontaneous reaction forward and the other spontaneous as well

Yes, that is why this is a horrible question. It definitely does not show an understanding of thermodynamics at all and for a test prep company, that's pretty bad. Anyway, you would need to assume that there is enough thermal energy not just for the delta G of the "D" reaction but enough for the delta G (double dagger) for that reaction.

 

[NextStepTutor_2]

The correct answer given is D. My initial interpretation was that since D is unfavorable in the forward direction, it wouldn't proceed at all. Based on the way the question is worded I guess D is still correct, except it would proceed the fastest in the reverse direction.

If the question specifically said forward, C would be correct. Yes?

View attachment 201861

The potential outcome of a reaction is usually influenced by two factors:

1. the relative stability of the products (thermodynamic factors)
2. the rate of product formation (kinetic factors)

Long story short, kinetics =/= thermodynamics. This is a poorly worded Q without sufficient information to make a definitive answer. We would expect energetically favorable (spontaneous) reaction to proceed faster but they do not have to be, and vice versa.

A good rule of thumb to use is:

At low temperatures, the reaction is under kinetic control (rate, irreversible conditions) and the major product is that from fastest reaction.

At high temperatures, the reaction is under thermodynamic control (equilibrium, reversible conditions) and the major product is the more stable system


A good "guess" would be a reaction with a lower Ea AND a favorable energy change moving forward. This diagram is also very tough to follow since they made the lines all the same color and pattern. This is not AAMC like, luckily.

Hope this helps, Good luck!