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Determining Spontaneity of a Reaction

Posted: Wed Feb 06, 2019 5:00 pm
by Layal Suboh 1I
Hello,

Why do we need to know both enthalpy and entropy to determine if the reaction will occur spontaneously? If the reaction is exothermic, won't the products be favored?

Thanks

Re: Determining Spontaneity of a Reaction

Posted: Wed Feb 06, 2019 6:31 pm
by David Effio 1H
In terms of entropy, if the DeltaS value is positive, the reaction is spontaneous, while if the DeltaS value is negative, the reaction is non-spontaneous. This is a good way to remember this.

Another way is when measuring the change in volume, if V1 is greater than V2, then the reaction is non-spontaneous. This is because you needed to put in energy to shrink the total volume occupied.

Meanwhile, if V1 is less than V2, then the reaction is spontaneous, as atoms naturally will occupy space on their own, without needing an energy input.

Yes, you are right if the reaction is exothermic, it means the products are favored, and therefore it is spontaneous!

Re: Determining Spontaneity of a Reaction

Posted: Thu Feb 07, 2019 12:23 am
by inlovewithchemistry
The LA in a review session I went to today also said that Gibbs free energy, or delta G, is a concept that helps us determine spontaneity. We haven't learned about it yet in lecture, but I think this would be a good reference point for your question.

Re: Determining Spontaneity of a Reaction

Posted: Fri Feb 08, 2019 9:05 pm
by EllaBerry
We use Gibbs Free energy to determine spontaneity. Gibbs free energy is determined using entropy and enthalpy, and if the value of Gibbs free energy is negative, than the reaction is spontaneous. Therefore, entropy and enthalpy help to determine whether a reaction is spontaneous.

Re: Determining Spontaneity of a Reaction

Posted: Sun Feb 10, 2019 8:08 pm
by Andrew Bennecke
Usually, we would use Gibbs Free Energy to find the spontaneity of a reaction; however, since we won't be using Gibb's on the midterm, the 7th edition indicates another way in table 4I.1. If delta S total > 0, then the reaction is spontaneous.