Spontaneity of Reactions

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Isabella Shahmirza 2H
Posts: 30
Joined: Fri Sep 25, 2015 3:00 am

Spontaneity of Reactions

Postby Isabella Shahmirza 2H » Sun Jan 24, 2016 12:20 am

When delta S is positive and delta H is positive, this means that the reaction or physical change is spontaneous at high temperatures. But isn't it possible for delta G to be a positive value from this, which would make the reaction not spontaneous. Therefore, is that why the reaction depends on a high temperature in order for it to be spontaneous?
Similarly, when delta S is negative and delta H is negative, in order for the reaction or physical change to be spontaneous is when the temperature is at low temperatures, because if not, then delta G would be a positive value and not spontaneous?

Heerali Patel 3A
Posts: 26
Joined: Fri Sep 25, 2015 3:00 am

Re: Spontaneity of Reactions

Postby Heerali Patel 3A » Sun Jan 24, 2016 12:26 am

Yes, you are correct. When delta S and delta H are both positive, a spontaneous reaction can only occur at high temperatures because this is the only way T*delta S will remain higher (in value) than delta H, such that delta G will be negative and the reaction will be spontaneous. Similarly, when both delta S and delta H are negative, temperature must remain low for spontaneity because, in this case, delta H must be higher than T*delta S so delta G can be negative for a spontaneous reaction.

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