Why should deltaG=0?

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Tasnia Haider 1E
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Joined: Sat Jul 22, 2017 3:01 am

Why should deltaG=0?

Postby Tasnia Haider 1E » Wed Jan 31, 2018 11:40 am

For the example shown in class today, why does deltaG have to be zero to find the minimum temperature?

Posts: 54
Joined: Sat Jul 22, 2017 3:00 am

Re: Why should deltaG=0?

Postby jillian1k » Wed Jan 31, 2018 12:19 pm

The example given asked at which temperature the reaction would spontaneously occur at 1atm. We know that for the rxn to occur spontaneously, it must have a negative deltaG (otherwise stated as deltaG<0). By stating deltaG as 0, we can substitute two known variables into the eqn for deltaG (deltaG=deltaH-(TdeltaS) and algebraically solve for T. Whereas, if we didn't state deltaG as 0, there would be two unknowns in this equation and we would have no way to proceed with the problem to find the answer.

By stating deltaG is 0 and using an equal sign to solve for T, the final answer for T tells us that this is the exact temperature required for the rxn to have a delta G of 0. Therefore, any temperature higher than this would result in a -deltaG because the (TdeltaS) term would be greater than the delta H term in the eqn for deltaG.

Sally Nason - 1K
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Re: Why should deltaG=0?

Postby Sally Nason - 1K » Wed Jan 31, 2018 1:53 pm

You can also think of it like delta G goes from negative to positive with 0 as the "threshold" so to find at what temperature this threshold is at, you set deltaG equal to 0.

Liz White 1K
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Joined: Thu Jul 13, 2017 3:00 am

Re: Why should deltaG=0?

Postby Liz White 1K » Wed Jan 31, 2018 2:27 pm

The above responses are correct. To elaborate further, solving for G = 0 will give us the boiling point at which the liquid turns into a gas. The question asks us to find the temperature at which the vaporization of Br2 is spontaneous. Since we don't have enough information to solve for any negative value G (where the reaction would be spontaneous), we can instead set G to 0, and solving for this gives us the minimum value for which the liquid bromine can become a gas. Therefore, anything above that minimum value, the boiling point, will result in the element being in its gaseous state. Any increase in temperature will result in a decrease in G, and when G is negative, the reaction is spontaneous.

Dang Lam
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Joined: Thu Jul 27, 2017 3:01 am

Re: Why should deltaG=0?

Postby Dang Lam » Thu Feb 01, 2018 5:27 pm

simple words: set delta G=0 to find the Temperature (T) that will make delta G equal 0. Given that, we know that if we increase T, delta G will be smaller and smaller. So anything Temperature that is > then the T you found will give delta G a negative number

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