Difference in values of Gibbs Free Energy


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Rachna Goli 3E
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

Difference in values of Gibbs Free Energy

Postby Rachna Goli 3E » Sun Feb 07, 2016 10:23 pm

Why is the Gibbs Free Energy of formation 3.11 kJ/mol for Br2 gas but only 0 kJ/mol for H2 gas?

Adam Kwok 3L
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Difference in values of Gibbs Free Energy

Postby Adam Kwok 3L » Sun Feb 07, 2016 10:31 pm

Because Br2 gas is not the standard state for bromine at 25°C—it's standard state is liquid, this means that the enthalpy of formation is not zero. In addition, the entropy is not zero because the formation reaction is Br2 (l) --> Br2 (g). Thus the Gibbs Free Energy of formation is not zero. At 25°C, hydrogen is in its standard state as a gas, so the formation reaction is H2 (g) --> H2 (g). Therefore, the enthalpy of formation and entropy are zero, and the Gibbs Free Energy of formation is zero. You can think of it also in terms of no spontaneity for an element in standard state to react if left by itself. The Gibbs Free Energy of formation of any element in its standard state is zero.
Last edited by Adam Kwok 3L on Sun Feb 07, 2016 10:46 pm, edited 1 time in total.

VictoriaRoderick_3A
Posts: 42
Joined: Fri Sep 25, 2015 3:00 am

Re: Difference in values of Gibbs Free Energy

Postby VictoriaRoderick_3A » Sun Feb 07, 2016 10:31 pm

I believe it is because H2 is the natural state of Hydrogen, while Br2 is not the natural state of Bromine.


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