Meaning of Spontaneity






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Henry Krasner 1C
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Joined: Fri Sep 28, 2018 12:15 am

Meaning of Spontaneity

Postby Henry Krasner 1C » Sun Feb 17, 2019 1:57 pm

Why is the change in free energy spontaneous when delta G is less than zero, but NOT when it is greater than zero?

Nathaniel 2E
Posts: 27
Joined: Mon Jun 25, 2018 3:00 am

Re: Meaning of Spontaneity

Postby Nathaniel 2E » Sun Feb 17, 2019 2:22 pm

The equation of the change in Gibbs Free Energy can be of help:

delta G = delta H - T*delta S

The change in the free energy of the system measures the balance between the two driving forces that determine whether a reaction is spontaneous, enthalpy and entropy.

The change in enthalpy is favorable for the reaction when it is less than 0, and unfavorable when greater than 0. The change in entropy is favorable for a reaction when it is greater than 0, and unfavorable when less than 0.

Delta G is negative for any reaction for which delta H is negative and delta S is positive. Delta G is therefore negative for any reaction that is favored by both the enthalpy and entropy terms. We can therefore conclude that any reaction for which delta G is negative should be favorable, or spontaneous.

Conversely, delta G is positive for any reaction for which delta H is positive and delta S is negative. Any reaction for which delta G is positive is therefore unfavorable.

Andie Jian 1D
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Re: Meaning of Spontaneity

Postby Andie Jian 1D » Sun Feb 17, 2019 2:45 pm

A reaction is spontaneous if the entropy of the universe is increasing (Stot=Ssys+Ssurr). We went over in lecture how this equation can be related to the equation for Gibb's free energy. Therefore, it makes sense that Gibb's free energy should be negative if a reaction is spontaneous, because if the entropy of the universe is increasing with every spontaneous reaction, then the free energy within the system should also be decreasing. For example, with many spontaneous reactions in the body, they are exothermic and therefore give off heat, but that energy given off in heat is unable to do work now. So, the entropy of the universe has increased due to it receiving the heat given off by the reaction, but the free energy of the system has decreased.

Jonas Talandis
Posts: 60
Joined: Fri Sep 28, 2018 12:16 am

Re: Meaning of Spontaneity

Postby Jonas Talandis » Sun Feb 17, 2019 4:52 pm

Like all concepts and equations in thermodynamics, etc., they always refer back to the system itself (e.g. Work the system did on the surroundings, work the surroundings did on the system). Therefore, if DeltaG is negative, we know that it represents that the system does not need to use energy to carry out the reaction. If it's positive, that's how much energy is required to carry out the reaction (activation energy).


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