Please help explain Gibbs Free Energy to me...

[Cofo]

Jesus Christ...I have a feeling I am making this WAY harder than it really is. 🙁
As of now, my understanding is that G = free energy. Am I correct or incorrect, and if I'm incorrect...tell me what G actually is. i just think my brain is fried right now

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[wantVCUdental]

I have been studying all day...and now I'm stuck trying to figure out Gibbs Free Energy.
If the Standard Gibbs Free Energy (change in G) is negative...is this basically saying that there is a LOSS of free energy? And if so...wouldn't this mean the reaction is not spontaneous? Jesus...I have a feeling I am making this WAY harder than it really is. 🙁
As of now, my understanding is that G = free energy. ...

Hi Cofo,
The easiest way to understanding the spontaneity of Gibbs free energy can be found in the formula that calculates G
G=H-TS
T=temperature
S=entropy (measure of disorder)
Let's ignore H in our analysis and say that G is solely dependent on -TS
G=-TS
the key is looking at entropy. The world is going from a more ordered state to a less ordered state, so the level of disorder is constantly increasing, therefore S>0. Temperature is measured in Kelvin, it's always positive. So in a NATURAL spontaneous process(which means S>0) the Gibbs free energy is a negative value.
because G=-TS, T>0, S>0, that must mean G<0
To reinforce this point,consider DNA/Protein synthesis, these processes take ATP (energy) to run, and they are building from small to large, therefore entropy is decreasing. From this we can also see that S<0 is a non-spontaneous process that takes energy.
And when S<0
G=-TS, T>0(ALWAYS), S<0, G>0 therefore non-spontaneous
hope this helps

 

[AF11]

Hi Cofo,
The easiest way to understanding the spontaneity of Gibbs free energy can be found in the formula that calculates G
G=H-TS
T=temperature
S=entropy (measure of disorder)
Let's ignore H in our analysis and say that G is solely dependent on -TS
G=-TS
the key is looking at entropy. The world is going from a more ordered state to a less ordered state, so the level of disorder is constantly increasing, therefore S>0. Temperature is measured in Kelvin, it's always positive. So in a NATURAL spontaneous process(which means S>0) the Gibbs free energy is a negative value.
because G=-TS, T>0, S>0, that must mean G<0
To reinforce this point,consider DNA/Protein synthesis, these processes take ATP (energy) to run, and they are building from small to large, therefore entropy is decreasing. From this we can also see that S<0 is a non-spontaneous process that takes energy.
And when S<0
G=-TS, T>0(ALWAYS), S<0, G>0 therefore non-spontaneous
hope this helps

Holy crap you're a good teacher. Couldn't have put it better myself!

 

[Cofo]

thank you wantVCUdental. 👍

 

[redchesus]

and they are building from small to large, therefore entropy is decreasing.

Can you elaborate on this? I always thought more complex molecules had more disorder, so a macromolecule like a protein would have absolute entropy (S) larger than that of the smaller amino acids

so wouldn't the change in entropy (&#916;S) be >0 not <0?

or is my reasoning wrong?

 

[wantVCUdental]

Can you elaborate on this? I always thought more complex molecules had more disorder, so a macromolecule like a protein would have absolute entropy (S) larger than that of the smaller amino acids

so wouldn't the change in entropy (&#916;S) be >0 not <0?

or is my reasoning wrong?

No, your reasoning is correct, there are two things that you need to consider when you look at entropy. 1. number and 2. size.
If we get down to the details, number is the biggest factor. And that means going from 1 molecule to 2 molecules that's a definite increase in entropy. and going from 2 molecules to 1 molecule is a definite decrease in entropy. That's where i drew the example of DNA/Protein synthesis.

Now, what you are saying is also correct. However, your concept only applies to what are called homologous series. For example, molecules with CnH2n+2, Methane, Ethane, Propane are a homologous series.
If we look at methane, there is not much entropy based on this molecule, however looking at ethane, a bigger molecule, there is an increase in entropy, because now you have introduced 1 more level of rotation across the C-C bond.
Now going from ethane to propane, you are again increasing the possibility of disorder, because now there are 2 C-C bonds that can rotate.
Therefore if you increase the size of a molecule in a homologous series, you also see increase in entropy 😀

In the case where you have both a change in number of molecules and a change in size of a molecule(like DNA synthesis), the former is always the predominant factor.
Hope i've explained that clearly.
Good luck

 

[UCB05]

There's a lot of different concepts floating around, so I thought I might try to help clear things up. Gibbs free energy generally refers to the energy of a system and is represented by the equation delta(G) = delta(H) - Tdelta(S). The delta means you're comparing to different states, so you have to know what it is you are talking about first.

In a molecular comparison, S represents the internal entropy of a molecule, which refers to its degrees of freedom. A methane molecule has simple C-H bonds which have no complex orientation. An ethane molecule has the same C-H bonds, but also has a rotating C-C bond, which increases the molecule's degrees of freedom (staggered, eclipsed, gauche, etc), so it will have greater internal entropy than methane. An ethene molecule has a stiff double bond with no rotation, so it will have less entropy than ethane.

when dealing with multiple atoms within a system, as VCU said, this trumps molecule size. imagine an ethane molecule in a box. it is one simple molecule bouncing around with internal rotation. However, imagine 2 methane molecules in the same box. Neither have simple intramolecular rotational states, but there are infinitely more spatial states in which the two molecules can exist within the box, states that are impossible for a single molecule to obtain.

delta(H) represents change in internal energy. This can be found in bond energy, thermodynamic and electrical favorability/unfavorability. ATP has high internal energy due to 3 electron-dense clouds of phosphates held right next to each other. breaking of a phosphate bond represents a huge drop in (H) and therefore delta(G).

Next is systemic free energy as a whole. delta(G) measures the difference in energy of two different states and the propensity of one state converting to another. It is similar to potential energy in that regard. A rock at the top of a cliff has more potential energy than a rock on the ground. Given the means, that rock will want to spontaneously fall down the cliff, but you'll likely never see the rock on the ground fall up onto the cliff. Likewise, a state with higher energy will prefer to achieve a state of lower energy, hence why spontaneous = -delta(G).

There are two terms to the equation: delta(H) -Tdelta(S).
delta(H) < 0, Tdelta(S)>0 -- delta(G)<0, always spontaneous
delta(H) > 0, Tdelta(S)<0 -- delta(G)>0, always nonspontaneous
delta(H) > 0, Tdelta(S) >0 -- delta(G)<0 and spontaneous when the Tdelta(S) term is greater than delta(H), which occurs when T or delta(S) is very high
delta(H) < 0, Tdelta(S)<0 -- delta(G)<0 and spontaneous when delta(H) is much more negative than Tdelta(S) is positive.

Also remember two things: spontaneity =/= reaction speed, and delta(G)=0 represents equilibrium, not no reaction. Forward and reverse reactions proceed, but at equal rates.

I hope this helped and didn't make things more confusing~

 

[AfroChapin]

If the Delta H is positive and the Delta S is negative the reaction is non-spontaneous at all temperatures.

If I plug in some numbers the Delta G comes out to be negative, making it spontaneous?

 

[TurkTurkleton]

If the Delta H is positive and the Delta S is negative the reaction is non-spontaneous at all temperatures.

If I plug in some numbers the Delta G comes out to be negative, making it spontaneous?

delta G = delta H - (T)(delta S)

Look at it like this - there are 4 different ways you can arrange the signs for delta H & delta S, giving varying values of delta G

The first two are clear and what we're more familiar with:

(-) delta H and (+) delta S --> (-) delta G always, spontaneous at all temps
(+) delta H and (-) delta S --> (+) delta G always, nonspontaneous at all temps

but then things get a little complicated and you have to consider the effect of temperature

(-) delta H and (-) delta S --> (+) and (-) delta G values, spontaneous only at low temps
(+) delta H and (+) delta S --> (+) and (-) delta G values, spontaneous only at high temps

[High and low temps are relative, obviously.]

Temperature influences how much effect delta S has on the overall delta G. A higher temperature will increase the effect of delta S. A lower temperature will result in delta S having a smaller impact.