Chemistry / Thermodynamics : what is gibs free energy?
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it's denoted by (change of)G
and by equation
(change of)G = (change of)H - T x (change of)S
(change of)S = entrophy , aka "disorder number"
(change of)H = enthaply , energy change of equation
(change of)G is -ve ==> reaction is spontenous forward
(change of)G is +ve ==> reaction is spontenous backwards
question : what does this (change of)G signify?
the excess energy left in the molecules after the surroundings has taken up some of it after a reaction?
the energy created after a reaction?
the energy contributed by the surroundings?
i read my textbook many times liao even more blur! pls try to explain in a way i can understand this concept
experts pls teach a supid idiot like me!
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Wah, this is the chemistry portion of thermodynamics. Chemistry portion of thermodynamics = sianz~~~
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hahaha mi havent study G yet.. exam oli up till entropy and enthalpy... the rest are chemical kinetics.. u study hard ah, nxt time when i reach that GIBs part i dun know i come and ask u wahahaha.
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Han Hao,
You see, Gibbs Free Energy is a State Function.
Useful energy, or energy available to do work, is of central interest because there is never a shortage of useful work to be done, and usually an insufficiency of useful energy to do it. Chemical reactions can be made to do work, or to serve as pathways for the conversion of heat into work. Energy which is, or which can be, available to do useful work is called Gibbs free energy or more often simply free energy. Free energy is called Gibbs free energy and given the symbol G in honor of Josiah Willard Gibbs, a professor of chemistry at Harvard University who almost singlehandedly developed both the concept and the quantitative equations which describe it.
Free energy is a state function because it is formally defined only in terms of state functions, the state functions enthalpy and entropy, and the state variable temperature. The definition of free energy is:
G = H - TS
In the above equation H is the enthalpy, S is the entropy, and T is the absolute temperature.
A change in free energy dG would then be given by dG = dH - d(TS). Since temperature is a state variable and entropy is a state function they are separable, and so dG = dH - TdS - SdT. However, at constant temperature the change in temperature dT is zero, so the term SdT is zero also. For any constant-temperature (isothermal) change, then dG = dH - TdS. Rearrangement of this equation gives, for larger changes, the Gibbs-Helmholtz equation:
DH = DG + TDS.
In a different section it was shown that TdS is an energy, the energy not available for doing work. The equation above therefore has the physical meaning
total energy available as heat = DG + energy not available for doing work
It therefore follows that DG must be the energy which is available for doing work. This free or available energy which can do useful work is then a most useful thing to know or to have.
The conventions of heat flow q are also those of enthalpy change. When DH is negative, heat is evolved (or given off) in the exothermic process or reaction, while when DH is positive, heat is absorbed in the endothermic process or reaction from the surroundings. Free energy change also follows the same conventions. When DG is negative, energy is evolved (given up) in the process or reaction and the change can do useful work on something else. This is a spontaneous process, and is sometimes called an exergonic (energy-giving) process. When DG is positive, energy is absorbed in the process or reaction and useful work must be done on the materials so that the reaction will occur. This is a nonspontaneous process, and is sometimes called an endergonic (energy-requiring) process.
To know more about chemical dynamics, goto this website:
http://www.psigate.ac.uk/newsite/reference/plambeck/chem2/ua102.html -
thnx Dr Pepper! i understand liao!!!!!!!!!!!Originally posted by DrPepper:The conventions of heat flow q are also those of enthalpy change. When DH is negative, heat is evolved (or given off) in the exothermic process or reaction, while when DH is positive, heat is absorbed in the endothermic process or reaction from the surroundings. Free energy change also follows the same conventions. When DG is negative, energy is evolved (given up) in the process or reaction and the change can do useful work on something else. This is a spontaneous process, and is sometimes called an exergonic (energy-giving) process. When DG is positive, energy is absorbed in the process or reaction and useful work must be done on the materials so that the reaction will occur. This is a nonspontaneous process, and is sometimes called an endergonic (energy-requiring) process.
thnx a million!
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dr pepper expert ah hahaha. gr8 man! i got problem i dun know can ask you liaoz.
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so, is that why fuel is not always fully utilised and much of the heat given off can't be used for driving the engine of a car...sorry, never study thermo b4 and a n00bz in chem...but just want to know...thanks
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Dr Pepper...do u do microbiology as well?
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Gibb's free energy is a measure of whether something wanna happen. if G is < 0, means umm like change in entropy times temperature is more than Heat cuz G = H - TS right. then if entropy liddat means then thing wanna happen cuz like entropy just makes things screw up. for example, your room very neat and tidy after u clean, then a few days later become messy and dirty right? thats cuz your room got entropy. G < 0 means the thing wanna happen. if G > 0, that means the thing probably wont happen. for example u release a balloon the air inside will want to come out, you dont see air outside trying to get inside the balloon right. liddat lor I also dunno wtf is Gibb's free energy lol next week got chemistry exam wtfwtfwtf.Originally posted by HanHao:it's denoted by (change of)G
and by equation
(change of)G = (change of)H - T x (change of)S
(change of)S = entrophy , aka "disorder number"
(change of)H = enthaply , energy change of equation
(change of)G is -ve ==> reaction is spontenous forward
(change of)G is +ve ==> reaction is spontenous backwards
question : what does this (change of)G signify?
the excess energy left in the molecules after the surroundings has taken up some of it after a reaction?
the energy created after a reaction?
the energy contributed by the surroundings?
i read my textbook many times liao even more blur! pls try to explain in a way i can understand this concept
experts pls teach a supid idiot like me! -
exam on thur izzit?Originally posted by the.raven:Gibb's free energy is a measure of whether something wanna happen. if G is < 0, means umm like change in entropy times temperature is more than Heat cuz G = H - TS right. then if entropy liddat means then thing wanna happen cuz like entropy just makes things screw up. for example, your room very neat and tidy after u clean, then a few days later become messy and dirty right? thats cuz your room got entropy. G < 0 means the thing wanna happen. if G > 0, that means the thing probably wont happen. for example u release a balloon the air inside will want to come out, you dont see air outside trying to get inside the balloon right. liddat lor I also dunno wtf is Gibb's free energy lol next week got chemistry exam wtfwtfwtf.