[pjoseph]

Dear All

Can't get my head around this.

Entropy of the universe (system+surroundings) always increases.
But why?
G = H - TS
G = Energy available for work
H = Enthalpy of reaction
How can the energy available be greater than the enthalpy of the reaction?
There must be some input from the 'rearrangement' of the molecules?
Ok I can accept that.
When S is positive for a reaction there is more energy from the work than H dictates.
How can it come from a a reaction becoming more disordered?
This requires breaking bonds eg. Solid to liquid which REQUIRES energy. Logically more disordering reactions should be less spontaneous due to the additional energy required to break up the ordered molecules.

Thereby making it more difficult to break things up than to put them together.
Please Help

[tamim83]

It is easier to look at history and experimental observations, then try and make the math work.  Historically, one of the early attempts to come with criteria for spontaneity came from Berthelot in 1867.  He proposed that all exothermic reactions are spontaneous.  Yet, we know that there are some spontaneous endothermic reactions and processes as well.  For example, boiling water is endothermic, yet it is spontaneous.  There are other examples as well so they came to the conclusion that  :delta:H<0 is not an all inclusive requirement for spontaneity. 

Now if you look at the Gibb's free energy:
 :delta:G =  :delta:H - T :delta:S.

you can see that T :delta:S is a correction to Berthelot's hypothesis.  We are saying that exothermic reactions that have a positive  :delta:H will be spontaneous.  Also, endothermic reactions with a large positive  :delta:S will be spontaneous.  So now spontaneity depends on both the breaking and making of bonds as well as entropy.  There is a competition between entropy and enthalpy.   In addition, temperature comes into play.  Bond formation is spontaneous when it is enthalpy driven, even though it is disfavored entropically. 

Hope this helps some. 

[pjoseph]

I understand the maths works but explaining it doesnt make sense. When a chemical reaction becomes more disordered it must break bonds. In doing this it must require energy meaning it should be LESS Spontaneous, not more as the maths dictates from this equation.

This goes against all basic thermodynamics regarding bond breaking /making doesnt it?

[Borek]

When a chemical reaction becomes more disordered it must break bonds.

The more I am trying to understand what you mean, the less I understand. Chemical reaction doesn't become disordered. Chemical reactions are not more/less disordered.

This goes against all basic thermodynamics regarding bond breaking /making doesnt it?

Well - not. Quite the opposite. 2nd law was tested and tested and proven to work, so it goes against your understanding of the problem. But as I am not able to understand your understanding, I have no idea where your mistake can be.

[Juan R.]

It is a basic law of Universe that the entropy always increases, but nobody knows why for sure. Some people is trying to explain why, but the topic is very difficult and there is not consensus. The chemist Ilya Prigogine has tried for years to derive the Second law of thermodynamics from some more basic principle. This is also know as the problem of the arrow of time. His last theories are presented to public in his book the End of Certainty.

http://order.ph.utexas.edu/Prigogine.htm

http://www.amazon.com/End-Certainty-Ilya-Prigogine/dp/0684837056

[McCoy]

When a chemical reaction becomes more disordered it must break bonds.

The more I am trying to understand what you mean, the less I understand. Chemical reaction doesn't become disordered. Chemical reactions are not more/less disordered.

Hi Borek, what do you mean you don't understand what the guy said?
Well it's true that there're some 'disorderness' that arise as a result of chemical reactions. A more basic one will be oxidation of glucose: C₆H₁₂O₆(s) + 6O₂(g)  :rarrow:6H₂O(l) +6CO₂(g)...obviously, there's disorderness here.

[Borek]

Just because disorder grows as the result of the reaction, doesn't mean reaction is disordered.

[McCoy]

Just because disorder grows as the result of the reaction, doesn't mean reaction is disordered.

Okay, now I got your point. However, the chaos that results from some chemical reactions is what the poster mean.
Cheers.