The two e's

[vrus]

What are entropy & enthalpy ? Are they related ? If so, how ? And what is 'Gibb's free energy' ? Thnks.

[insane_alien]

enthalpy entropy and gibbs free energy are all related by

i use d instead of delta here(need to learn latex)

 

dG=dH-TdS

 

where

dG = change in gibbs free energy(J)

dH = change in enthalpy(J)

T = temperature(kelvin)

dS = change in entropy(J)

[vrus]

enthalpy entropy and gibbs free energy are all related by

i use d instead of delta here(need to learn latex)

 

dG=dH-TdS

 

where

dG = change in gibbs free energy(J)

dH = change in enthalpy(J)

T = temperature(kelvin)

dS = change in entropy(J)

 

Yeah I heard about that formula, but what do they actually mean ?

[insane_alien]

i'm not sure about gibbs energy(chem teacher just said "nevermind as long as you use it")

when enthalpy goes down(a negative value for dH) you get heat out so its exothermic.

the opposite is endothermic.

the entropy is ... well i'm sure someone can explain it in more detail than i could

[jdurg]

Gibbs Free Energy is a measure of the spontonaity(sp?) of a reaction. If dG is negative, then the reaction will occur spontaneously at that temperature. If it's positive, then energy needs to be put into the system in order for the reaction to take place. The equation dG = dH - TdS says that when the enthalpy goes down (i.e. reaction is exothermic) and the entropy goes up (i.e. the system becomes more disordered), the reaction will be spontaneous at that temperature.

 

As an example, we'll look at the conversion of oxygen gas (O2) into ozone.

 

The reaction 3O2(g) => 2O3(g) has a dH of +284.6 kJ, and a dS of -0.1398 kJ/K. Right away, we can see that this will probably NOT be a very spontaneous reaction.

 

The equation dG = dH - TdS gives us a value of +326.28 kJ for the change in Gibbs Free Energy. This tells us that the reaction of oxygen gas forming ozone is NOT spontaneous and is NOT energetically favorable. It also tells us that in order for the reaction to occur, you MUST put energy INTO the system. Real life experiences tell us that this is correct as ozone doesn't form on its own. It only forms if there is a lot of energy put into an oxygen rich atmosphere. (Typically when there's an electrical discharge).

 

So simply put, the dG value tells you if the reaction will occur on its own, or if energy must be put into the system. A negative value indicates a spontaneous reaction. A positive value indicates a non-spontaneous reaction.

[insane_alien]

oh yeah now i remember

[vrus]

A negative value indicates a spontaneous reaction. A positive value indicates a non-spontaneous reaction.

 

Thanks. That really clears things up. However, how would you define spontaneous ? I've never really used that word before. I think I know, but its better if you clear it up.

 

Another thing; doesn't entropy have many different meanings ? Someone told me it it is a calculation of all the possible ways a reaction can go ?

What do you say about this ?

[jdurg]

Entropy is a measure of disorder. It's kind of an abstract value and I'm not fully sure how it's directly measured, but basically speaking the more entropy a system has, the less ordered the molecules/atoms are. Solids have low entropy because the atoms are aligned in a set pattern, while gases have a lot of entropy because they can go in any which way.

 

Being spontaneous means the reaction will proceed on its own without any need to input energy. If you mix hydrogen gas and oxygen gas, it will slowly happen, but the two will react and form water. (If you add a catalyst or a little bit of heat, then it proceeds quite rapidly). As a result, the Delta G for the reaction of hydrogen and oxygen is pretty negative. On the other hand, if you have hydrogen fluoride dissociating into hydrogen and fluorine gas, the Delta G for that reaction is VERY positive. That means that no matter how long you wait, you will NEVER see the HF dissociate into H and F. The only way it will happen is if you put energy into the system.

 

Here's another analogy. Let's say that you have a block of ice outside on a warm day as well as a beaker of water. The 'reaction' of the ice melting into water is a spontaneous occurance. It might take a while, but if you wait long enough all the ice will melt. The delta G of the reaction H2O(s) => H2O(l) at 300K is negative. The solid will spontaneously form the liquid. On the other hand, the reaction of H2O(l) => H2O(s) is NOT spontaneous. You can wait all you want, but the water will not solidify at 300K. The delta G for the 'reaction' is positive. You have to put energy into the system in order to get the water to solidify.

[CanadaAotS]

i never got all that stuff about entropy, but it makes sense now, thanks

[vrus]

Entropy is a measure of disorder. It's kind of an abstract value and I'm not fully sure how it's directly measured' date=' but basically speaking the more entropy a system has, the less ordered the molecules/atoms are. Solids have low entropy because the atoms are aligned in a set pattern, while gases have a lot of entropy because they can go in any which way.

 

Being spontaneous means the reaction will proceed on its own without any need to input energy. If you mix hydrogen gas and oxygen gas, it will slowly happen, but the two will react and form water. (If you add a catalyst or a little bit of heat, then it proceeds quite rapidly). As a result, the Delta G for the reaction of hydrogen and oxygen is pretty negative. On the other hand, if you have hydrogen fluoride dissociating into hydrogen and fluorine gas, the Delta G for that reaction is VERY positive. That means that no matter how long you wait, you will NEVER see the HF dissociate into H and F. The only way it will happen is if you put energy into the system.

 

Here's another analogy. Let's say that you have a block of ice outside on a warm day as well as a beaker of water. The 'reaction' of the ice melting into water is a spontaneous occurance. It might take a while, but if you wait long enough all the ice will melt. The delta G of the reaction H2O(s) => H2O(l) at 300K is negative. The solid will spontaneously form the liquid. On the other hand, the reaction of H2O(l) => H2O(s) is NOT spontaneous. You can wait all you want, but the water will not solidify at 300K. The delta G for the 'reaction' is positive. You have to put energy into the system in order to get the water to solidify.[/quote']

 

 

That's a great explanation! Thanks again!

[jdurg]

No problem. Glad to have helped. Entropy is just a really odd concept that can be hard to grasp. Probably because you really can't directly measure the amount of 'disorder' of something.

 

If you examine the Gibbs Free Energy equation, you'll see that it actually makes a lot of sense. The equation says that reactions which give off heat and result in an increase in the amount of disorder will be energetically favorable. That is, they'll tend to happen spontaneously. If you look at explosive compounds, you'll see that they all give off a lot of heat (they're very exothermic) and the products are typically all gases. As a result, the entropy goes up dramatically. This creates a very positive value for TdS which results in a very negative Delta G. As a result, virtually are explosive compositions are inherently unstable and will decompose over time. The most sensitive mixtures have an incredibly negative Delta G value, hence why they are so unstable. (I tried to find the entropy of formation of nitrogen triiodide in order to give an example, but I don't have a CRC handy so I can't look that up. The dH of the decomposition of crystalline NI3 is VERY large and negative, and all the products are gases, so one would expect the entropy of that reaction to be VERY large and positive. As a result, the Delta G is VERY large and negative and the reaction proceeds spontaneously.)

 

Knowing that equation will also help explain why some reactions are considered spontaneous even though the enthalpy is positive. A reaction can have a very positive Delta H, but if the entropy of the system increases, then at certain temperatures the reaction will spontaneously occur. It's a neat equation when you think about it.