Science
15 Mar 09
27 Mar 09
Originally posted by AThousandYoungThe energy input is called the activation energy
That's right. Some increases in entropy require energy input. I guess you've disproven my hypothesis.
There is amajor difference between a spontaneous process and a feasible process.
Spontaneous means will occur without any external input (e.g. diffusion of gases)
A feasible process e.g. Mg + O2 > 2MgO
Is feasible but not spontaneous. (i.e it leads to an increase intotal entropy but has a large activation energy).
27 Mar 09
Originally posted by twhiteheadCrystal formation can be a feasibe process. Crystal formation is course a decease in entropy of the system, but if the increase in entropy (from the exotherimic reaction) compensates this decrease then it will be feaaible!
The Second Law applied to everything. Obviously you must take into account the total entropy.
Are you claiming that crystals cannot form in a closed system? Or are you saying it takes work to form crystals? I disagree on both points.
To give another example, the effects of gravity frequently results in a system becoming more ordered. Simply take a gla ...[text shortened]... t its core and water on its surface, it would not slowly turn into a perfectly uniform mud ball.
27 Mar 09
Originally posted by dannyUchihaIt decreases the entropy of the system.
Of course it will form water.
You have contradicted yourself and reassured my point.
Hydrogen exists as H2 and oxygen as O2. The formation of water needs the breaking of both bonds and the formation of the H-O bonds present in water. This leads to an increase in entropy, just like I suggested.
With this we can see that neither my point nor the second law of thermodynamics are being contradicted here.
The entropy of H2 and O2 is greater than that of H2O, however the heat energy given out increases the entropy of the surroundings by a greater amount leading to an increase in the TOTAL entropy!!
28 Mar 09
1 edit
Originally posted by najdorfslayerCrystals can form in isolated systems. For example, under certain conditions micelles can form regular lattices.
Crystal formation can be a feasibe process. Crystal formation is course a decease in entropy of the system, but if the increase in entropy (from the exotherimic reaction) compensates this decrease then it will be feaaible!
29 Mar 09
Originally posted by najdorfslayerNo, the energy input for melting ice is not activation energy.
The energy input is called the activation energy
There is amajor difference between a spontaneous process and a feasible process.
Spontaneous means will occur without any external input (e.g. diffusion of gases)
A feasible process e.g. Mg + O2 > 2MgO
Is feasible but not spontaneous. (i.e it leads to an increase intotal entropy but has a large activation energy).
29 Mar 09
2 edits
Originally posted by AThousandYoungFair enough I was thinking more of chemical process.
No, the energy input for melting ice is not activation energy.
Activation energy only really applies to a chemical proces as during melting no chemical bonds are beng broken.
All this means is once you go above 273K then the increase in entropy of the system outweighs the decease in entropyof the surroundings.
29 Mar 09
Originally posted by najdorfslayerIt can apply to phase transitions, as well. At atmospheric pressure, water will not freeze a few degrees below freezing point. It needs a catalyst of some sort, usually this is a wall or a contamination in the water. Likewise, if you have some very pure water you can heat it above 100 degrees centigrade if heated quickly enough.
Fair enough I was thinking more of chemical process.
Activation energy only really applies to a chemical proces as during melting no chemical bonds are beng broken.
29 Mar 09
Originally posted by KazetNagorraInteresting, what is the explanation behind this?
It can apply to phase transitions, as well. At atmospheric pressure, water will not freeze a few degrees below freezing point. It needs a catalyst of some sort, usually this is a wall or a contamination in the water. Likewise, if you have some very pure water you can heat it above 100 degrees centigrade if heated quickly enough.
29 Mar 09
2 edits
Originally posted by najdorfslayerIt has to do with interfacial energies. For a liquid to freeze, solid particles have to form inside the liquid. But there is an energy associated with the interface between the solid and the liquid, so this only becomes favourable if the solid particles has a certain characteristic size, say R, which can overcome the inferface "penalty". The interfacial energy scales with R^2, but the energy loss from freezing is a volume related effect and scales with R^3. So for smaller characteristic sizes, the inferface energy will dominate and the solid particle will only grow in size if a thermal fluctuation (the chance of which is related to the Boltzmann factor) causes it to go beyond the limit.
Interesting, what is the explanation behind this?
29 Mar 09
Originally posted by KazetNagorraMmm interesting, this is going outside the realms of my expertise I think 🙂
It has to do with interfacial energies. For a liquid to freeze, solid particles have to form inside the liquid. But there is a negative energy associated with the interface between the solid and the liquid, so this only becomes favourable if the solid particles has a certain characteristic size, say R, which can overcome the inferface "penalty". The int ...[text shortened]... ion (the chance of which is related to the Boltzmann factor) causes it to go beyond the limit.
You are obviously a physicist, I am a chemist! Normally I wouldn't teach it into this depth. (As this is more crossing over into physics).
30 Mar 09
1 edit
Originally posted by KazetNagorraHow far have you got? I have an interest in Relativity theory and have recently bought 'Gravitation' by Misner et al. Hard going but fascinating after the more accessible texts which do not really get to grips with subject.
Indeed, I am a physicist.
30 Mar 09
Originally posted by James DiracNot very far, I'm an undergraduate student, graduating next year in applied physics. Don't know too much about relativity, I have a fair understanding of special relativity but haven't really studied the maths behind the general relativity theory (it's pretty complicated anyway and involves tensor analysis, urgh).
How far have you got? I have an interest in Relativity theory and have recently bought 'Gravitation' by Misner et al. Hard going but fascinating after the more accessible texts which do not really get to grips with subject.
30 Mar 09
Originally posted by KazetNagorraYes. I thought, mistakenly, that a pretty good grasp of Special Relativity theory would provide a fairly painless transition to the General theory. Ha ha, but I have become addicted!
Not very far, I'm an undergraduate student, graduating next year in applied physics. Don't know too much about relativity, I have a fair understanding of special relativity but haven't really studied the maths behind the general relativity theory (it's pretty complicated anyway and involves tensor analysis, urgh).