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Offline PFScience

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Chemical potential energy query
« on: June 19, 2015, 04:49:31 PM »
During my studies and general course reading, I have come across some conflicting messages regarding the nature of chemical potential energy. Can anyone help clarify this?

I like the basic concept of PE as being energy stored due to overcoming a given force... the classic example being the ball at the top of the hill - it got there by some means, that involved work done overcoming gravity.... fine no probs.

I also like the notion of gases having more PE due to the atoms by definition having to have more of it in order so that they can overcome the VDW forces that would otherwise force them into states of liquidity or solidity.

Now where the conflicts arise in terms of PE is where bonding is concerned. Many sources cite PE as being stored in chemical bonds but I have seen others and my uni lecturer has stated, that this is misleading.. PE is NOT stored in chemical bonds. My lecturer in fact went on to discuss how the phrase "bond breaking" is also misleading as it gives the wrong visual impression as to what is going on during reactions. He prefers the phrase "atom separation".

In my mind this is how I see it.... please if poss give me some feedback on it and as to which camp you believe is correct!!

*When you have covalently bonded atoms, for example, there is a high electron density between the 2 nuclei and it is the electrostatic attraction between this negative charge and the positive charge of the 2 adjacent nuclei that bonds them together.

*When energy is applied to this molecule, the electrons begin to absorb energy and in doing so can be promoted to higher energy levels. This promotion to higher energy levels occurring in both atoms has the gradual effect of forcing the 2 nuclei apart, thus increasing the internuclear distance and hence decreasing the coulombic force of attraction occurring between the opposing charges of the electrons and the nuclei.

*Eventually with enough energy the 2 atoms are pushed apart and the bond is "broken". The individual atoms due to the energy they have absorbed now have a higher level of PE within their electron cloud. How much energy is required depends on the electronegativity difference and/or the ratio of bonding/antibonding within the molecule.

*When the atoms proceed to form new bonds in order to become the final product, energy will be released, the quantity depending on the strength/stability of the new bond being formed. In an endothermic reaction, the products will have a higher level of PE than the original state and in an exothermic reaction, the products will have a lower level of PE than the original state.

*Since electrons in atoms have KE and PE, during bond formation, the decrease in PE is accompanied by an increase in KE. However, the magnitude of KE increase is only half that of the PE decrease - the remainder being released as light and/or sound energy.

Thoughts??


« Last Edit: June 19, 2015, 05:12:07 PM by PFScience »

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Re: Chemical potential energy query
« Reply #1 on: June 19, 2015, 06:14:04 PM »
Many sources cite PE as being stored in chemical bonds
Yes, I think this is a decidedly confusing and misleading this to say. In general, I think you're on the right track. Potential energy as a concept is only meaningful when compared to some reference state. I think the word "stored" in the quote above is the biggest offender. We can say that some reactant has more potential energy than some product, but - as you say - bonds by their definition are always at a lower energy state than separated atoms. So no energy was "stored" to get there. Of course, most substances aren't formed from individually separated atoms - they are formed from other molecules that are also made of bonds.  We can manipulate chemical systems to "store" chemical energy in the form of particularly unstable molecules, even through thermodynamically unfavorable processes. This is the very basis of making explosives - we store chemical energy in an unstable molecule, that is then delivered elsewhere.

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*Since electrons in atoms have KE and PE, during bond formation, the decrease in PE is accompanied by an increase in KE.

In general, true. In practice, the excess kinetic energy is dispersed in the form of heat to neighboring molecules. In many gas phase reactions, for instance, products will not form without the presence of spectator gasses, which can absorb some of the excess kinetic energy and "trap" the formed molecule in a potential well. (In other words, if two atoms smack into each other, they will have so much kinetic energy that they can just bounce off each other. In the presence of a third "spectator" atom, the activated complex loses some of it's kinetic energy, and then the two atoms don't have enough kinetic energy to overcome the electrostatic forces that hold them together. At this point a "molecule" is formed. Depending on the density of nearby atoms, the remaining excess kinetic energy is quickly lost via subsequent collisions, subject to the statistical (temperature dependent) average, of course.

What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline PFScience

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Re: Chemical potential energy query
« Reply #2 on: June 20, 2015, 06:05:43 AM »
Thanks... much appreciated. Things are making more sense in my head.  ;)

One of my memories of doing my chemistry A-Level was that quite a few of us asked our teacher.."If chemical bonds contain so much PE, then why is it not released after the bonds have been broken??". We never did get a satisfactory answer !!

Offline PFScience

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Re: Chemical potential energy query
« Reply #3 on: June 20, 2015, 03:18:12 PM »
One final query that has popped into my head if you could confirm for me please!

Upon the forming of a new bond with the accompanying decrease in PE and increase in KE. The increase in KE (vibrational/transitional/rotational) is that of the product molecule itself (within the reaction system) which due to further collisions then transfers this thermal kinetic energy out into the surroundings? Hence the reason why in exothermic reactions, the surroundings experience an increase in thermal energy whilst the system experiences a decrease  :rarrow: ie) the thermal energy has transferred from an area of relative heat excess (system reaction) to one of relative heat absence (surroundings) ? 

Sorry if Im not quite on track.... Im trying to get everything nailed down before I create some definitive notes!

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Re: Chemical potential energy query
« Reply #4 on: June 20, 2015, 04:22:59 PM »
If two particles are moving towards each other (each with a characteristic kinetic energy) and there is an electrical attraction between them, they will begin to accelerate toward each other because of the electrostatic force acting on them. The potential energy is going down during this acceleration and kinetic energy is increasing (of each particle). A classical analogy might be if you were to slide two magnets toward each other. Essentially a portion of the potential energy of the system has been transferred to kinetic energy (meaning, potential/kinetic energy of all particles under consideration). In case where the two atoms are particles that are forming a bond, once a bound state is formed (enough kinetic energy is lost to the environment such that neither atom has enough kinetic energy to fly off into the sunset), that kinetic energy can be in the form of internal vibration, rotation, and of course translation. As the molecule collides with other nearby energy, that excess is gradually (well, often on the order of femtoseconds, if you consider that gradual) dispersed so that the kinetic energy of the system is more or less the same (on a time average) as other particles in the local environment.

It's important to acknowledge the statistical mechanical reality, though. Collisions are happening all the time, transferring kinetic energy to and fro. So although equilibrium might be perceived as a "static" situation, this is only true in the sense that there's is an ensemble average of kinetic energy determined basically by the temperature. In reality there is a distribution of kinetic energies among all available particles at any given time. It is this distribution that allows reactions to occur - at any given time, enough kinetic energy is transferred to some systems that allow them to cross a potential barrier.
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Offline PFScience

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Re: Chemical potential energy query
« Reply #5 on: June 21, 2015, 09:47:09 AM »
Thank you for this..

So would it be accurate to say that the conversion between PE and KE as the acceleration occurs between reacting atoms, is analagous to the conversion of chemical energy to KE as a sprinter accelerates through a race? The atoms has PE "stored" throughout its electron cloud and the sprinter has chemical energy "stored" throughout their muscles.

Also during an exothermic reaction, it is stated that the enthalpy (H) (or total internal energy, U) of the system decreases whilst that of the surroundings increases. But to explain the system further, it experiences a v.v.small increase in KE but a significant decrease in PE - hence the overall decrease in enthalpy (H)?




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Re: Chemical potential energy query
« Reply #6 on: June 22, 2015, 03:47:43 PM »
So would it be accurate to say that the conversion between PE and KE as the acceleration occurs between reacting atoms, is analagous to the conversion of chemical energy to KE as a sprinter accelerates through a race? The atoms has PE "stored" throughout its electron cloud and the sprinter has chemical energy "stored" throughout their muscles.
I don't think this is a great analogy. The body is a complex system. It's true chemical potential energy is being converted to kinetic energy in a way, but the process is far more complicated and deliberate than a simple spontaneous process described by collision of two atoms to form a diatomic molecule.

Quote
Also during an exothermic reaction, it is stated that the enthalpy (H) (or total internal energy, U) of the system decreases whilst that of the surroundings increases. But to explain the system further, it experiences a v.v.small increase in KE but a significant decrease in PE - hence the overall decrease in enthalpy (H)?
I'm not sure I follow what you're asking. Chemical potential is formulated in most cases by the Gibbs energy, which includes both enthalpy and entropy.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline PFScience

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Re: Chemical potential energy query
« Reply #7 on: June 23, 2015, 02:13:24 PM »
I don't think this is a great analogy. The body is a complex system. It's true chemical potential energy is being converted to kinetic energy in a way, but the process is far more complicated and deliberate than a simple spontaneous process described by collision of two atoms to form a diatomic molecule.

I do some tutoring with younger students and I was trying to come up with a relateable analogy!

I'm not sure I follow what you're asking. Chemical potential is formulated in most cases by the Gibbs energy, which includes both enthalpy and entropy.

I was trying to discuss how in an exothermic reaction, although the system is described as losing heat, there is a v.small increase in KE due to the increased KE via the initial bonding. Yes, this molecule then collides with others which dissipates this increase, but nevertheless the KE value (which is an average) has increased v.slightly. ??

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Re: Chemical potential energy query
« Reply #8 on: June 24, 2015, 09:19:01 AM »
I do some tutoring with younger students and I was trying to come up with a relateable analogy!
I think balls and hills (and maybe magnets) are the best analogies to use, because gravity is the closest thing to a force field that people can relate to from their daily lives.

I was trying to discuss how in an exothermic reaction, although the system is described as losing heat, there is a v.small increase in KE due to the increased KE via the initial bonding. Yes, this molecule then collides with others which dissipates this increase, but nevertheless the KE value (which is an average) has increased v.slightly. ??
I suppose this is true, but recall in most cases molecules have to absorb kinetic energy from other nearby molecules to make reactions go in the first place (i.e, activation energy).
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline PFScience

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Re: Chemical potential energy query
« Reply #9 on: June 25, 2015, 04:44:43 PM »
I suppose this is true, but recall in most cases molecules have to absorb kinetic energy from other nearby molecules to make reactions go in the first place (i.e, activation energy).

In relation to this, if when particles approach each other to bond there is a conversion of PE to KE which results in a mutual acceleration and subsequent release of KE into the surroundings once bonding has occurred, does it thus follow that the reason for endothermic reactions becoming colder as an experiment progresses is due to a conversion from KE to PE in the 'bond breaking' phase? Logically, any reduction in temperature must be to do with a reduction in KE.   

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