Reactions

[aommaster]

thanx YT2095. First time i got to object to something said!!! Anyway, no ones per

[Dudde]

VendingMenace said in post #20 :

 

I think you are missing my point. My point is this; you can have a reaction in which there no heat of reaction. That is, the net delta H for the total reaction is zero. That is what I am saying.

 

...(really long post)...

 

Hope that helps

 

I realize what you were saying, and I agree that the net enthalpy would indeed be zero

 

....I can't remember my original point to begin with though, dangit

maybe it was just that energy will be given off/taken in during any reaction, it doesn't matter what the end/start energies are (if I sit down, then get up and poke you in the eye, then sit down again, I've still poked you in the eye, even though my starting and ending positions were the same)

[VendingMenace]

Hydronium ions (H30+), in water is by the process of Grotthuss mechanism, the protons transfer from one water molecule to the next. The transferral is directed to proton motion through H20. Transport of Hydronium ions in ice is very close say directionally proportional, to H30 transport in water. If Hydronium molecules are supposed to extinct movement at 190 K (theory), then this implies that H30 motion that is an activated process. Similar to the defect in the Hydrogen-bonded structure (the D defect).

 

Of course if you are talking about an activated process then you are talking kenetics, not thermodynamics and this really has no bearing on the discussion. The discussion has been revolving around two point (actually the same point, but written differently).

 

The first poiint (a question) is as follows...

 

Is there a reaction that does not produce or absorb heat? In other words, is there a reaction that isn't exothermic or endothermic in where the bond energies on both sides aer equal?

 

THis is a question that is clearly a thermodynamic question. It is concerned with the left and right side of the chemical equation. The products and reactants. Not how you got form one to the other. Again, he is asking about edothermic vs. exothermic. As i tried to point out in my above post, /_\ H (cool way to write it, man) is a state variable and does not depend on path. Thus, any argument that deals with mechanisms (that is anything that tries to address activated process, activation energy, ect.) is not addressing the main question; can there be a reaction that has a /_\ H of zero?

 

Now, of course i can write a reaction that will have a delta H of zero. That is really not a problem. I have already written two. I think the more interesting question, which comes out of the above on is as follows...

(edited slightly)

 

can it be proved that two different chemicals must have different energies?

 

Now, this is clearly a question of bond energies of the two chemicals. That is, can two different chemicals have the same exact potential energy (wich is stored as heat energy, enthalpy)? Now again, we are talking about the realative /_\ H between these chemicals. There is no need to talk about ketetics of an enterchange between the two. Thus, activation process and activation energies are right out again.

 

So we see that what you said in quotes above does not apply to this question.

 

Energy change, the result of breaking and forming bonds, the energy to break a bond is known as bond-dissociation energy. When this energy, needed for bond-dissociation is less than the energy produced by the bonds formed the reaction will give of energy.

 

This comes close to addressing the question. I say close, becuase it does not really. What you have done is cleverly hide a discussion of activation energy. You have done this by splitting a reaction into two parts. A reaction where A disociates into X and Y and a second reaction where X and Y combine to form B.

 

However, do to thermodynamic considerations, we can just write this as the net equation A --> B. Then look at the total change for A to B.

 

The original stipulation was that a reaction can occur in wich the bond energies on both sides are equal (see above quote of original question). Thus, i said that there could be a reaction A goes to B, in which the bond energies of A and B were the same.

 

By breaking down the reaction into composite parts, you are looking at A goes to X and Y. THere are no constriants on X and Y, so it is true that they could be of different energy. However, this is inconsiquential. I am not concerned with X or Y. Rather A and B. I can ignore any trasient species that i wish to. Again, this is a direct result that /_\ H is a state variable.

 

Thus, while this does apear (at least at first) to adress the question. It really does not. Again, it breaks down to an analysis of reaction rates -- kenetics. But we are talking about thermodynamics. I must stress again that the question is; can it be proved that two different chemicals must have different energies?

 

An exothermic reaction, is the process of releasing energy, so the P (product) bond-dissociation is greater than the R (reactant) bond-dissociation. Whereas an Endothermic reaction absorbs energy, the R bond-dissociation energy is greater than the energy released, by the P bond dissociation.

 

THis is not so much an argument as it is a definition then.

 

This energy change in a reaction, is known as the enthalpy of reaction, the symbol is H, the change of enthalpy is delta H, /_\H. Exothermic reactions, have negative enthalpy change, the molecules, atoms and bonds loose energy to the surroundings. On the other hand Endothermic reactions, have positive enthalpy change, the molecules, atoms and bonds absorb (gain), energy from their surroundings.

 

Law of conservation of energy IS the amount of energy in the universe id CONSTANT. Energy is ONLY transferred in chemical REACTIONS.

 

Entropy, S, is the (m) measure of chaos, large values for entropy, are equal to large amounts of disorder, this is not directed to the conservation law.

 

/_\G = /_\H - T/_\S

 

/_\G Free energy, Gibbs theorizing, the concept of free energy and entropy, it’s unit is calorie or kilocalorie, this energy is used for a specific process and is available from a process (a previous post of mine shows Gibbs equations).

 

These are a series of definitions and are not really arguments. But thanks for taking the time to make sure everyone understand the definitions.

 

EVEN in exergonic and endergonic, Reactant + reactant = Product + product + ENERGY RELEASED, the energy is still there.

 

well, um, yeah. That is kinda the definition then isn't it? Endergonic means a positive change in free energy, exergonic means a negative change in gibss free energy. So, of course enedergonic and exergonic reactions are accompanied by change in energy. That is the definition.

 

Summery:

 

All chemical REACTIONS, are accompanied by change in energy, EVEN in exergonic and endergonic,

 

First off, this really doesn't make sense. BY DEFINITION, exergonic and endergonic reactions are accompanied by a change in energy. The must be.

 

Secondly, you have not shown that all reactions are accompanied by a change in energy. What you have done is shown that there is an activation energy that must be crossed. And i am assuming that you mean to claim that this is the energy change the must accompany a chemical reaction. However, this is not quite true. There is no theorectical lower limit to the activation energy of a complex. Thus, it is quite easy to imagine a complex (or several complexes) for wich the activation energy between them is zero. We can further stipulate that some of these complexes are at at the same energy. Now between these complexes we have a reaction that has a /_\H of zero and does not have any activation energy. Thus there is no energy accosiated with the trasition.

 

Now it may seem a bit like cheating to just make up these complexes that have no activation energy. However, it really is not. They are just theoretical complexes, and we are considering theory here aren't we? What they really allow me to do is to show that any sort of kenetic argument truly does not apply at all to the original question. And so we find that we can ignore activation energies (like i have been saying all along) and move on to the real question. That question has been, and remains;

 

can you prove that two different chemicals must have different energies?

 

Now remember this is a thermodynamic question. Please try to refrain from any discussion of the path taken between these two chemicals, as such a path is nessesarily meaningless for the purposes of this discussion.

 

cool

[wolfson]

Ok the answer to his/her, question, Mr. Vending, reactions of different chemicals will always result in exo or endothermic process’s, however there are thermoneutral reactions where the transition state structure is roughly ½ way along the reaction coordinate and will equally resemble the structure of the reactants and products, and have either small (fast) or large (slow) barriers, but you should consider stating that all reactions (of different reactants) require energy or “give out” energy during their reactant process, by means of theorizing ideological thermoneutral reactions, below undergraduate education would not be plausible.

 

P.S. I like that “Christmas effect” on the home page…

[VendingMenace]

reactions of different chemicals will always result in exo or endothermic process’s

 

My whole point is that i am not convinced that this is nessesarily always the case. Can you provide some proof of this? That is proof that the delta H for a reaction between different chemicals cannot be zero?

[Dudde]

because if they're reacting, they're absorbing or expelling energy, which makes it endo- or exothermic

 

there a reaction that does not produce or absorb heat? In other words, is there a reaction that isn't exothermic or endothermic in where the bond energies on both sides aer equal?

that's what I was trying to answer

[aommaster]

oh ok thanx for the help