Jump to content

A few chem questions unclear to me...


Recommended Posts

No, the acid does not exist as such. The acid H2CO3 cannot be isolated. When CO2 is dissolved in water, then indeed the liquid becomes a little acidic, but the real reaction mechanism from CO2 and water to the acid is not really simple. The net equilibrium will be something like, without the H2CO3 species present in solution:

 

Actually, carbonic acid has been isolated before. It has the structure O=C-(OH)2 according to the response given in this Q&A.

 

The article ""On the surprising kinetic stability of carbonic acid (H2CO3)", Loerting T., Tautermann C. S., Kroemer R. T., Kohl I., Hallbrucker A., Mayer E., and Liedl K. R., Angew. Chem. Int. Ed., 2000, 39, 891-894" is also further proof that carbonic acid exists as a species.

 

So I would say, yes, H2CO3 is an actual acid and has been isolated as such.

Link to post
Share on other sites
  • Replies 85
  • Created
  • Last Reply

Top Posters In This Topic

Actually' date=' carbonic acid has been isolated before. It has the structure O=C-(OH)2 according to the response given in this Q&A.

 

The article ""On the surprising kinetic stability of carbonic acid (H2CO3)", Loerting T., Tautermann C. S., Kroemer R. T., Kohl I., Hallbrucker A., Mayer E., and Liedl K. R., Angew. Chem. Int. Ed., 2000, 39, 891-894" is also further proof that carbonic acid exists as a species.

 

So I would say, yes, H2CO3 is an actual acid and has been isolated as such.

Wow, this is interesting! I have some older books and they all state that H2CO3 does not exist in the free state. So, the isolation must have been very recent. Somewhere in the year 2000 or just before? I unfortunately have no access to the article you mention, but from the weblink I can grasp the idea. Do you know something about the physical properties of H2CO3 (is it a gas, a liquid?), or was the isolation only just a few molecules?

 

It is nice to read about such compounds, which seem so familiar, while in reality they have eluded chemists up to very recently.

 

Now its time to wait till the first auctions on eBay appear for ultrapure carbonic acid, totally free of water :D :D. Maybe an interesting exercise for you... probably even harder than storing fluorine gas if only a few molucules of water cause an autocatalytic destruction of the compound :):D

Link to post
Share on other sites

I am not sure. I would think from the structure of carbonic acid being akin to a ketone with the -OH groups in place of hydrocarbon groups, that the compound would be a liquid. I would also think that carbonic acid exists in an aqueous state, but in fleetingly small quantities. (I.E. it forms, decomposes, forms, decomposes, etc. etc.). That's something I'd have to look up.

Link to post
Share on other sites
No' date=' not all reactions are reversible. E.g. burnt wood and the gaseous reaction products cannot revert to wood and oxygen.

 

 

No, this is exactly what makes the computations of chemical reactions and structures so difficult. When a molecule or set of molecules need to be modelled, then the number of states for even very simple systems can be incredibly large. The same energy in a system can represent many many different configurations.

 

In practice, one also frequently observes, that reactions are not 'clean'. Especially, when the different energy levels for differnet compounds are very close to each other, then a reaction can produce many different compounds (there are many competing reactions). An example of this is reduction of HNO3 by zinc. This reaction can yield NO, NO2, NH4(+), NH3OH(+), N2O, all in one 'dirty' mix. What is formed precisely, depends on temperature, pressure, concentration of reactants, etc.[/quote']

 

So, how can you tell whether a reaction is reversibie?? secondly, my teacher told me that chemical reaction is always dynamic equilibrium. When does that apply??

 

ANd, can you name one factor affecting the reaction equation beside energy??

Link to post
Share on other sites

So, how can you tell whether a reaction is reversibie?? secondly, my teacher told me that chemical reaction is always dynamic equilibrium. When does that apply??

 

ANd, can you name one factor affecting the reaction equation beside energy??

All the reactions are reversible. Even, there is only an atom, it still get a chance to lose electrons.

Link to post
Share on other sites
All the reactions are reversible. Even, there is only an atom, it still get a chance to lose electrons.

I was tempted to give this answer also at the original question, but I decided not to do so. From a pure theoretical/academic point of view you are right, but from a practical point of view I still would say that not all reactions are reversible. Have you ever seen appear wood from ashes and smoke :D?

 

If you want to explain why certain reactions are reversible and why others are not, then the concept of equilibrium does come into play again.

 

In general, one can say that the closer the energy levels of the reactants and the products are, the easier the reversible reaction can occur, but this is not a guarantee that the reversible reaction also does occur always.

 

Another factor, which determines whether a reaction is reversible or not is whether the reaction products remain available in the system or not. E.g. if one of the products is a gas and is completely insoluble, then the reaction is driven completely to one direction. Another way, in which this can occur is that one of the reaction products is taken away.

 

An example of this is the equilibrium reaction of dichromate and chromate:

 

(1) Cr2O7(2-) + H2O <---> 2HCrO4(-)

(2) HCrO4(-) <---> CrO4(2-) + H(+)

Reaction (1) is almost completely at the left, a solution of a e.g. potassium dichromate is only very faintly acidic.

 

Now, if some excess lead nitrate is added, then the incredibly insoluble PbCrO4 is formed, which settles as a solid. This drives reaction (2) to the right and that in turn also drives reaction (1) to the right. The liquid becomes almost colorless and all dichromate is converted to chromate in the form of lead chromate. The liquid also becomes quite acidic.

 

An other example is addition of a bicarbonate to an acidic liquid.

The equlibrium HCO3(-) + H(+) <---> H2O + CO2 is driven to the right, because most of the CO2 escapes from the system as bubbles. If the system were kept under high pressure and no space for formation of gas bubbles, then the reaction would really be an equilibrium.

 

Yet another factor is whether there are mechanistic pathways for reactions to occur. Even an energetically very favourable reaction does not need to occur if there is no suitable mechanism for the reaction to occur (this is why perchlorates are so inert in aqueous solutions). So, if there is a reaction with roughly equal energy levels at both sides of the arrow and there is mechanistic pathway for one direction, while there is not one for the other direction, then on a macroscopic level we would observe the reaction as being non-reversible.

Link to post
Share on other sites
What do you mean by "energy level"??

 

bond energy??

Energy, available in molecules and so on has no real macroscopic equivalent, I think it is OK to call this 'bond energy'. The best comparison with macroscopic energy is potential energy. This potential energy can be released in chemical reactions, producing heat (and possibly light).

 

A real nice example is white phosphorus, where the four P atoms are arranged in such a way, that there is considerable bond strain. Compare this with a mechanical spring, which is about to break apart in two pieces due to over-stressing. This is what makes white phosphorus so reactive. When the bond (spring) breaks apart, then a lot of potential energy is converted to another form of energy (fast motion macroscopically, which is heat at the atomic/molecular level).

 

When the spring snaps, then it may hurt you, when the P4-bonds break apart then it may really hurt you :D. I hope that this example makes things a little more clear to you ;).

Link to post
Share on other sites

In general, all reactions will occur above 0K.

 

Some reactions tend to occur more completely than others, and some, more rapidly than others. The completion of a reaction is linked to the thermodynamics (specifically, the Gibbs' Free Energy) and the rapidity is related to its kinetics (specifically the rate constant, which is a function of reaction conditions - temperature, pressure, catalysis, etc. - as well as molecular geometry and the molecularity of the reaction).

Link to post
Share on other sites

So, how do you know what products are the reactants likely to form??

 

btw, what is "thermodynamics" in general?

 

secondly, kinetics? does that mean both potential and kinetic energy?

 

Albert

Link to post
Share on other sites
So' date=' how do you know what products are the reactants likely to form??

 

btw, what is "thermodynamics" in general?

 

secondly, kinetics? does that mean both potential and kinetic energy?

 

Albert[/quote']

Thermodynamics has to do with energy levels, kinetics has to do with mechanisms. I'll try to explain by means of a macroscopic equivalent.

 

Suppose you have a marble on the top of a high hill, with the hill being in an otherwise flat landscape, the landscape being a completely flat floor.

This marble has quite some potential energy E=m*g*h, because the hill is high. (m = mass of marble, g is gravitational acceleration, h is height of top of hill, relative to the flat landscape).

 

Thermodynamically speaking, it is more feasible for the marble to be lower, at the flat floor. As you can see from the formula of the potential enery, it does not depend on the precise shape of the hill, whether there are obstacles on the hill etc, the only thing which matters is the marble and the height of the hill.

 

Whether the marble really goes downward is a matter of kinetics. If the hill is completely smooth and has no bumps, then the marble very easily rolls downwards, so there is a mechanistic path for the reaction to occur (marble going from top of hill to flat floor). In order to start the reaction, one only has to softly tap the marble and there it goes...

 

Now suppose the hill is covered with high and rough vegetation. Despite the fact that the marble has high potential energy, it cannot easily go downwards, it gets stuck in the high vegetation. Now the reaction cannot occur and the marble remains at the top.

 

Now suppose there is some third object, being a kind of animal that moves away vegetation. Suppose this animal moves away vegetation, just in front of the marble, allowing it to roll downward, then it creates a path for the marble to move downward, without itself being used by the marble. When the marble it at the bottom of the hill, then the animal still exists.

This is exactly what a catalyst in chemistry does. It provides a pathway, such that a reaction, which otherwise would occur with difficulty can occur more easily.

 

Chemical equivalents are numerous. E.g. hydrogen peroxide (being the marble at the top of the hill) has a high potential energy, but that energy is not released, because there is no path for this. It simply remains hydrogen peroxide. Now, if some manganese dioxide (being the animal) is added, then the hydrogen peroxide violently decomposes to water and oxygen (water and oxygen representing the marble at the flat floor, at the bottom of the hill), while at the end of the reaction, the same amount of manganese dioxide still is present (the animal is not used up).

 

 

A nice example of another catalyst for decomposition of hydrogen peroxide is the ion I(-) in the absence of any acid.

 

Hydrogen peroxide reacts with iodide ion, forming hypoiodite ion:

 

I(-) + H2O2 --> IO(-) + H2O

 

Hypoiodite ion in turn reacts quickly with hydrogen peroxide as follows:

 

IO(-) + H2O2 --> [i.O2](-) + H2O

[i.O2](-) --> I(-) + O2

 

As you can see, iodide ion provides a path for quick decomposition of hydrogen peroxide, the net reaction being 2H2O2 --> 2H2O + O2 and I(-) left behind.

 

Well, this has become quite a lengthy post. I hope it helps you understand the concept of thermodynamics and kinetics.

Link to post
Share on other sites

thanks, woelen.

 

Your explanation,Straight to the point, Worths than a thousands books if I could read. :)

[metaphor]

 

Since it is too late at night, I will read it tommorow.

 

Do you major Chemistry in university?? in US? in IVY LEAGUE?

 

Go and entiltle yourself "Chemistry Expert" like JDurg. :)

 

Albert

Link to post
Share on other sites

thanks woelen,

 

DQW said all reactions will occur above 0k.

 

take, for example, water, and oxygen, do they react??

 

btw, woelen, can you be more specific at the potential energy?? in terms of chemistry?? not physics?? ie, e = mgh

 

Secondly, Can I say acid generally have high thermodynamics??

 

Albert

Link to post
Share on other sites
btw, woelen, can you be more specific at the potential energy?? in terms of chemistry?? not physics?? ie, e = mgh

PE=mgh , gravitational potential energy=mgh.

The potential energy in the microscopic world is the energy used to break the bonds. and thus that is stored in the substance

For instance, for a water to become vapour, you know that the volume is increased largely. With the same amount of molecules, you know that the distance between any two molecules may be lengthened in order to provide a larger volume.

Take water molecules as example again

In chemistry, molecules are bonded with intermolecular forces, (i.e. Van der Waals' Forces).To break the bonds, in other words, to seperate them, energy must be input.

Energy is then converted into their potential energy rather than their kinetic energy.

Hence, gas has a larger potential energy than liquid.

 

take, for example, water, and oxygen, do they react??

I would think that the product is hydrogen peroxide at a particular temperature or with some kinds of catalase. However, I don't know how to predict it. I just know the reaction by memory. Hope someone can help me.

Link to post
Share on other sites
thanks woelen' date='

 

DQW said all reactions will occur above 0k.

 

take, for example, water, and oxygen, do they react??[/quote']

Let's go back to the metaphore of the marble and the hill and add one other concept, that of temperature. Think of the flat floor and the hill being in a box and the box being shaken at high frequency around a certain equilibrium position. The higher the temperature, the higher the amplitude of the shakes of the box. If we go back to the hill with vegetation, then by means of shaking the marble may go down, albeit slowly. With more shaking (higher temperature) it may go down faster, because sometimes the marble is lifted over the vegetation. This is what higher temperature in chemistry does as well. It may make paths available, which are not available at lower temperature (e.g. burning of wood works at high temps, not at low temps).

 

Now let's go back to your question on reaction of water and oxygen. In the hill/marble metaphore this corresponds to the marble on the floor. Because of the shaking, the marble sometimes bumps from the floor a little bit upwards. Suppose that the shaking is sufficiently high and that the marble is lifted up so high that it reaches the top of the hill, then you would have the reverse reaction. On the other hand, the marble will not remain there for long, also because of the violent shaking.

 

As you can imagine, the higher the hill, the smaller the chance that the marble accidently reaches the top of the hill. This is exactly what happens in real chemistry. The larger the energy difference between the compounds at the left and the right side of the arrow, the smaller the chance that the low-energy compounds can be converted to high energy compounds.

 

For 2H2O2 <---> 2H2O + O2, the energy at the left is much higher than the energy at the right. So, the hill is high. The chance that by accident the marble ever reaches the top of the hill by means of shaking is very small, but theoretically speaking, it is not zero.

In practice, hence, you could say that the reaction 2H2O + O2 --> H2O2 does not occur, theoretically speaking there might be a few molecules which change into H2O2, but the number of them will be so low, that in practice it will not be detected.

 

If you take the reaction H2 + I2 <---> 2HI, then the energy levels are fairly close and the reaction proceeds in both directions when H2 and i2 are gaseous. In the hill/marble metaphore this can better be represented as a highland and a lowland, with the high area just a little higher than the low area. Now, when marbles are at the low area and the box is shaken, then there is a reasonable chance that some marbles go to the higher area. When shaking continues, of course also marbles will go back from high to low and hence, you get an equilibrium. If all marbles atrted high, then you also get an equilibrium and you can imagine that the final equilibrium does not depend on the initial configuration of where marbles are, it only depends on the precise shape of the landscape and the shaking.

 

 

 

 

btw, woelen, can you be more specific at the potential energy?? in terms of chemistry?? not physics?? ie, e = mgh

As I stated earlier already in another post, the potential energy, stored inside a molecule cannot simply be described by means of a simple formula, such as given for the marble/hill metaphore. You need quantum mechanics to perform such computations.

 

What you see in practice is that for many reactions the amount of energy is given, e.g.

 

2H2O2 --> 2H2O + O2 + xx KJ/mol (I don't know xx, many textbooks contains tables for many common reaction or half-reactions).

 

What is important, it that xx does not depend on the path of the reaction (e.g. use of a catalyst or not). The hill/marble metaphore also shows this. Whether the marble moves in a straight line from top to floor or through a shaky and lengthy path, in both cases it looses the same height h and hence it looses the same amount of potential energy.

 

 

Secondly, Can I say acid generally have high thermodynamics??

 

Albert

What do you mean with high thermodynamics? Assuming that you mean high energy level, then I would say 'moderate'.

 

Acid/base reactions can be fairly energetic (e.g. add solid NaOH to concentrated HCl), but in general redox reactions are much more energetic.

 

NaOH+HCl can lead to boiling and splattering, but many redox reactions release so much energy that fire-like phenomena can be observed (e.g. mix of KClO3 and S, or mix of KMnO4 and glycerin).

 

If I would classify (roughly) reaction classes according to energy release, then I would come up with the following (ordered from high to low):

 

redox

acid/base

coordination

precipitation

 

But beware, this is just a rough list. There certainly are redox reactions, which are less energetic than many acid/base reactions, it is just the overall/on average behaviour.

 

Wilco

Link to post
Share on other sites
I would think that the product is hydrogen peroxide at a particular temperature or with some kinds of catalase.[/b'] However, I don't know how to predict it. I just know the reaction by memory. Hope someone can help me.

It might be, but only with considerable input of energy. As I explained in my lengthy post, a catalyst does not affect the energy released (or needed) for a reaction.

Link to post
Share on other sites
catalyst does not affect the energy released (or needed) for a reaction.

A catalyst does effect the amount of energy needed for a reaction by lowering the activation energy.

 

~Scott

Link to post
Share on other sites
A catalyst does effect the amount of energy needed for a reaction by lowering the activation energy.

 

~Scott

 

Yes, you're right, but that is something different than the net energy released or needed by a reaction. In my hill/marble analogue, the activation energy could be the energy needed to tap the marble or in the case of vegetation, to lift the marble over the vegetation, but still the total energy converted from potential to kinetic energy remains the same. This is exactly the same in chemistry. Any energy, initially put in the system for passing the activation barrier is regained, so it does not contribute to the net energy released (or taken) by the reaction.

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.