View Full Version : Specific heat of ammonia?


Facial
03-11-07, 12:02 AM
What accounts for the fact that ammonia has a higher specific heat than water? Few liquids have sp. heats higher than that of water's.

Clearly we see that it virtually the same molar mass (17g/mol as compared with 18g/mol for water) and exhibits hydrogen bonding to a lesser degree. Then clearly, from a molecular scale, ammonia should be more readily heated than water. Yet it isn't so. How come?

Chatha
03-14-07, 12:22 PM
The specific heat of water and ammonia are near identical. Ammonia being a little higher due to molar mass and more hydrogen atoms. Hydrogen bonds are strong because Hydrogen is the smallest atom, this takes it cloeser to another atom in any dipole/dipole bond, giving it a stickier bond. Hydrogen bonds in water are responsible for water's weired features, like why ice floats on water. Water is one of the very few sustances that is more dense when liquid than solid. This is due to the freedom of Hydrogen bonds in liquid water.

Billy T
03-23-07, 01:06 PM
Hydrogen bonds in water are responsible for water's weired features,...This is due to the freedom of Hydrogen bonds in liquid water.Perhaps, but I think that a strange way to state the fact. More accurately is the fact that the angle between the two protons (with O at the vertex) is 105 degrees, not 180 so that H2O is permanently polarized and tends to be in reality strings of nH2O where n is a small integer. When warm, (above 4C) these strings are very dynamic and n tends to be smaller, but as you cool below 4C they become more like short straws and have some space between them so the volumetric density begins to drop and at 0C the space in the solid version is quite significant and ice floats.

The strength of the H bond has little to do with all this.

Chatha
03-23-07, 01:19 PM
Perhaps, but I think that a strange way to state the fact. More accurately is the fact that the angle between the two protons (with O at the vertex) is 105 degrees, not 180 so that H2O is permanently polarized and tends to be in reality strings of nH2O where n is a small integer. When warm, (above 4C) these strings are very dynamic and n tends to be smaller, but as you cool below 4C they become more like short straws and have some space between them so the volumetric density begins to drop and at 0C the space in the solid version is quite significant and ice floats.

The strength of the H bond has little to do with all this.
He's talking about specific heat. The energy difference between NH3 and H20 is in both their intermolecular force and bond. Remember energy is always equal to bonds and forces.

Billy T
03-23-07, 01:29 PM
What accounts for the fact that ammonia has a higher specific heat than water?...Basically this is because there are more "modes" of vibration in NH4 than in H2O molecule.
When energy can be transferred between the different modes (and it always can to some extent) they share the available energy equally (when thermal equilibrium is reached at least). Temperature is the KE of the molecule - in some sense just three modes (motion in the three orthogonal directions.)

I will just illustrate the two pure vibration modes of H2O (no flexing or rotation and straighten out the true 105 degrees - see other post.):

H -- O ----H and half cycle later: H ---- O --H

plus

H --- O --- H and half cycle later: H ---- O ---- H

In this second case the Hs move towards each other half of the cycle and away from each other the other half. In the first mode, the space between the Hs does not significantly change.

I would not be begin to attempt to show even half of the modes of vibration

....... H
........|
H - - N - - H
........|
....... H

can have!

Each of these modes get the same fraction of the energy added to the molecule as each of the three KE modes. (Called "Equal partition of Energy" law) so with a fixed amount of energy (heat) added the KE modes of NH4 each get less - I.e. the increase in KE is less (Ergo the specific heat, which the amount of heat required to warm a gram one degree C, if memory serves, is greater.)

PS the "dots" in my picture of NH4 above are just because this form reduces several spaces to one and I needed to avoid that to "type a picture."

By edit: after looking at my pictures, it is obvious that NH4 can do any pure vibration that H2O can with both the "vertical" and "horizontal" modes.

Thus I will guess how the specific heats should be (no doubt wrongly as I am neglecting rotational modes):

Assume we added 30 units of energy to each molecule. Then each of the 3 H2O KE modes gets 6 and both of the two Vibration modes (V modes hereafter) gets 6.

But as there are 4 V modes in NH4, the total modes is 7 not 5 and each of the KE modes gets only slightly more than 4 units of energy.

If we try to correct for the neglected rotational modes assuming there to be three rotational ones as three rotational axis exist then the energy is shared in 8 modes in H2O and in 10 modes in NH4

Thus if each of the 10 modes of NH4 gets one unit of energy in each mode, then the 8 modes of H2O will each get 1.125 units of energy. I.e. the heat capacity of NH4 should be roughly 12.5% greater than water's. - How does this compare with the facts? (I am too lazy to look them up. - I almost always rely on my reasoning and memory.) Also note that I did not include the slight difference in the molecular masses. There will be more molecules in a gram and hence more modes in a gram of the lighter molecule. You make this correction. ("Exercise left for the student."):D

Billy T
03-23-07, 02:11 PM
... Remember energy is always equal to bonds and forces. That too is not very accurate. For better understanding, even numerical estimation, See post 5, (made while you were posting #4.)

Chatha
03-23-07, 05:40 PM
Billy T,

I really don't understand your posts, seems like advanced chemistry of some sort. Nevertheless ,the greatest amount of energy in heating any substance comes from the bonds and intermolecular forces. H20 has a higher specific heat than solid Gold or Mercury because of its bonds and hydrogen forces, primarily the forces.

Basically this is because there are more "modes" of vibration in NH4 than in H2O molecule Yes, but I think we undergrades call it "space". Liquid water has more Hydrogen bonds spread over per area than in solid form. In solid form the bonds form a specific arrangement or crystals, leaving more "empty" or weak space. At least thats what my books say.

Billy T
03-23-07, 09:31 PM
Billy T,

I really don't understand your posts, seems like advanced chemistry of some sort. Nevertheless ,the greatest amount of energy in heating any substance comes from the bonds and intermolecular forces. ...Try again. It is simple, not advanced chemistry - Not even chemistry - just relatively simple physics.

You appear to be confusing force with energy. I.e. the FORCE or STRENGTH of the bond has little, if anything, to do with THERMAL ENERGY or HEAT.

Heat is Kinetic Energy ( the energy of motion). When the molecule is a single atom, there is only the translation energy of motion (until the temperature is very high and the bound electrons can be excited out of their "ground states").

If two atoms make the molecule, then as energy is added to a large collection of them, not only do they move faster, but they vibrate or dynamic oscillate their separation. (The possible vibrations are quantized - part of quantum theory you can neglect at your stage of understanding. Also neglect, for now, the quantized rotations that also store energy.)

The essence of my post about heat capacity, specific heat, etc. is that if there are many different distinct ways (standardly called "modes," just as a violin strings vibrate in certain "modes") to store energy and because all modes "share" the energy available equally, the complex molecules have many internal modes (vibration, etc) soaking up large part of the energy available.

Thus their heat capacity PER MOLECULE is much greater than water's. Their SPECIFIC heat capacity is typically much less than water’s as they are more massive. (Many fewer total number of modes in a gram.) If you want high SPECIFIC Heat, then that will be found in complex molecules with a lot of hydrogen (low weight so lots of the molecules in a gram)

I am sure the specific heat capacity of H2 must be the highest possible, but I have never read that. NH4 has high specific heat also for reason I explained (and am sure any one of reasonable intelligence at least in high school can understand.) I.e. If part of the heat added is going into internal storage (vibrations) then less is available for making the molecules move faster. - It is really very simple.:)

Chatha
03-25-07, 02:46 PM
Try again. It is simple, not advanced chemistry - Not even chemistry - just relatively simple physics. If you read the posts well enough you will also find out that its also basic elementary chemistry. And it gives a better and simpler insight to why specific heat varies from compoud to compound.


You appear to be confusing force with energy. I.e. the FORCE or STRENGTH of the bond has little, if anything, to do with THERMAL ENERGY or HEAT. You are VERY wrong, I was giving you credit until now. In chemistry, forces and bonds are directly proportional to thermal, hardness, and electric conductivity. Solids have a lower specific heat than liquids because solids are more tightly packed together, this is the reason why metals are better conductors. Water has a high specific heat because of its random hydrogen bonds, which occupies more space than when crystallized. A system in entropy is usually more effective in filling space than when in certain pattern. Additional reason why the hydrogen bonds in ice is less effective resides on the fact that water expands when cooled, creating more space in an already arranged molecule, and making it less dense. The best analogy to understand water and ice density is this; having no plan is sometimes more effective than sticking to only a certain plan.


Heat is Kinetic Energy ( the energy of motion). When the molecule is a single atom, there is only the translation energy of motion (until the temperature is very high and the bound electrons can be excited out of their "ground states").

If two atoms make the molecule, then as energy is added to a large collection of them, not only do they move faster, but they vibrate or dynamic oscillate their separation. (The possible vibrations are quantized - part of quantum theory you can neglect at your stage of understanding. Also neglect, for now, the quantized rotations that also store energy.)

When heat is applied to a molecule, the particles vibrate if its in a solid state or more tightly packed together. Then it consequently begins to move faster and further apart, changing phase to liquid and then gas. This is all we need to know about Specific heat. Quantisizing the energy from atom to atom is irrelevant in understanding specific heat because it obeys all phases of matter.


The essence of my post about heat capacity, specific heat, etc. is that if there are many different distinct ways (standardly called "modes," just as a violin strings vibrate in certain "modes") to store energy and because all modes "share" the energy available equally, the complex molecules have many internal modes (vibration, etc) soaking up large part of the energy available.
On the same token, a well tuned string of a violin will vibrate less, and less enegry will be required to pluck it. By the reason, matter will strong bonds will have lower specific heat capacity than matter with weaker bonds and forces. You can never escape bonds and forces when talking about heat conductivity.



Thus their heat capacity PER MOLECULE is much greater than water's. Their SPECIFIC heat capacity is typically much less than water’s as they are more massive. (Many fewer total number of modes in a gram.) If you want high SPECIFIC Heat, then that will be found in complex molecules with a lot of hydrogen (low weight so lots of the molecules in a gram) A higher molar mass is directly proportional to a higher heat capacity (heat capacity is different from specific heat capacity. Heat capacity is specific heat capacity multiplied by mass in Kg). More important than amu is bonds and forces, which determines the specific heat capacity experimentally. Hydrogen bond is the second strongest force in nature after Ionic-Dipole forces. Its the reason why Water has a higher specific heat capacity than SnO and most metalic oxides. London dispersion are the weakest.


I am sure the specific heat capacity of H2 must be the highest possible, but I have never read that. NH4 has high specific heat also for reason I explained (and am sure any one of reasonable intelligence at least in high school can understand.) I.e. If part of the heat added is going into internal storage (vibrations) then less is available for making the molecules move faster. - It is really very simple.:)
I get your drift, we are saying the same thing, try to read my posts closer. What you are trying to explain is phase change. In a phase change, under constant temperature, there is a frame in time when the temperature of the molecules does not increase, this is the phase where the molecules are too busy breaking apart or absorbing energy or vibrating (as you call it). What causes the difference in variations in internal storage? Bonds and forces. If all bonds and forces were the same, and all molecules weight the same, all matter will have the same specific heat capacity. liquid Water and ice water have different specific heat capacities because of their arrangement. 99% of the conductivity characteristics of all matter has to do with its arrangements (if its a solid), its bond strength, and its intermolecular forces. Virtually all matter has varied specific heat capacity in all three phases of matter. Metals and solids absorb heat better because they are tightly packed, so they have a lower specific heat capacity, in fact some solids will immidiately start reflecting heat back to you right after you begin to heat them. Why is all this so hard to understand?

http://en.wikipedia.org/wiki/Specific_heat_capacity

Chatha
03-25-07, 03:09 PM
The best way to understand specific heat capacity is this. There are three phases of matter excluding ionized gas. Solid..Liquid..Gas. Solids have the lowest heat retention because they are closely packed, so the best phase of matter for the highest specific heat would be gas, right? But gases are too fast, and collide more easily, negating the concept that free space will increase its specific heat. So we are back at Liquids, a little bit of a solid and a little bit of a gas. The liquids with the strongest intermolecular forces have the highest specific heats because it takes a lot of energy to absorb or break the forces and set it up for a consequence phase change to gas. Ionic-dipole liquids should have the highest heat capacities (liquid salt solutions), its harder to heat a salt water than pure water.

Billy T
03-25-07, 03:17 PM
... You can never escape bonds and forces when talking about heat conductivity...This is about the only thing in your entire post that is completely true , but I doubt if you understand why. (Even this is only accidently true as you said only "bonds" not "hydrogen bonds" or "atom bonds" and thus the "outer shell electron bonds" are accidently (and correctly) included.

The bonds of the outer shell electrons (not the hydrogen atom bonds as you seem to think) do determine whether or not a sold (or liquid) is a good or poor conductor of both electricity and usually heat.

If all the electrons are bound, each to a particular atom, then the substance is always a poor conductor of electicity and generally speaking, of heat as well. This is because free (not locally bound electrons) are required for electrical condution but heat can also be conducted by "phonons" (propagating latice vibrations). Usually phonon conduction of heat is much less efficient; however, it can be quite high in defect-free crystals, like clear diamonds. (The defects found in most solids "scatter the phonons" -sort of like trying to propagate an energy wave down a rope which has some bricks tied to the rope at random locations, compared to a rope with none, in which the wave will travel to the far end with little disipation of the energy in the wave.)

All this gets quite interesting in semi-conductors. - This is far above your current level of misunderstanding, but later, when you understand the basics, read about the "Fermi levels" and their structures, especially about "Brillion zones" and "anisotropic Fermi surfaces"

PS the phonons can even scatter the electrons doing the electrical conduction and thus make the conductivity lower. For example, measure the electerical resistance of a cold 100W incadensent light bulb and calculate the power it would disipate when 110Volts* is applied. You will (if you do it correctly) be surprised to find it is mych more than 100W! - This is because when it is "white hot" in the socket, the filament has much greater population of phonons and they are scattering the electrons trying to conduct the electricity - I.e. because of the photon -electron scattering, the electrical resistance of the hot filament is almost double that of the cold filament! Don't take my word for it - do it and see I am right about this also.
*Or 117V - whatever is the rated voltage printed on the bulb.

Billy T
03-25-07, 03:29 PM
...The liquids with the strongest intermolecular forces have the highest specific heats because it takes a lot of energy to absorb or break the forces and set it up for a consequence phase change to gas....More nonsense. this is called he "heat of vaporazation" (540 cal/gm for H2O, if menory serves) and has essentially nothing to do with specific heat. It has much more to do with "surface tension" which is large for H2O as it is a permanetly polar molecule (the 105 degress between the two protons, as I explainded to you in an earlier post. BTW this polar structure of lipid type molecules is why your body's cells have a self constructing walls. In polar water also at the air interface there is a "surface wall" (surface tension) and it takes a lot of energy for each molecule becoming "steam" (actually water vapor in the air) to escape thru this surface tension "wall."

Again strong hydrogen bonds have nothing to do with it.

Chatha
03-25-07, 03:40 PM
This is about the only thing in your entire post that is completely true , but I doubt if you understand why. Are you kidding? I have explained why to you, and even improved on your notion of particle vibration, which you apperantly did not understand fully. Vibration comes before motion, usually from solid state. Its not really advanced physics.




Even this is only accidently true as you said only "bonds" not "hydrogen bonds" or "atom bonds" and thus the "outer shell electron bonds" are accidently (and correctly) included.
Whether or not the outer shell electrons are included is irrelevant in determining specific heat. Specific heat is determined by the amount of force/bonds needed to be broken and absorbed for increased temperature and phase change respectively.


The bonds of the outer shell electrons (not the hydrogen atom bonds as you seem to think) do determine whether or not a sold (or liquid) is a good or poor conductor of both electricity and usually heat. Its amazing you are reading my mind now. Conductivity of heat id different from conductivity of electricity, infact the principle by which they work is very different. Electricity needs free electrons to travel, because electricity itself is basically movement of electrons. Heat is an accumulation of energy, it works better with solids because solids are basically accumulated.



If all the electrons are bound, each to a particular atom, then the substance is always a poor conductor of electicity and generally speaking, of heat as well. This is because free (not locally bound electrons) are required for electrical condution but heat can also be conducted by "phonons" (propagating latice vibrations). Usually phonon conduction of heat is much less efficient; however, it can be quite high in defect-free crystals, like clear diamonds. (The defects found in most solids "scatter the phonons" -sort of like trying to propagate an energy wave down a rope which has some bricks tied to the rope at random locations, compared to a rope with none, in which the wave will travel to the far end with little disipation of the energy in the wave.)

All this gets quite interesting in semi-conductors. - This is far above your current level of misunderstanding, but but later, when you understand the basics, read about the "Fermi levels" and their structures or anisotropic surfacesThanks, but actually its not far above my level. There is nothing you have said here that I don't know or haven't read. You omitted the fact that supercooled materials can also be good conductors. Plus, its very difficult for me to take a man who says straucture of molecues and bonds do not affect specific heat, and someone who says the specific heat of water in all three phases is the same. Stick to physics. Thought its a shame someone so good in physics can't phatom basic chemo-thermodynamics.

Chatha
03-25-07, 03:49 PM
More nonsense. this is called he "heat of vaporazation" (540 cal/gm for H2O, if menory serves) and has essentially nothing to do with specific heat. It has much more to do with "surface tension" which is large for H2O as it is a permanetly polar molecule (the 105 degress between the two protons, as I explainded to you in an earlier post. BTW this polar structure of lipid type molecules is why your body's cells have a self constructing walls. In polar water also at the air interface there is a "surface wall" (surface tension) and it takes a lot of energy for each molecule becoming "steam" (actually water vapor in the air) to escape thru this surface tension "wall."

Again strong hydrogen bonds have nothing to do with it.
LOL. Heat of vaporization is the amount needed to vaporize a substance. I never gave any reference to that. My chemistry text book were written by 3 different professors. Besides, who do you think the world will believe? The man who believes heat conductivity is determined by freedom and movement of electrons, or the rest of the world that thinks its determined by packaging and forces of phases of matter? In a way its the same thing because most solids are good conductors of heat, and most solids have free flowing 3D electrons. My point is this, foam can be made out of a solid with free flowing electrons, though it does not conduct heat. Like I said, it all falls on the arrangement and bonds of the material, you are basically saying the same thing but you are making a deal out of it, and sometimes you really don't understand its all thermodynamics. When it comes to energy, the packaging determines the efficiency. Let the ridiculusness and irrate rants begin.:eek:

Chatha
03-25-07, 03:53 PM
...Factors that affect heat capacity
Degrees of freedom:
have internal structure because they are composed of atoms. These atoms have different ways of moving within molecules. Heat energy stored in these motions does not contribute to the temperature of a substance.Degrees of freedom: Molecules are quite different from the monatomic gases like helium and argon. With monatomic gases, heat energy comprises only translational motions. Translational motions are ordinary, whole-body movements in 3D space whereby particles move about and exchange energy in collisions (like rubber balls in a vigorously shaken container). These simple movements in the three X, Y, and Z–axis dimensions of space means monatomic atoms have three translational degrees of freedom.[2] Molecules, however, have various internal vibrational and rotational degrees of freedom because they are complex objects; they are a population of atoms that can move about within a molecule in different ways (see animation at right). Heat energy is stored in these internal motions. For instance, nitrogen, which is a diatomic molecule, has five active degrees of freedom: the three comprising translational motion plus two rotational degrees of freedom internally. Not surprisingly, nitrogen has five-thirds the constant-volume molar heat capacity as do the monatomic gases.[3] See Thermodynamic temperature for more information on translational motions, kinetic (heat) energy, and their relationship to temperature.
Molar mass: When the specific heat capacity, c, of a material is measured (lowercase c means the unit quantity is in terms of mass), different values arise because different substances have different molar masses (essentially, the weight of the individual atoms or molecules). Heat energy arises, in part, due to the number of atoms or molecules that are vibrating. If a substance has a lighter molar mass, then each gram of it has more atoms or molecules available to store heat energy. This is why hydrogen—the lightest substance there is—has such a high specific heat capacity on a gram basis; one gram of it contains a relatively great many molecules. If specific heat capacity is measured on a molar basis (uppercase C), the differences between substances is less pronounced and hydrogen’s molar heat capacity is quite unremarkable. Conversely, for molecular-based substances (which absorb heat into their internal degrees of freedom), those with very high molecular weights — like gasoline — can store a great deal of energy per mole and yet, can be quite unremarkable on a mass basis.
Hydrogen bonds: Hydrogen-containing polar molecules like ethanol, ammonia, and water have powerful, intermolecular hydrogen bonds when in their liquid phase. These bonds provide yet another place where kinetic (heat) energy is stored...

http://en.wikipedia.org/wiki/Specific_heat_capacity

Billy T
03-25-07, 04:16 PM
http://en.wikipedia.org/wiki/Specific_heat_capacity

Exactly what I TOLD YOU (IN POST 5)!

Yes, I even did a calculation for NH4 vs. H2O in post 5, using the “degrees of freedom” approach.Your wiki reference is correct that there are only two rotational degrees of freedom for both O2 and N2, but I correctly used three vibration and three rotational degrees of freedom of both H2O and NH4 and even mentioned that O2 and N2 are symmetric molecules, and thus (like NH4) not permanently polarized, but water is etc.

I bet you do not know why N2 and O2 have only two degrees of rotational freedom whereas both H2O and NH4 have three. After you admit you have no ideas about this, I will tell you why.

BTW, O2 and N2 do actually have three also. It is only that in the normal temperature range, one of the three is always unexcited (always in the ground state) and thus never absorbs any energy, as if it did not exist.

Billy T
03-25-07, 04:48 PM
Can not delete what was here. Instead of editing, forum software made my edit into the next post.

Billy T
03-25-07, 04:51 PM
... someone who says the specific heat of water in all three phases is the same. ...I did not say that and it is not true and I know why.

In an earlier post, I explained that water is not really H2O molecules but nH2O molecules where n is a small integer. I.e liquid water is a constantly changing collection of short (small n) polymers of the basic H2O molecular unit as that unit is highly polar. - One unit's "O end" tends to temporarily join with the two protons (105 degrees apart only) at the "end" of another unit. This permits the polymer nH2O to have additional vibrational modes (more ways to internally store energy and thus a higher heat capacity than isolated H2O molecules) Hence, the specific heat of steam, which is only isolated H2O molecules is only about half that of water.

Further more, because of my deep understanding of all this, I predict that the molar specific heat of water is slightly lower at 90C than it is at 10C. This is because at 10C the typical value of “n” in the distribution of nH2O molecules is larger than the typical "n" at 90C. (A "mole" being defined by weight, 18 grams, not by Avocado’s number of nH2O molecules. (Because of the dynamic polarization into nH2O, a "mole of water" actually has far fewer than 6.??e23 discrete molecules. - Memory, after 40+ years, has lost the decimals. 03? 23? Something like that.)

Chatha
03-25-07, 05:24 PM
But degrees of freedom is only one of the reasons for specific heat capacity. Plus I could ask my chemsitry prof for the variability of freedom, or even look it up muself, thats no problem. My question is, why is this more important than the other two factors? Plus, if H20 and NH3 each have 3 degrees of freedom, why does NH3 still have a higher specific capacity?

After you admit you have no ideas about this, I will tell you why. I have no idea, please enlighten.

Chatha
03-25-07, 05:45 PM
Basically, what I understand is that there are three translational motions, as in the 3 dimensions of space particles can move; X,Y,Z. Then there are other internal motions refered to as rotational or internal motions, which all store energy. The more degrees of freem a substance has, the higher the specific heat because the more nodes to store energy. Mono atomic substances have only 3; translational, thus they have the lowest SHC. Many molecules have 5 degrees, which is why they have 5/3 the SNC of their monoatomic references. My guess is that the internal degrees of freedom comes from the inertia/angular momentum of collision with other atoms, but I may be wrong. What's your opinion?

Chatha
03-25-07, 06:55 PM
After reading your posts carefully, and reading about "degrees of freedom", I believe you do have a deep understanding of this topic. However you did make a mistake of refering ammonia as NH4 instead of NH3, though I don't know for sure if this automatically affects the modes of vibration. Nontheless your assessment was flawless. So let me try to paraphrase "degrees of freedom". Vibrational nodes store energy, and the more vibrational systems you have, the more energy is absorbed. Like you said, and with reference to your first post, hydrogen bonds have very little to do with all this. Pardon my differences, you have to understand that when you are an undergrad, they tell you to always accept "ideals" without real indepth analysis. However, hydrogen bonds still have something to do with specific heat, as its another way to store energy...after V modes, though insignificant in the discussion between NH3 and H20. Water having a total maximum of 6 V modes, Librations(3) and 3 internal motions(asymetric, symetric, and bent). Ammonia probably having 2 more modes from asymetric stretch of additional atom---not sure

Billy T
03-25-07, 07:44 PM
... I believe you do have a deep understanding of this topic. However you did make a mistake of refering ammonia as NH4 instead of NH3, though I don't know for sure if this automatically affects the modes of vibration. ...No, it was just a careless typo, but even if NH4 did exist, it would have the same number of rotational modes, 3, as NH3, but there would be more vibrational modes. Thanks for catching my typo.

Later by edit: -no, not a typo. I must confess I was confused. - I drew ammonia as if there were four Hs in post 5. I think my confusion was caused by recalling that it had a quadrahedron shape. Now, giving it more thought, I think the three H are in a plane and the N can be on either side of this plane. If that is correct, Amonina should be polar and some IR frequency should pump the N from one side to the other - Is this correct? I am not much of a chemist but bet NH3 is a very strong absorber for some narror band of IR.

Also thanks for reminding me that the first mode I illustrated in post 5's typed drawing is called "asymmetric stretch" and the second is the "symmetric stretch" mode. I should have given those standard names.

As I may not get back here and I like to teach I will tell you why N2 and O2 have only 2 rotational degrees of freedom and confess that I spoke of them as symmetric molecules with no permanent polarization mainly to help you guess foolishly that the fact they are un-polarized might have something to do with it. It does not. CO is a permanently polarized molecule (the O end being more negative) and also has only 2 rotational modes. It has to do with the fact that the energy levels are quantized and in the classical analogy of a rotator, the energy is (1/2)I w^2 (w is usually written as the Greek omega, but I forget how to do that here.) Thus if I, the moment of inertia about one axis of the rotation, is very small then the angular rate, w, must be very great to have the energy of the first excited state. Hence in a collisions it is essentially impossible to spin any linear (diatomic) molecule about the axis passing thru the two atoms fast enough*. Since no energy can excite even the first excited state about this axis it is as if this rotational mode did not exist.

In contrast almost any molecule with three or more atoms will have a non- zero I about all three axises, so all three can store energy. Perhaps there are some co-linear three atom molecules, but not with oxygen as the central atom as no pair of the oxygen "orbitals," where atoms can bind, are 180 degrees from one another. Lynis Pauling (only winner of two Noble Prizes on different areas) got one for his work on these "orbitals." - I think he wrote a book, "The nature of the chemical bond" which discusses all this if you want to know much more than I do.

It is always good to question what you are taught, but give your professors the benefit of the doubt when you simply do not understand.
---------------------------
*Sort of like trying to spin a very small diameter steel rod about its axis by hitting it. - Nearly impossible, but easy to make it tumble about either of two axis which are orthogonal to the rod’s axis.

Billy T
03-25-07, 08:44 PM
Basically, what I understand is that there are three translational motions, as in the 3 dimensions of space particles can move; X,Y,Z. Then there are other internal motions refered to as rotational or internal motions, which all store energy. The more degrees of freem a substance has, the higher the specific heat because the more nodes to store energy. Mono atomic substances have only 3; translational, thus they have the lowest SHC. Many molecules have 5 degrees, which is why they have 5/3 the SNC of their mono-atomic references. My guess is that the internal degrees of freedom comes from the inertia/angular momentum of collision with other atoms, but I may be wrong. What's your opinion? Mostly correct. You are learning, and that makes me feel good, if I am partly why.

Many of the most common molecular gasses are dia atomic and thus effectively have only two rotational modes. Hence the 5 degrees of freedom. Typically the rotational energy levels are much closer to the ground state and thus easier to excit. Here your earlier concern about the strength of the bond does have some validity. The strong bond and light weight of hydrogen does make the first excited vibrational state more widely separated from the ground state. (Sort of like a stiff spring and light weight has a higher frequency of vibration than a soft spring and/ or heavy weight. Crudely speaking, the higher the lowest quantized mode of vibrations frequency is, the greater is the energy needed to excite it and the less likely it is to store any added energy. Thus, the effect of the strong H bond is just the opposite of what you were thinking.)

I.e. at room temperatures the added energy goes mainly into translation and rotation, not vibration but to fully understand this you will need to learn about the statically distribution of energy (in thermal equilibrium) among the various possible energy level states. lets not go into that now.

From what you have already learn, you can now understand why CO2 and H2O are so important in the "green house” gases despite being only a very small fraction of the atmosphere (but much more of the story is related to fact that both are polar molecules and thus can interact with electromagnetic radiation, especially IR and non-polar O2 and N2 can not easily interact with IR radiation.)

Chatha
03-25-07, 10:39 PM
http://www.ems.psu.edu/~bannon/moledyn.html
http://www.pha.jhu.edu/~broholm/l37/node5.html#SECTION00011030000000000000
The one place I am confused is why diatomic molecules have 2 rotational energy. My guess is that the third unexcited axis has to do with low angular rate, since linear molecules are 180 degrees. Like you said, its almost impossible to make a micro diameter rod spin by simply hitting it. It seems to me that the Inertia or angular momentum needed for the x axis is inefficient, but from the visual references I posted. I can't really understand. I understand all the basics but I just can't put it together. But the one thing I can understand is that NH3 must have more derees of freedom since it has more atoms. Thanks for the advice.:D

Chatha
03-26-07, 12:15 PM
For me, the best way to begin to understand degrees of freedom is, s= C/(R/2), where R is the gas constant, C is the specific heat under constant pressure @25 degrees celcius, and s is the degrees of freedom for the molecule. Solving the equation I got 18 for water, which means each aton has 6 degrees of freedom. I got 19 for ammonia, which means each atom has 5 degrees of freedom. I can also see why degree of freedom is probably more important than Hydrogen bonds or molar mass.