N to N2

[observer1]

I still have a doubt
What is the 9000C for? Is that the temperature when N2 break?

[exchemist]

5 hours ago, observer1 said:

I still have a doubt
What is the 9000C for? Is that the temperature when N2 break?

There is no magic temperature at which the bonds suddenly break.

At any given temperature, molecules have a statistical range of velocities. When you dissociate a substance by heating, what happens is some of the molecules get enough thermal kinetic energy for the bond holding the atoms together to break. If a dissociated atom later on encounters another dissociated atom then, unless the atoms have between them more energy than the bond energy, they will recombine. So what you have is a dynamic process, in which some molecules are splitting into their constituent atoms, and some atoms are recombining into molecules. This is a chemical equilibrium: N₂ <-> 2N.

Whether most of the substance is in the form on the left hand side or the form on the right hand side depends on the bond energy, the entropy of the two states and the conditions (temperature and pressure). As you raise the temperature, the average velocity of the molecules increases. That means that the fraction of them with an energy greater than the bond energy increases. This results in a greater fraction of them being in the dissociated state at any given moment.  A strong bond energy favours the left hand side, while a significant entropy of dissociation favours the right hand side, more so at higher temperatures.   

What I am pointing out is that - because the bond energy in this case is so high - it is not until you reach about 9000C that you will have equal amounts of molecular and atomic nitrogen. In fact, I've now got a more accurate value for the entropy of dissociation, 115J/K-mol, which leads to temperature of more like 8200C for a 50:50 mixture of atomic and molecular nitrogen. (This is in good agreement with @studiot's earlier post in this thread, in which he quoted a textbook saying that at 8000C, there would be 40% dissociation.) 

By the way, catalysts are irrelevant to this. A catalyst does not change the thermodynamics of a reaction, which is what determines the equilibrium state. It just accelerates the rate at which it gets to equilibrium from an initial set of reactants. In the Haber process, the catalyst accelerates the reaction by avoiding having to dissociate the nitrogen molecule into atoms before reacting with hydrogen. It does this by forming new bonds between nitrogen and the metal surface. The energy of the bound molecules and atoms stays lower, throughout the process, than if they had to be free atoms. As a result, a greater fraction of the molecules have enough energy to react - so it goes faster. 

 

Edited August 23, 2022 by exchemist

[observer1]

ok so at 8,200C i will have a 50:50 mixture of atomic and molecular nitrogen, so is it straightforward like if i double the temperature (theoretically) to 16,400C, will i have a 100% atomic nitrogen?

[studiot]

19 minutes ago, observer1 said:

ok so at 8,200C i will have a 50:50 mixture of atomic and molecular nitrogen, so is it straightforward like if i double the temperature (theoretically) to 16,400C, will i have a 100% atomic nitrogen?

Can't seek how you make that out when exchemist is trying very very hard to help you with some great explanations. +1

 

How is this a response to being told

2 hours ago, exchemist said:

What I am pointing out is that - because the bond energy in this case is so high - it is not until you reach about 9000C that you will have equal amounts of molecular and atomic nitrogen.

 

 

Edited August 23, 2022 by studiot

[observer1]

see i am a child still. I learnt NONE of what we were discussing in school. I don't know some of what he was saying.
I am uneducated compared to him on this topic and in the thing i want nearly complete atomic nitrogen. Is it possible theoretically or is 50:50 the maximum ratio

@studiot

[exchemist]

1 hour ago, observer1 said:

ok so at 8,200C i will have a 50:50 mixture of atomic and molecular nitrogen, so is it straightforward like if i double the temperature (theoretically) to 16,400C, will i have a 100% atomic nitrogen?

It's not that simple, because it's to do with the statistical distribution of kinetic energy among the atoms and molecules and how that alters with temperature. But certainly there would be a lot more atomic N, if nothing else were going on at such enormously high temperatures (see later in the post).

There are two formulae in chemical thermodynamics which enable us to work it out. The first I've already mentioned: ΔG =  ΔH - TΔS. We now have the values for this reaction, so we can say ΔG = 945 x 1000 - (16400 +273)x115, which gives a value of  ΔG = - 9.75 x 10⁵ . (I add 273 to the temperature as it needs to be absolute, i.e in K rather than deg C.)

The second formula is ΔG = -RTlnK, where K is the equilibrium constant, in this case (p(2n))²/p(n2), p(2n) being the partial pressure of atomic N and p(n2) being that of molecular nitrogen. So lnK = -  ΔG/RT = 9,75 x10⁵/(8.32 x (16400 +273))  ~ 7. So that makes K ~ 1000. So there will be approx √1000 times, i.e. about 30 x, as much atomic N as molecular N2 at that temperature........

...or would be, if there were no other processes set in train by such an astronomically high temperature. However, at such a temperature you would no longer have entirely nitrogen atoms! The 1st ionisation energy of N is 1400kJ/mol. You would have a lot of N+ ions and free electrons, i.e. a plasma.  This temperature is about 3 times that of the surface of the sun ( refer  @chenbeier 's earlier post ).

So there you have it. The bonding in nitrogen is so strong that to break it you need to start breaking up the atoms themselves and have to resort to stellar temperatures.  Best to look elsewhere for a practical way to store energy.   

[observer1]

so there is not practical way of using N2 as fuel ryt?

[exchemist]

52 minutes ago, observer1 said:

so there is not practical way of using N2 as fuel ryt?

Well, there are fuels that makes use of the bond energy of nitrogen, which seems to be the idea you are pursuing.

Hydrazine for instance (H2N-NH2) has an enthalpy of combustion of 620kJ/mol, i.e. about 2/3 that of the N-N triple bond, and that is partly because it forms N2 as well as water when it burns, converting an N-N single bond into the triple bond. Hydrazine is also used as a rocket fuel without combustion, being decomposed by a catalyst into N2, NH3 and hydrogen. Again the formation of N2 is responsible for a large part of the heat output. 

But you can't separate nitrogen into atoms and store it in that form for fuel. That can't be done. 

By the way, I see you are still at school. I thought you might be. The reason I went into the thermodynamic equations was just in case you were starting to work with them in school - and because I thought it might be fun to apply them to this problem of yours. It is the kind of thing you will learn about if you study chemistry in the 6th form, at about the age of 16-17.  If you have not got to that stage, don't worry. 

 

Edited August 23, 2022 by exchemist

[observer1]

oh well.......

there goes my idea of using the energy released when N becomes N2 for fuel.

[observer1]

is there a way to close the topic or should i just leave it @exchemist

[exchemist]

7 hours ago, observer1 said:

is there a way to close the topic or should i just leave it @exchemist

Just leave it. Someone else may came along and add something. They don’t normally get closed unless a moderator considers the discussion has become pointless or objectionable.

[observer1]

k

[sethoflagos]

On 8/23/2022 at 1:11 PM, exchemist said:

...or would be, if there were no other processes set in train by such an astronomically high temperature. However, at such a temperature you would no longer have entirely nitrogen atoms! The 1st ionisation energy of N is 1400kJ/mol. You would have a lot of N+ ions and free electrons, i.e. a plasma.  This temperature is about 3 times that of the surface of the sun ( refer  @chenbeier 's earlier post ).

So there you have it. The bonding in nitrogen is so strong that to break it you need to start breaking up the atoms themselves and have to resort to stellar temperatures.  Best to look elsewhere for a practical way to store energy.   

+1

For the full shebang try https://publications.csiro.au/rpr/download?pid=csiro:EP116380&dsid=DS1