sulfuric acid is the king
Hazard to Self
Posts: 94
Registered: 11-1-2017
Member Is Offline
|
|
Inorganic Mechanism or ?
I really don't know how to name this topic or where to put it...
But it's interesting...
2 KNO3 + 5 C = 2 KCN + 3 CO2
KNO3 + 2 C = KCON + CO2
4 KNO3 + 5 C = 2 N2 + 3 CO2 + 2 K2CO3
2 KNO3 + 4 C = 3 CO + K2CO3 + N2
4 KNO3 + C = CO2 + 2 K2O + 4 NO2
Same reactants,different products.
Some are possible some impossible?
Why?
Most of the time will be 4 KNO3 + 5 C = 2 N2 + 3 CO2 + 2 K2CO3
What's the mechanism?
Thanks!
|
|
|
ninhydric1
Hazard to Others
Posts: 345
Registered: 21-4-2017
Location: Western US
Member Is Offline
Mood: Bleached
|
|
Some are impossible if you calculate the enthalpy, entropy, and Gibbs-free energy of the reaction. The reason the equation 4 KNO3 + 5 C = 2 N2 + 3 CO2
+ 2 K2CO3 is the most common is because it has a high entropy (it produces 6 moles of gas and 2 moles of solid from 9 moles of solid is how I think of
it) and it has a negative enthalpy and Gibbs-free energy.
The reason 4 KNO3 + C = CO2 + 2 K2O + 4 NO2 doesn't work is because K2O has a high Gibbs-free energy, even if the reaction as a positive entropy. The
same thing too for CO.
This is the thermodynamic explanation. It might be wrong since my thermodynamic knowledge is a bit rusty.
|
|
|
CRUSTY
Hazard to Others
Posts: 139
Registered: 5-6-2016
Location: Nearby
Member Is Offline
Mood: High-Order
|
|
That's the gist of it. Part of the issue as well comes with activation energy. If these were all spontaneous at room temperature (ΔG<0), which
none of them are, but if they were, then the reaction with the lowest ΔG would dominate. However, since this is not the case, activation energies may
take precedent in some situations over the differences in Gibbs free energy, varying based on mechanisms.
|
|
|
sulfuric acid is the king
Hazard to Self
Posts: 94
Registered: 11-1-2017
Member Is Offline
|
|
2 KNO3 + 5 C = 2 KCN + 3 CO2
I have got negative Gibbs free energy for this reaction.
That's why everything is strange.
|
|
|
ninhydric1
Hazard to Others
Posts: 345
Registered: 21-4-2017
Location: Western US
Member Is Offline
Mood: Bleached
|
|
That reaction is actually possible at high temperature. Calculate the enthalpy and you will see it requires a lot of additional energy.
|
|
|
sulfuric acid is the king
Hazard to Self
Posts: 94
Registered: 11-1-2017
Member Is Offline
|
|
The problem is that by the H,reaction is spontaneous at room temperature...
2KNO3+5C--->2KCN+3CO2
2(-493)+0--->2(-113)+3(-394)
-986--->-1408
rH=-1408-(-986)
rH=-422
|
|
|
DraconicAcid
International Hazard
Posts: 4357
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
That means it's thermodynamically spontaneous, but it doesn't actually mean that it will happen, especially if there are other reactions that are more
spontaneous. I don't have the numbers handy, but as an example, the rxn:
2 Ag+(aq) + 2 I-(aq) --> 2Ag(s) + I2(s) is spontaneous, having a negative Gibb's free energy. But we all know that if you mix solutions of silver
ions and iodide ions, you don't get a redox reaction at all- you get the precipitation of silver iodide (because that reaction has a more negative
deltaG).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
ninhydric1
Hazard to Others
Posts: 345
Registered: 21-4-2017
Location: Western US
Member Is Offline
Mood: Bleached
|
|
Additional energy as in activation energy. My mistake.
|
|
|
AJKOER
Radically Dubious
Posts: 3026
Registered: 7-5-2011
Member Is Offline
Mood: No Mood
|
|
Pressure could also be a factor. In a system under high pressure, the reaction forming the least gas volume (or a reduced volume) may come into play
irrespective of its unfavorable thermodynamics.
Note, ∆H is the change in enthalpy between reactants and products, which is the heat given off or absorbed by a reaction at constant PRESSURE.
Similarly, a system subject to a very high temperature could select, from a set of possible reactions, the one consuming a lot of energy thereby
reducing stress.
Note, the equation ∆G° = ∆H° - T∆S° can be somewhat deceptive as it implies a simple linear relation with temperature as the only variable.
However, in reality, both ∆H° and ∆S° change as a function of T if there is a difference in the heat capacities of the reactants and products.
See https://ocw.mit.edu/courses/biology/7-51-graduate-biochemist...
One of my readings, as I recalled involving products of heating NH3/O2, demonstrated in a real world situation, that thermodynamics can, at times,
provides only a rough guide if T and P are changing (predicting chemical reaction software incorporate changes in T and P, see, for example, http://www.sciencebysimulation.com/chemreax/Faq.aspx ).
[Edited on 1-10-2017 by AJKOER]
|
|
|
DraconicAcid
International Hazard
Posts: 4357
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
Enthalpy of reaction won't tell you if the rxn is spontaneous.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
sulfuric acid is the king
Hazard to Self
Posts: 94
Registered: 11-1-2017
Member Is Offline
|
|
If somebody of you really understands subject,then please,tell me at which conditions will that reaction happen 
Thanks!
|
|
|
ninhydric1
Hazard to Others
Posts: 345
Registered: 21-4-2017
Location: Western US
Member Is Offline
Mood: Bleached
|
|
Here is the reference that the reaction can work:
http://www.sciencemadness.org/talk/viewthread.php?tid=23
Now, here's the thermodynamic explanation.
If you calculate Gibbs-free energy, enthalpy, and entropy, you can plug those values into one of the most valuable equations in thermodynamics:
ΔG=ΔH-TΔS
ΔG=Gibbs-free energy
ΔH=Enthalpy
T=Mininum temperature at which the reaction is SPONTANEOUS
ΔS=Entropy
Once you find T, you know the temperature at which the reaction is spontaneous. This temperature is partially the reason activation energy is needed.
I recommend looking into thermodynamics. It helps explain many reactions that occur in inorganic chemistry.
|
|
|
sulfuric acid is the king
Hazard to Self
Posts: 94
Registered: 11-1-2017
Member Is Offline
|
|
I already wrote that the reaction is spontaneous at room temeprature...
Positive S and negative H is always spontaneous at all temeratures...
I am still waiting for the answer at which conditions will that reaction work.
So if somebody can tell exact data...that will be very nice to see
|
|
|
DraconicAcid
International Hazard
Posts: 4357
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
Yes, it's thermodynamically stable at all temperatures. But that doesn't mean it will actually happen, so much as assuring us that the reverse
reaction will not happen. We cannot calculate the conditions under which is will occur.
Calculate the Gibb's Free Energy of 4 KNO3 + 5 C = 2 N2 + 3 CO2 + 2 K2CO3 I can almost promise you that it will be more favourable under nearly all
conditions.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
sulfuric acid is the king
Hazard to Self
Posts: 94
Registered: 11-1-2017
Member Is Offline
|
|
Yea i calculated...it's 6x more favourable...
So what is meant by spontaneous?
|
|
|
DraconicAcid
International Hazard
Posts: 4357
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
If a reaction is spontaneous, that means the products are more stable than the reactants- if a favourable mechanism can be found, he equilibrium
constant for the reaction would be greater than 1. Just like the water in the Great Salt Lake would be more stable in the Pacific Ocean than where it
is. However, that doesn't mean that there is a mechanism that will allow it- just like the Great Salt Lake doesn't flow into the Pacific because
there's a bunch of frikkin mountains in the way.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
