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 - 26-9-2017 at 15:48
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 - 26-9-2017 at 16:44
This is the thermodynamic explanation. It might be wrong since my thermodynamic knowledge is a bit rusty.
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 - 30-9-2017 at 16:19
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 - 30-9-2017 at 18:44
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 - 1-10-2017 at 04:59
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=-422DraconicAcid - 1-10-2017 at 10:26
I have got negative Gibbs free energy for this reaction.
That's why everything is strange.
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).ninhydric1 - 1-10-2017 at 10:27
Additional energy as in activation energy. My mistake. AJKOER - 1-10-2017 at 11:19
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 - 1-10-2017 at 11:41
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 - 1-10-2017 at 14:25
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 - 1-10-2017 at 14:40
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.sulfuric acid is the king - 1-10-2017 at 17:39
Yea i calculated...it's 6x more favourable...
So what is meant by spontaneous?DraconicAcid - 1-10-2017 at 17:48
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.
