Sciencemadness Discussion Board - Some things I don't understand.

Some things I don't understand.

Adas - 14-11-2011 at 02:23

I have some well-known chemical reactions here and I don't understand why they can work:

Ca(OH)2 + K2CO3 ----> 2 KOH + CaCO3 - Why? Doesn't Potassium have lower electronegativity than Calcium?

HCl + KClO4 ----> KCl + HClO4 - Isn't HClO4 stronger acid?

KCl + NaClO4 ----> KClO4 + NaCl - The same

+ one off-topic question: Will Cu(NCO)2 decompose to give CuCNO and (CNO)2 ?

LanthanumK - 14-11-2011 at 08:10

There is no reduction or oxidation going on. It is the formation of insoluble CaCO3 that drives the reaction forward.

I'm not sure about this one.

KClO4 is relatively insoluble, so it precipitates in concentrated solution and drives the equation forward.

DJF90 - 14-11-2011 at 11:18

All are solubility driven reactions. The HCl + KClO4 is driven by the fact that KCl is insoluble in conc. HCl (or conc. Perchloric acid, cant remember) if I recall correctly.

dann2 - 14-11-2011 at 18:09

Quote: Originally posted by LanthanumK

....
KClO4 is relatively insoluble, so it precipitates in concentrated solution and drives the equation forward.

Just wondering if you know is there a Delta T associated with that reaction.
A guy once said that if you add some concentrated KCl solution to a concentrated solution of Sodium Perchlorate (roughly proper ratios) that the mixture would be liable to boil out (splatter) of the container.
Might try it out myself sometime.

Dann2

Adas - 17-11-2011 at 03:52

And what about my off-topic question?

hissingnoise - 17-11-2011 at 04:48

Quote:

+ one off-topic question: Will Cu(NCO)2 decompose to give CuCNO and (CNO)2 ?

Fulminic acid (HCNO) is even more unstable than its salts!
I certainly wouldn't attempt its isolation . . .

ScienceSquirrel - 17-11-2011 at 04:52

Quote: Originally posted by dann2

Quote: Originally posted by LanthanumK

....
KClO4 is relatively insoluble, so it precipitates in concentrated solution and drives the equation forward.

It seems unlikely, such high energy releases are usually associated with adding strong caustic alkalis like sodium hydroxide or strong acid such as oleum to water.

hissingnoise - 17-11-2011 at 05:27

Quote:

A guy once said that if you add some concentrated KCl solution to a concentrated solution of Sodium Perchlorate (roughly proper ratios) that the mixture would be liable to boil out (splatter) of the container.

This metathesis reaction is used routinely to prepare KClO<sub>4</sub> from the sodium salt!
It produces an instant 'snowstorm' as KClO<sub>4</sub> precipitates!
No exotherm whatever!

dann2 - 2-4-2012 at 15:49

No exotherm whatever MFA Hissingnoise!!!!!!!!!

I took a concentrated solution of Sodium Perchlorate and a concentrated solution of KCl. The moles of salts were equal. The solutions were at 10°C. I dunked one into the other and got the usual instant 'yougart' mixture. The temperature also increased to 32°C.

You get the snow storm with K Chlorate not Perchlorate too!

Dann2

(I know your still there Hissingnoise,...........lurking........... I have extra (triple X, like the mints) sensory perception)

[Edited on 2-4-2012 by dann2]

Bot0nist - 3-4-2012 at 12:08

Pulverulescent is hissingnoise dann2.

dann2 - 3-4-2012 at 14:18

Get away outa that with ya!

He's just an imposter!

Anyhow I once read (Alan Yeates page) that if you were to take a hot concentrated solution of Sodium Perchlorate (would contain very little water as NaPerk is very soluble) and add it to a hot concentrated solution of KCl (proper stiocometric amounts) that was not a wise thing to do because the whole shabang would boil out on top of you when you mixed them together.
Looks like it would acually happen if the solutions were up above (roughly) 60 or 70°C
Dann2

franklyn - 20-10-2012 at 01:07

I don't understand this

Can somebody explain what is going on here.
In the first case :
Hydrazine which is the weaker base is displaced as expected by Ammonia.
Hydrazinium Sulfate is decomposed by liquid Ammonia , replacing and liberating
Hydrazine to form Ammonium Sulfate
N2H4 •H2SO4 + 2 NH3 => N2H4 + (NH4)SO4

In the next case :
Ammonia which is a stronger base than Hydrazine is displaced by it.
Ammonium Nitrate is decomposed by Hydrazine , replacing and liberating
Ammonia to form Hydrazinium Nitrate
NH4NO3 + N2H4 => NH3 + N2H4 •HNO3

__________________________________

My query relates to another post in which I assert that Ammonia is a weaker
base than Methylamine so it cannot displace it to form NH4Cl with HCl.
According to this _

CH3NH2 •HCl + NH3 => CH3NH2 + NH4Cl

Which appears analogous to the displacement of the weaker base Hydrazine
N2H4 •HCl + NH3 => N2H4 + NH4Cl

How can this be !
This reference cited by AJKOER
http://www.docstoc.com/docs/68188299/CHAPTER13
Scroll down to number 15 - Ammonolysis

.

PHILOU Zrealone - 25-10-2012 at 02:12

Quote: Originally posted by franklyn

I don't understand this

Can somebody explain what is going on here.
In the first case :
Hydrazine which is the weaker base is displaced as expected by Ammonia.
Hydrazinium Sulfate is decomposed by liquid Ammonia , replacing and liberating
Hydrazine to form Ammonium Sulfate
N2H4 •H2SO4 + 2 NH3 => N2H4 + (NH4)SO4

In the next case :
Ammonia which is a stronger base than Hydrazine is displaced by it.
Ammonium Nitrate is decomposed by Hydrazine , replacing and liberating
Ammonia to form Hydrazinium Nitrate
NH4NO3 + N2H4 => NH3 + N2H4 •HNO3

__________________________________

My query relates to another post in which I assert that Ammonia is a weaker
base than Methylamine so it cannot displace it to form NH4Cl with HCl.
According to this _

CH3NH2 •HCl + NH3 => CH3NH2 + NH4Cl

Which appears analogous to the displacement of the weaker base Hydrazine
N2H4 •HCl + NH3 => N2H4 + NH4Cl

How can this be !
This reference cited by AJKOER
http://www.docstoc.com/docs/68188299/CHAPTER13
Scroll down to number 15 - Ammonolysis

.

Le Chatelier's rule/principle for reactions in equilibrium...
A reaction system in equilibrium placed in a closed reactor with stable parameters (P,V,T) tends to a stable proportion of reactants (concentration)... A static equilibrium.

But if the system is open... V tends to be infinite and if a compound is volatile (gaseous) the reaction /equilibrium will be shifted to that side of the reaction and go to near completeness. The same kind of theory applies to precipitation reaction where the precipitate goes to a concentration of "zero" and is considered as dropped out of the system.

Here N2H4 is much less volatile than NH3...
So in an open system despite NH3 is more basic...N2H4 will remain because NH3 will escape faster and continuously from the reactor.
In a closed reactor:
N2H4(l) + NH4Cl(s) <==--> N2H5Cl(s) + NH3(g)
In an open reactor:
N2H4(l) + NH4Cl(s) <--====> N2H5Cl(s) + NH3(g)

In the case of NH3 and CH3-NH2... CH3-NH2 is more basic than NH3 but both are very volatile... so the question is more delicate.
In a closed reactor:
NH3(g) + CH3-NH3Cl(s) <==--> NH4Cl(s) + CH3-NH2(g)
In an open reactor:
NH3(g) + CH3-NH3Cl(s) <==?==> NH4Cl(s) + CH3-NH2(g)

Nicodem - 25-10-2012 at 06:42

Quote: Originally posted by franklyn

My query relates to another post in which I assert that Ammonia is a weaker
base than Methylamine so it cannot displace it to form NH4Cl with HCl.
According to this _

CH3NH2 •HCl + NH3 => CH3NH2 + NH4Cl

Which appears analogous to the displacement of the weaker base Hydrazine
N2H4 •HCl + NH3 => N2H4 + NH4Cl

Those are just equations which essentially just define the reactants and the products, but say nothing about the synthesis. Several times I already tried to explain that bare chemical equations like those do not describe the chemical system. The problem is that it is the whole system (solvent, temperature, reagent ratios, rate and order of addition, catalyst, stirring...) that defines the reaction results. For example, if you add methylamine hydrochloride to aqueous ammonia, no product will form. But if you add methylamine hydrochloride to ethanolic ammonia, I believe it is quite likely that you will obtain a precipitate of NH4Cl. All the parameters are the same, only the solvent changes and the change can be as dramatic as that.

Also, you say ammonia is a weaker base than methylamine. That is true for aqueous solutions where ammonia is about 10-times weaker base, but this is not necessarily true for other solvents. Compare the pKa values of NH₄⁺ and RNH₃⁺ ions in DMSO and you will see how things can change. Besides the proton transfer reactions are just one of the many possible driving forces for a product to form - they are not necessarily the determinative force.

franklyn - 25-10-2012 at 11:06

Thank you both for explaining that. This has actually been the
epiphany moment for me since I've been a member here.
It now seems like one also needs to have expertise in law
to be able to understand chemistry in depth.

.

AJKOER - 25-10-2012 at 11:39

Per this questionable source (http://www.hoboes.com/pub/Prohibition/Drug%20Information/Mis... ), "Methyl Chloride + Ammonia in an ether solution (CH3NH2 + NH4Cl)", the use of a different medium as was suggested by Nicodem above was a possible route .

[Edited on 26-10-2012 by AJKOER]