chief - 19-7-2007 at 23:53
How ecxactly do nitrates and/or manganates decompose ? :
- Does there exist a fixed phase-change-temperature , above which all the mass is decomposed,
OR
- does it decompose with a time-constant (temperature dependent) and thereby with a half-life-time, so that over a temperature-range the fresh and
the decomposed materials coexist?
chief - 23-7-2007 at 11:26
Hello there !! I have made some experiment with nitrates: NaNO3 melts at 306[Celsius] and decomposes over a temperature-range up to over 600[Celsius]
and still has oxygen in it !!
So its a question of reaction-dynamics to be optimized. Most probably manganates have too some temperature-time-range of decomposition, but if its as
wide as with the NaNO3 I don't know yet. Any Ideas?
Mario. - 13-1-2008 at 14:14
For example the KNO3 decomposes to KNO2 + O -> that is the main path of decomposition which can limit the burn rate of a propellant. NH4NO3 for
example starts to melt and decompose very, very early, BUT... there is a certain equilibrium that causes the decomposition products to be trapped in
the molten mass and form NH4NO3 back afterwards!
As in case of any other reaction, the faster you are able to remove the reaction products the faster the reaction will proceed. Imagine an
equilibrium, say 10:1 with 10 being the source and 1 being the product... the reaction goes very slowly and when there is 9% of the source converted,
the reaction seemingly stops. The same is tru with equilibriums which favour the decomposition products, say with equilibrioum 1:100. The closer the
product concentration will get to the 100:1 ratio the slower will the rection proceed.
Nitrates DO NOT compare to manganates. That is all. N is gas, Mn is metal. Consider that in reactions which involve production of gases. So there
always something stays inside, like MnO2 for example, which can be a good catalyst to many reactions. Manganates can be compared to Ferrates,
Chromates, Molybdenates, Technates, with the last hardest to compare to.
Af for your NaNO3 melting: If you had pure NaNO3 at the start, you proceeded to create NaNO3 + NaNO2 + NO2 + NO + other gases solved in that, plus
this all became a ionic solution of Na+, NO3-, NO2-, etc.. so that the actual properties of that mix vary every second. If all you want to do is to
increase decomposition speed, you may try to: remore products faster, increase reaction area/area of products removal, use catalysts to lower
activation energy of a favorable reaction, use other reactants to remove reaction products or to move the reaction equilibrium to the favored side.
etc, etc, etc...
As for the time constant... that is only the observed thing, not the actual... the molten mass can dissolve lot of gas. For example AN fertilizer is
manufactured in opposite manner than it decomposes... (not exactly) - the ammonium gas is bubbled into the HNO3 acid + molten AN mix.
[Edited on 13-1-2008 by Mario.]
NH4NO3 Decomposition
MadHatter - 13-1-2008 at 14:41
With careful heating, ammonium nitrate, will decompose to water and nitrous oxide. Heat
it too fast or too high, it explodes.