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Finding Right Reaction

by anubodh
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anubodh
#1
Nov5-13, 09:48 AM
P: 35
Sir, i desperately want to know a reaction in which the resulting product has more density than the reactants and which can be reversed easily (prectically) with minimal or no loss of energy.Please reply.Thanks..
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mfb
#2
Nov5-13, 12:30 PM
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Chemical reactions do not produce mass, if you don't change the volume (which is not part of the chemical reaction) you cannot change the density. Reversing reactions is often problematic, but that is a different issue. You can easily revert a physical change of the volume.
anubodh
#3
Nov6-13, 08:38 AM
P: 35
I know that mass remains conserved but what i am saying is that initially if we have a chemical of volume v in which we add another chemical of volume v1.When we add them the volume of the resulting product is same but the density of the product becomes twice of v.

mfb
#4
Nov6-13, 08:57 AM
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Finding Right Reaction

Then you have to compress the volume from v+v1 to v. That is not part of chemistry.
Ygggdrasil
#5
Nov6-13, 09:03 AM
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There are plenty of reactions that lead to an increase in volume (namely those that involve solids and/or liquids reacting to produce gasses (e.g. the decomposition of sodium azide into sodium and nitrogen gas, which was used to inflate airbags in cars). However, many of these reactions are not so easy to reverse. Of course, once you start asking for reactions that can be easily reversed with minimal or no loss of energy, you begin straying into the realm of perpetual motion machines and breaking the second law of thermodynamics.
Borek
#6
Nov6-13, 09:06 AM
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Technically any spontaneous reaction taking place in the gas phase, with stoichiometry described by

A + B → C

will do what you want with the density (final density being sum of densities of the reactants). For example phosgene synthesis (somehow at the moment I draw blank when it comes to safer examples):

CO + Cl2 → COCl2

Not that it will be easy to find one that will be reversible.
anubodh
#7
Nov6-13, 11:47 PM
P: 35
thanks for your replies.
Now,instead of naming the reaction, just tell me that is such a reaction is possible in which the final density being sum of densities of the reactants (or little less than sum of both) provided both the reactants and products in liquid state.
Borek
#8
Nov7-13, 04:32 AM
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Unlikely. Gas is a gas, liquid is a liquid, they behave differently.
mfb
#9
Nov7-13, 08:04 AM
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The final density has to be the same as the sum of the densities of the reactants, if you don't change the volume during the reaction.

It looks like you are thinking of something specific. Can you give more details what you are looking for?
DrDu
#10
Nov7-13, 09:02 AM
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Quote Quote by mfb View Post
Then you have to compress the volume from v+v1 to v. That is not part of chemistry.
What do you mean here? In most reactions, you work at fixed (namely ambient) pressure, so volume changes are natural in chemical reactions.
mfb
#11
Nov7-13, 09:19 AM
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Then you still have to get the temperature right, and make sure that nothing can escape from the reaction - in general, you won't get exactly v as result.
DrDu
#12
Nov7-13, 09:36 AM
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I am still thinking that the answer to the original question is relatively easy. E.g. the density of water is higher than that of ice at 0 deg. so melting ice will increase density and is easily reversible.
A mixture of alcohol and water is also more dense than the mean of the pure liquids and easy to separate. Mixing concentrated H2SO4 with water even leads to an absolute decrease in volume at high solution although separating into components is less easy.
epenguin
#13
Nov8-13, 05:33 AM
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I suppose any reaction in which there is a precipitate fits your bill. Almost by definition. When dissolved reactants react to form a solid product this is almost always denser than the solution, and can be separated by a simple laboratory centrifugation. Very common in laboratories.


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