# hard time to understand azetrope of hcl...



## ericrm (Oct 11, 2012)

L'acide chlorhydrique en tant que mélange binaire de HCl et H2O possède un azéotrope en ébullition à 108,6 °C pour une fraction de 20,2 % HCl.

i know its french but ... hcl as a binary solution hcl and h2o, have an azeortop in boiling at 108.6 for a fraction of 20.2 % hcl

do that mean that i can boil off water(only water) at 100%c in an 10% hcl solution until the 20.2% is obtain? i try realy hard to understand the azeotrop thing so if someone can tell me if i understand right or wrong pls


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## element47.5 (Oct 11, 2012)

from wikipedia (which explains the phenomenon quite well)

An azeotrope is a mixture of two or more liquids in such a way that its components cannot be altered by simple distillation. This happens because, when an azeotrope is boiled, the vapor it produces has proportionate constituents as the original mixture. 

Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is either less than the boiling point temperatures of any of its constituents (a positive azeotrope), or greater than the boiling point of any of its constituents (a negative azeotrope). 

A well known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by weight).[4] Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.2°C, which is lower than either of its constituents.[5] Indeed 78.2°C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures or pressure maximum azeotropes.

An example of a negative azeotrope is *hydrochloric acid at a concentration of 20.2%* and 79.8% water (by weight). Hydrogen chloride boils at −84°C and water at 100°C, but the azeotrope boils at 110°C, which is higher than either of its constituents. The maximum temperature at which any hydrochloric acid solution can boil is 110°C. In general, a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called maximum boiling mixtures or pressure minimum azeotropes.


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## ericrm (Oct 11, 2012)

i have already read wiki ,but it doesnt make me understand,so i cant answer my question...


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## butcher (Oct 12, 2012)

Similar we can make a strong moonshine by using the azeotrope when distilling ethanol, :lol: 

We can concentrate a dilute solution of nitric acid by evaporation up to its azeotropic mixture of about 68%, without loss of very much nitric acid, as long as we do not heat it to too high of a temperature at that concentration, as the boiling point of nitric acid changes with its concentration, evaporation of water from solution can occur well below the boiling point of nitric acid at that concentration.

Evaporation of the dilute nitric acid will vapor off mostly water (temperature kept below boiling point of nitric acid at that concentration) this will raise the concentration of the acid, and the procedure can continue to the azeotropic solution of 68% nitric is reached

I have tried to explain this in several posts, and gave boiling points and densities for different concentrations of nitric acid.
Those posts will have more detail than I will post here.

20% HNO3 boiling point 104 deg C
30% HNO3 boiling point 107 deg C
50% HNO3 boiling point 114 deg C
68% HNO3 boiling point 120 deg C
Notice how the boiling point raises as it gets more concentrated.

If we kept heating after azeotrope the fume would just vapor off 68% nitric acid until the pot was dry, we cannot concentrate past the azeotropic mixture.

For evaporation concentrating nitric acid keep the temperatures below boiling points for that concentration of acid, as water will vapor off well below the boiling point of nitric acid, too strong of heat will just boil off acid wasting it.

From this we see we can concentrate the 10% HNO3 to 68% HNO3. by heating around 80 deg C until concentrated, we can test concentration by the reaction with copper, or by using density measurements as the density changes with concentration.

The dilute nitric will also concentrate when heated and reacting with metals, so a dilute nitric can be heated to dissolve silver, it would also waste less nitric acid as NOx gas if not heated to hot, although it may take a little more time, as the silver would react with dilute solution and the nitric acid would be concentrating as the water vapors are evaporated from heated solution, dilute solutions also can keep the NOx gas in solution as nitric acid dissolve more silver, than with what might have been wasted as gas if we were using a more concentrated acid on the silver.


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## ericrm (Oct 12, 2012)

Butcher, once again your explanation are very clear and professionnal. but i want to know about HCl :| . the probleme with Hcl is that it is a disolve gaz and not a liquid of its own. alcool and water,sulfuric and water,nitric and water, nitric and sulfuric those are all homogenous mixture of 2 liquid... HCL is not, HCL is a solution of dissolve gazeous salt in water(i dont know if the therme salt is very well used here) this is why i dont know if its azeotrop will act the same way than the rest and apparently with the few answer i get i think im not alone lol


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## Geo (Oct 12, 2012)

Eric, i have no answer for your question. i would love to understand the mystery of chemical science myself. if that part baffles you, think about this. hydrogen is a gas. oxygen is a gas. together they create a liquid.this liquid is an acid and a solvent for almost everything but we take it for granted all the time and consider it harmless. i know that when you mix a gas with water, its not just mixing a gas with water. your mixing a gas with hydrogen oxygen,which is two gases in itself. when you add the gas to hydrogen and oxygen, you no longer have a separate gas and hydrogen and oxygen but have created a whole new compound with its own properties and aspects whether the gas your adding is hydrogen,chlorine or even oxygen.

im sorry, i feel dumber for even typing all that mess. :lol:


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## butcher (Oct 12, 2012)

Geo you have the idea you just need to study more on that idea, then you can get as confused as I do.

Nitrogen and oxygen both gases can combine to form many compounds (depending on the conditions that combined them) let’s look at nitrogen and oxygen, they can combine to make nitrogen monoxide (NO) a different gas.
Nitrogen monoxide (NO) combines easily with oxygen to form NO2 (note these so far are gases),
NO2 is a gas when it reacts with water it forms nitric acid HNO3 as it dissolves into solution.

as we distill a solution of nitric acid we can separate these gases and water, or these can separate, in chemical reactions, or with electrolysis. 

Actually water in solution is Hydrogen and oxygen as H2O or H3O (or even) H+ + OH- depending on conditions and as we know we can separate the ions or gases with electricity in an electrolytic cell.

Chlorine is a gas when it reacts with water it forms hydrochloric acid (HCl) and hypochlorous acid (HOCl)

This hypoclourous acid is very unstable in acid or light and will break down into hydrochloric acid (HCl) and OXYGEN (O2)
2HOCl --> 2HCL = O2 

if the water or solution was basic like with sodium hydroxide The hypochlorous acid HOCl would form sodium hypochlorite (NaClO) (bleach in solution).

Sulfur can combine with oxygen and form sulfur dioxide SO2 if this gas comes into contact with water it forms H2SO4 sulfuric acid.
Sulfuric acid has a high boiling point and a very high azeotropic mixture; we can concentrate it by evaporating water to about 98% after this we begin to break the acid down as we fume off SO2 gas.
HCl acid has an azeotope of about 20%.

chlorides can be oxidized one way is by using manganese dioxide (MnO2) (this can be found in batteries), this way we can oxidize a chloride like HCl acid to make chlorine gas.
4HCl + MnO2 --> MnCl2 + 2H2O + Cl2 gas

And as we seen in the reactions above we can reduce the chlorine back to a chloride like HCl


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