Madchemyst
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Question on Strong acid concentartion
Hi all,
I am just starting to get back into chemistry. It has been quite a while but the more I am getting into it the more I am enjoying it. I have been
reading and studying as much as playing and have a question. The question I have is why would you dilute a strong acid verses just using it
concentrated? Is it a cost thing or a technical thing to make more Hydronium ions available? If that is even correct? If it is a technical thing how
do you know which to use and what dilution? Thanks for any help and this is a great place to learn stuff....
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Molecular Manipulations
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It depends on what you're doing. Tests and chemical analysis usually can be done with very dilute reagents, while practical chemistry and chemical
synthesis are better suited with concentrated reagents because then you don't have to boil off as much water etc.
With very concentrated mineral acids like 98% sulfuric and 99% nitric acids there are very few actual hydronium ions because there's not enough water
to get protonated. This is rarely an issue because in a reaction the water is regerorated as the substance is being protonated.
It also depends on your' definition of concentrated and dilute.
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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gdflp
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Welcome to Sciencemadness
Another reason to use a dilute acid rather than a concentrated acid is as follows : Let's say that the desired product is a simple inorganic salt, for
example barium chloride. Adding concentrated hydrochloric acid to solid barium carbonate won't work well. Since there isn't enough water, eventually
the reaction will slow down or cease as the barium carbonate particles get coated in a layer of barium chloride which is unreactive towards the acid
and prevents further attack on the barium carbonate. If the acid is dilute enough however, the barium chloride product will dissolve in the water and
the reaction will proceed vigorously. Here's an example from woelen, one of our moderators, illustrating this.
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DraconicAcid
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And if you need ten millimoles of acid, it's far easier to measure out ten mL of 1.0 M acid than 0.8 mL of concentrated HCl.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Madchemyst
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Thanks so much for the replies and for the link gdflp. I will be trying this. Like so many things in life, nothing is simple. As the saying goes,
experience is the best teacher, along with folks willing to take their time to help.
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DraconicAcid
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The wisest people learn from other people's mistakes.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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aga
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... as well as their own.
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j_sum1
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Heh. I must be wise then.
I make enough mistakes.
Some of them in full view of others.
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BromicAcid
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One of the big mistakes I made early on with concentrated solutions was trying to use the most concentrated solution possible to affect a
precipitation. The issue there is that the first bit of material immediately starts crashing out material, you end up with something called occlusion
where you end up trapping acid or reagent within the crystals, sometimes you end up making an amorphous blob instead of good looking crystals. Simply
put, as many others have said above, it depends on what you are doing.
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Madchemyst
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The main reason for the question is I want to learn chemistry not just do cookbook experiments. However, in doing experiments from books, sometimes
it states to use Conc. HCl or add 10 ml of 1.0M HCL to water...etc. So I could not figure out why you would use one over the other. I started
reading more about acid -bases and although I now understand enough to know how much I don't understand, I still did not see the answer. It really is
a puzzle to say this is what I want and this is how I manipulate those little buggers we call atoms to get something else......... I have been
reading a text book to learn the theory, using the "Illustrated Guide to Home Chemistry Experiments" to learn technique and using the The Golden Book
Of Chemistry Experiments to learn some of the basic properties and reactions of elements. Any suggestions to this approach or other material would be
welcome. Thanks again all..
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gdflp
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The forum has a library of chemistry books which can be found here. Lots of the books are rather advanced, but if you see a certain compound that you are interested in preparing, there are a bunch of
preparative manuals that may have synthesis details. Some that I would recommend for Inorganic Chemistry are Inorganic Laboratory
Preparations by Gert Schlessinger, Handbook of Inorganic Preparative Chemistry by George Brauer and for organic chemistry,
one of the better ones is Practical Organic Chemistry by Arthur Vogel.
In addition, for a general overview of chemistry I would get a general chemistry textbook designed for freshman college students, don't get the newest
edition as it is more expensive and unnecessary since no major changes in basic chemistry have happened for a while A textbook one edition older can generally be had for around $10USD, I would
recommend looking on Amazon.
Searching the forum by typing : "search terms + site:www.sciencemadness.org/talk/viewthread.php" into google can yield a wealth of information, it is
a 12 year old forum and many topics have already been discussed. If you have any questions, let us know.
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