Sciencemadness Discussion Board - Acid/Base Chem Equations

Acid/Base Chem Equations

denis - 18-11-2008 at 20:25

I am utterly lost with the chemical equations for strong acids/bases.

For a strong base (HCl) the (dissociation) equation would be:
HCl(aq) + H2O(L) -> Cl(aq) + H3O(aq)

but for a stong base (Sr(OH)2) it would be:
Sr(OH)2 --H2O--> Sr + 2OH

Why is the H2O not included in the equation for strong bases (instead on the arrow) but on a strong acid, the H2O is included?

Sorry if this is a lame question, I just need help badly

Nicodem - 19-11-2008 at 00:39

Acid/base chemistry is about transfers of protons from one chemical species to the other (by Bronstein definition) or electron pair donation from one chemical species to the other (a wider definition by Lewis). I suggest you to use the Lewis concept since it is more general and actually makes more sense:
http://en.wikipedia.org/wiki/Acid-base_reaction
http://en.wikipedia.org/wiki/Lewis_acid
http://en.wikipedia.org/wiki/Br%C3%B8nsted-Lowry_acid-base_t...
etc.

Your equation about HCl reaction with H2O is obviously wrong since you wrote it as a homolytic dissociation. Always remember that any acid/base chemistry involves heterolytic bond dissociations and/or formation. It is always a pair of electrons that migrates, never a single electron. In this specific case the HCl dissociates heterolyticaly in water:
HCl + H2O <-> Cl- + H3O+

Acid/base reactions are equilibrium reactions with the difference in pKa's of involved acids determining the equilibrium constant (see http://www.lauher.com/che142/acidbase.pdf).
For example, the pKa of HCl in water is about -7 while that of H3O+ in water is -1.74. Thus the equilbrium constant of the reaction is about 105,26=5.5*106 (the "p" in pKa stands for the negative logarithm function (-log), therefore K=10-pKa

. This is a very large equilibrium constant and actually means that the reaction is leaned far to the right side toward the chloride and hydronium ions.
Your second equation is also wrong for similar reasons.
In these equations it is always implied the ionic species are solvated hence they only are true for diluted solutions where solvation is complete. For example, a 10M HCl is anything but completely dissociated since there is a deficit of H2O molecules to efficiently solvate all the chloride and hydronium ions, the polarity/dielectric constant of such mixture is also different from that of pure water, and therefore the pKa of HCl is not -7 any more in concentrated solutions. (see http://en.wikipedia.org/wiki/Solvation)

denis - 19-11-2008 at 21:37

Thats sort of beyond what im learning (High School/Grade 12 - Canada), the pKa stuff I mean, unless we're still going to be learning about it later, I dont know.

So HCl forms an equilibrium in water?
If so..their just teaching us wrong/useless stuff, to simply make it less confusing (but more confusing later on - college/university).. Just like how last year they taught us Standard Temp. Pressure and now we never use it (only SATP), and converting grams to # of particles (w/avagadros number), now we never use that either. Pointless education at its best... they never teach stuff we'll use later?

Basically right now we being taught that HCl and water will dissociate 100%, so that a solution of HCl is actually made up from H3O+ and Cl- ions..is that even partially correct, or later on will I find it is wrong? Dont tell me why its wrong/whats right, just yes or no, because ill probably mess up my next test from all the confusion

chemrox - 19-11-2008 at 22:10

I think I see your confusion. Water is involved in both because these are aqueous equilibria. However the basic species, OH- is formally an entity in this paradigm where as there is no such thing as H+. The acidic proton is always shown associated with a water molecule as H3O+. When I learned this system we used H+. HCl = H+ + Cl- for the dissociation of acids and NaOH = Na+ + OH- for the dissociation of bases. Often the expression (aq) was added to show that these are taking place in water. Your second expression doesn't balance. Go back and look again. Sr(OH)2 (aq) = Sr++ (aq) + 2(OH-) (aq)

Nicodem - 20-11-2008 at 00:49

Quote:

Originally posted by denis
Thats sort of beyond what im learning (High School/Grade 12 - Canada), the pKa stuff I mean, unless we're still going to be learning about it later, I dont know.

So HCl forms an equilibrium in water?
If so..their just teaching us wrong/useless stuff, to simply make it less confusing (but more confusing later on - college/university).. Just like how last year they taught us Standard Temp. Pressure and now we never use it (only SATP), and converting grams to # of particles (w/avagadros number), now we never use that either. Pointless education at its best... they never teach stuff we'll use later?

I don't know the educational system in Canada, but yes, even in my country we only briefly explain the pKa concept, reaction equilibriums and solvation to the pupils in what would be equivalent to high school. It is normal that you don't learn the concepts that are useful in research since you are not expected to be involved in research while in high school. You are thought the very basic stuff in a very, very simplified manner with lot's of approximations. For example, that they tell you the dissociation of HCl in water is 100% is not after all completely wrong. In an idealized model of high dilution you can approximate it to 100% since the undissociated form of HCl will only be present in the order of parts per million. This way of teaching is only wrong when you take it too seriously and don't consider it as a simplified theory. A theory is always a simplification of reality and needs not to be true anyway - there is always place for upgrading or even completely modifying it. For example, the Newton mechanics theory works perfectly and 'exactly' in all the cases, except for the smallest processes of low energy and low mass (of the order of eV and electron mass!) or extreme velocities (like approaching the speed of light), but people deal with flying baseballs more commonly than flying electrons or speeds in the range of hundreds of thousands kilometres per second in their normal life learning.

Quote:

Basically right now we being taught that HCl and water will dissociate 100%, so that a solution of HCl is actually made up from H3O+ and Cl- ions..is that even partially correct, or later on will I find it is wrong? Dont tell me why its wrong/whats right, just yes or no, because ill probably mess up my next test from all the confusion

Now they teach you so, but if you ever go to study chemistry you will later on find out that there are no such ions in water solutions. All ions interact with H2O molecules in a complicated manner. For example, you don't have "naked" Cl- anions in water, they are always solvated with H2O molecules trough hydrogen bonding and the net charge is neutralized trough solvent dipole orientation around the ion. Such solvated ions are impossible to characterize since the interaction is dynamic (no stable single species of hydrated ions form), therefore it is much easier to ignore such a state of things and teach at a more basic and what appears a terribly simplified level. However, trust me, at your age you don't want to learn these things at the top level of the theory.
But if you would ever decide to study chemistry you will have to learn these concepts in more depth or else you will never understand why is Cl- such a poor base in water, why does it have a poorer nucleophilicity in water than Br- and I- and why this is the opposite in aprotic solvents where Cl- is a stronger nucleophile than the other two...
Like Chemrox said, you can be happy they don't teach you that HCl dissociates to Cl- and H+ like they used to do only a few decades ago.

woelen - 20-11-2008 at 13:13

The problem with teaching such simplified models is not that they are simplified. Actually, that is good, otherwise young people never would grasp the subject. My problem is that these simplified models are presented as the whole story.

As a young boy I was teached all kinds of precipitation reactions, such as CuCl2 + 2NaOH --> Cu(OH)2 + 2NaCl. Now I see that this is a very strong simplification (in reality all kinds of mixed chloride/hydroxide/hydrated species occur and hence the color of the precipitate is not nice blue, but some greenish stuff is formed). These models were presented as the complete thing.

So, what I really would like to see is that young people are told also that the models they are learning are not the whole story, but simplifications, introduced to them in order to make picking up the subject feasible. A more complete picture will emerge at a later age and a complete picture will never be given, because it does not exist (this is the nature of natural sciences, any model, no matter how detailed, is an approximation of reality).

[Edited on 20-11-08 by woelen]

chemrox - 20-11-2008 at 15:16

I hope this is the case.. it's how I learned. Ideal Gas law comes to mind. The real lesson in learning to do chemistry calculations is a method of analytical thinking. I learned how to think in my first college chemistry courses. Along with equilibria, kinetics, dimensional analysis and all the rest, the real lesson ws how to think about a problem and arrive at possible solutions based on data and theory.