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I'm going to use an example (I could have used a different one); On the document it is about the study of the chemical balance of an acid-basic solution.
The chemists have written the chemical reaction equation to make a table of the evolution of pH during the reaction.
If I understand with this equation I'll be able to understand others. $\ce{CH3COOH + H2O ->CH3COO- + H3O+}$

Here is what I already understand:

  • This is the equation of methanoic acid with water
  • I understood why the general formula of methanoic acid is $\ce{CH3COOH}$

What I want to know is how do we know that

  • $\ce{CH3COOH}$ changes into $\ce{CH3COO-}$ ?
  • $\ce{H2O}$ changes into $\ce{H3O+}$?

In general, how does one find the products when one knows the reagents? Does reagents molecules always correspond the same ions?

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Correct me if I'm wrong here -- you want us to explain how chemists come up with reactions? Currently this question is hard to understand, and possibly too broad. Please edit it and make it clearer, and I'll reopen it if it's up to the mark. –  ManishEarth Jan 26 '13 at 13:16
    
It is still rather broad, but I'm not sure how it can be made more specific. I'll leave this open for now --despite being broad, it is a nice, conceptual question. We'll see what sort of answers it gets :) (I may write one up myself later) –  ManishEarth Jan 27 '13 at 10:24
    
A note: I really doubt that understanding how your specific reaction was determined will explain how reactions are determined in general. Tere are various ways for predicting reactions, your methanoic acid reaction above is predicted from one of the simpler ones. –  ManishEarth Jan 27 '13 at 10:25

1 Answer 1

up vote 3 down vote accepted

If I went into specifics, this answer would span a few thousand books. Instead, I'll give an overview:

There are many ways of determining the products given the reagents:

Actually reacting the reactants

One can, of course, actually make the reaction take place. Once you've done that, you need to identify the contents of the resulting "soup". A few methods for this (You may have to separate the products first) :

NMR Spectroscopy

Here, a nifty technique is used to determine the magnetic properties of a compound. From these properties, one can make reasonable guesses at the substructures existing in a molecule, and check against a list of possible products. This is a very powerful technique, and usually gives good results.

Qualitative analysis

Here, we use various chemical reagents on the products, and analyse these products, making conclusions about the nature of the original products.

Microscopy

Here, using an STM, one figures out the structure of the molecule. This isn't used that often, but is an interesting technique.

There are many other such techniques, with varying accuracies and applicabilities.

Predicting the products in the absence of experiment

A lot of chemical research is into improving its (already vast) predictive power. The end-game of most sciences, after all, is to be able to predict stuff. So, in many cases, we can just predict how the reaction proceeds, without actually making the reaction occur in a lab.

Figuring out a reaction mechanism

This generally occurs in organic reactions. Here, you break the reaction into elementary steps. Each step can be explained by looking at electronic attraction/repulsion, as well as the increase in stability (which can generally be estimated by taking into account the inductive, mesomeric, hyperconjugation, and steric effects, though other things may effect the overall stability as well). From these steps, one gets a proposed mechanism, which may be verified.

Knowing solubilities and other numerical values

In ionic/inorganic reactions, the path a reaction takes generally hinges on the solubility of its final products. This is sometimes the case for other parameters as well. Knowing various numerical values lets one compare two possible reaction paths

Computational chemistry

Here, one uses the equations of quantum mechanics (Schrodinger's equation and a few of its cronies), and solves them for the given problem using numerical methods.


The particular reaction you have given

This one can be easily explained by resonance. In $\ce{CH3COOH}$, the $\ce{H}$ is loosely bonded, as the $\ce{C=O}$ bond resonates with the $\ce{C-O}$ bond. This makes it easy for the $\ce{H}$ to float away as $\ce{H+}$, leaving behind a pretty stable $\ce{CH3COO-}$ (made stable by resonance of the negatively charged lone pair over the oxygen atoms).

So far, we know that $$\ce{CH3COOH -> CH3COO- + H+}$$

Now we know that it is behaving like an acid. It is extremely well known that water forms a Hydronium ($\ce{H3O+}$) ion by self-dissociation, and this is accelerated by making the medium acidic (All free $\ce{H+}$s find the nearest $\ce{H2O}$ molecule and form $\ce{H3O+}$). $\ce{H3O+}$ is stable due to resonance, again.

So we now know that $$\ce{H+ + H2O -> H3O+}$$

Put the two reactions together and you get $$\ce{CH3COOH + H2O -> H3O+ + CH3COO-}$$

Though, in this case, the historical method of finding out the reaction may have been different -- acetic acid has been well known and studied for a very long time.

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Manishearth, thank you for your answer. I'm going to assimilate your answer. I need time to think about that. If i have questions i'll ask you then. Thank very much. –  user1147 Jan 27 '13 at 19:34

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