[minimal]

I apologize for the amateur nature of this question but, how can you tell where a reaction will take place - and what exactly will happen? Is it a question of electronegativity and perhaps steric hindrance?
For instance, lets say a compound has a NH3+ and COOH group attached to it, and NaOH is added. Will the amine group lose a H because of the positive charge? Or will the carboxyl group lose an H because of the higher electronegativity of Oxygen? Or perhaps, will both happen, just at different percentages?
If there are other factors that can be present that may not necessarily have been present in my example please inform me of them.
I know that questions aren't supposed to be too broad, and this one is, but I'm looking for broad answers: for instance, (review your electronegativity, lewis structures, etc), any sort of areas to look further into would be appreciated
Thanks

[tamim83]

For an example like the one you gave my answer would be look at the pKa.  It also depends on the pH of the solution that the molecule is in.  It also depends on the concentration of the NaOh (i.e. how much are you changing the pH). 

I really think this is a case by case thing.  It really depends on what kind of reaction you are looking at and what the species are on the reactants side as well as reaction conditions (pressure, pH, temperature, etc.).  This gets more difficult with organic reactions but can be very straight foward for simple reactions.  Sorry if this seems really vague, but I really do believe it all depends on the reaction you are looking at. 

[minimal]

yeah I think I've noticed that general trend, however I was a bit dubious as to whether prediction of products was merely the memorization of many different laws, and figuring out which took precedence.  It seems strange to me because obviously chemistry is subject to the laws of physics, and physics seems to be governed by more concise (if non-deterministic) laws.

[tamim83]

Well, whenever you hit organic chemistry then yes there are some uh "rules of thumb" to go by when it comes to where the reaction takes place on a molecule.  I can pretty much look at a reaction and tell by looking at a molecule's fuctional groups or the reagents and predict products.  But at a general chemistry level, you are not really going to get anything like that. 

Chemistry does follow er "laws of physics" and certain rules.  It is just in chemistry you are more likely to find deviations and exceptions to rules.   

[minimal]

For an example like the one you gave my answer would be look at the pKa.

Ok, one more question then, isn't the pKa determined experimentally? If so, how are we able to predict what the products of various reactants will be?

[Mitch]

Which reactants?

[minimal]

Which reactants?

well see this is kind've my point, like I was asking him I was wondering if there are sort of uber governing rules that exist no matter what the reactants are.  I know with certain functional groups there's predispositions for whatever reactants to act at certain sites, but according to his answer it kind've confirmed my unhappy suspicion that that's not really the way it works.
And my second question then is, since pKas are experimentally determined (I believe), there has to be ways where you *know* what the products of the reaction are going to be according to various variables right? (like reactants, temperature, concentration, pH etc).  Is it really just subtle nuances of various laws? 
And if certain reactants are predisposed to react at a given site, lets say a carboxyl group, but there are other functional sites that they can work at, they will react at both sites right? Just in different percentages (lets say 80% for the carboxyl group and 20% for a methyl group - and these are completely arbitrary examples)

[Yggdrasil]

Mostly, these rules are determined empirically and become the "rules of thumb" which organic chemists learn in their classrooms.  There are general trends and paterns which one can abstract from these empirical results (e.g. aldehydes are more reactive than most other carbonyl compounds, resonance stabilization of the base decreases the pKa of the acid), but these results are mostly empirically obtained.

In some instances you can do some kind of calculations to predict whether a reaction will happen given certain concentrations, temperatures, etc. (for example, by looking at the deltaG of the reaction), but usually this still relies on values obtained empirically (for example, standard deltaH and deltaS values are empirically derived for the most part).

When Schrödinger described his equations governing quantum mechanics, a famous physicist (or maybe chemist) said that all of chemistry had now become reduced to a mathematical problem.  Unfortunately, these mathematical problems are very complex and chemists must still rely on empirical methods to predict reactivity.  We are still waiting for the development of a computer powerful enough to be able to completely describe all possible reactions in the chemical universe.