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The book I'm studying with often confronts me with exercises that involve solving reactions such as $\ce{CA(OH)_2 + CO_2 -> ?}$.

Now, most of the time, the questions are just repetitions of material that has been taught, that reaction in particular has been shown many times in the chapter before the questions, $\ce{... -> CaCO_3 + H_2O}$, but not explained.

When I look up such reactions on the Internet to try to understand, I mostly come across people who ask others to balance the equations for them. But that is not really interesting for me, as I find balancing a trivial mathematical task.

I am, however, failing to understand why this reaction (or many other reactions) happen the way they do. My book has also mentioned that you can combine two materials, like $\ce{CO + H2}$, in different ratios, under different temperatures, pressures, and with different catalysts, to get different results (methane, methanol, and many others).

So my question is -- when asked to solve a reaction, and given only the reactants (or possibly also other factors such as mentioned above), is there a mathematical principle (for example, relying on differences in electronegativity) to solve the reaction besides being completely familiarised with molecular orbital theory and quantum mechanics? Because so far, the only alternative I can see is memorising the reactions.

I realise this question is rather broad, and possibly there is no satisfying answer to it or any answer besides to go study quantum mechanics, but to provide some context, I'm a student at high school studying chemistry not as my main subject (so on a rather elementary level), and I'm looking for a perspective how to understand the material I learn and not memorise it.

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    There are some general principles, so after a while you would begin to make your own generalizations and recognize patterns, but initially there is quite a lot to memorize. E.g. organic chemistry strongly relies on polarity, which is indeed related to electronegativity, but steric effects and much more complex quantum mechanics gets involved at some point, so much so that people often just have to run an experiment and see what happens. In inorganic chemistry, how would you guess that iron (2+) ions react with sulphide to give an insoluble iron sulphide precipitate? It's just experimental... – user6376297 Jul 05 '17 at 17:09
  • And in your example, why does calcium hydroxide react with carbon dioxide to give calcium carbonate? We can explain it a posteriori, but you could not safely generalize to similar systems. E.g. I am pretty sure that silicon dioxide and magnesium hydroxide would not react in the same way. So no, it's not like mathematics where an all-encompassing rule can be stated and applied universally: chemistry is still an experimental science. At least, that's the way I see it. – user6376297 Jul 05 '17 at 17:14
  • possible duplicate of https://chemistry.stackexchange.com/questions/50684/how-can-i-predict-if-a-reaction-will-occur-between-any-two-or-more-substances – Mithoron Jul 05 '17 at 19:40
  • Possible duplicate of How can I predict if a reaction will occur between any two (or more) substances? – Buttonwood Jul 05 '17 at 20:26

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