[hasankamal007]

Good morning. I have few questions regarding chemical reactions.

•First of all, why do chemicals even react?

•Compounds having already complete octet react. But why? If the octet of every atom in that compound is already complete, they shouldn't react.

•And what determines the higher reactivity a substance might have over the other substances?

•And is there a method or 'idea' from which we can tell what would the product of a reaction be?
For example, CaO + H₂O -> Ca(OH)₂.
Suppose you don't know the RHS of above equation. Can you apply some chemistry and actually make out the product of the reaction?

A broader view:
All my teachers say that this reaction results in this product and blah blah. But they dont tell me why only that product. Why not something else? What decides the chemical formula of the product?

These why's and how's bother me.
Thanks in advance!

[raghav_anand98]

I'm only answering the question for which I know the answer;

•First of all, why do chemicals even react?
They react because they are more reactive!! One element has greater affinity towards another element and so i reacts. That's where the metal reactivity series come in..

•And what determines the higher reactivity a substance might have over the other substances?
This is determined by the valence electron or the electron in the outermost shell. Both the no. of electrons in the outermost shell and the distance of the electrons from the nucleus of the atom determine a substances reacivity. Substances having 1 or 7 electrons in the outermost shell are highly reactive because they 'want' to give up their electron or gain that one electron to fulfill the octet rule. Also the further the electron is from the nucleus the more reactive the METAL. This rule does not apply for non-metals..

[Arkcon]

•Compounds having already complete octet react. But why? If the octet of every atom in that compound is already complete, they shouldn't react.

For this one, your terminology is a bit off.  We consider atoms to have a "complete octet", or else, they've become ions.  Molecules, even if they're not charged, aren't said to have a complete octet. 

[Arkcon]

•And is there a method or 'idea' from which we can tell what would the product of a reaction be?
For example, CaO + H₂O -> Ca(OH)₂.
Suppose you don't know the RHS of above equation. Can you apply some chemistry and actually make out the product of the reaction?

There are a number of chemical concepts that can be used to predict reactions.  Some you will learn in time, some are very advanced.  Often however, starting historically, and continuing to the present day, these things were determined experimentally.

[fledarmus]

Chemicals react because it is energetically favorable for them to react. Just like rocks fall because it is energetically favorable for them to fall. The specifics of which chemicals will react under what conditions to form which products will be a major part of your study of chemistry, but in the end it all comes down to energy. Rocks have a certain amount of energy, called potential energy, bound up in their position, which can be released when they fall, and increased if energy is added to lift them higher; chemicals have energy bound up in them based on their arrangement of bonds and other interactions, which can be released by rearranging those bonds into more stable configurations, or increased by adding energy and forming less stable configurations.