Importance of equilibrium constant [closed]








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Here i am going to let out a doubt of mine which has been bothering me since high school about equilibrium constant. Let's say we have a reaction.


$A+B\rightarrow C+D$. When we say reversible reactions, we say that $A$ and $B$ react to form $C$ and $D$ and after that $C$ and $D$ again react back to form $A$ and $B$. *When does this reacting back to form $A$ and $B$ occur?*


Now,the equilibrium stage of a reaction. What's the importance of the equilibrium constant and why was the constant defined to be $\frac{[C][D]}{[A][B]}$. I have always struggled understanding why the equilibrium constant is defined in that way.




The reverse reaction occurs simultaneously to the forward reaction, if all products are present.


We say the reaction is at [chemical equilibrium](https://en.wikipedia.org/wiki/Chemical_equilibrium) when rates of both reactions are equal and the net change is zero. The fact this equilibrium is dynamic and not static can be observed via [isotopic](https://en.wikipedia.org/wiki/Isotope) exchange.


The [equilibrium constant](https://en.wikipedia.org/wiki/Equilibrium_constant) is defined that way (variants use partial pressures, molar fractions or activities), because, well, that expression is at the given temperature constant, assuming the equilibrium state.


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