How do you know if a reaction will take place or not

[usermanual]

how do you know if a reaction will take place or not
like i know that i have to use the activity series and all but isn't that only when the reaction thing is single displacement where there 1 element in the first thing and then 2 element in the second compound so like this (a+ bc= ac + b) so for example Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g) so this reaction occurs because Zn is more to the left of the activity series compared to H but my question is what if its a double displacement reaction so AB+ CD -> CB + AD
for example i have Na2S + H2O how would i kno if a reaction takes place or not?
or do i always assume that when there are 2 elements in each reactant/ compound, that a reaction will always occur?

srry if this question is kinda confusing
it's just that i don't know the correct chemistry terms to use

and also I am in grade 11 chemistry so please easy with the chem terms

 

[alxm]

There are two factors in whether a reaction will occur or not: Thermodynamics and Kinetics.

By thermodynamics I mean whether or not the products of the reaction have lower energy than the reactants. You can calculate that using heats of formation. For redox reactions you can use electrochemical potentials. (since the energy of the reaction is \Delta G = -nFE)

By kinetics I mean the reaction rate. If the products have lower energy, it's energetically beneficial to react, but it doesn't say anything about the rate at which that occurs. For instance, graphite has lower energy at room temperature/pressure than diamond does. But you don't see diamonds spontaneously turning into lumps of coal! Because it happens so very very slowly. The kinetics of a reaction you really have to measure experimentally.

Now if you look at Na2S, what can happen when you put it in water?
Well you could have the dissociation Na₂S --> 2Na⁺ + S^2-. From experience one can predict this - simply because few sodium salts are insoluble, and indeed, sodium sulfide is water soluble.

But the sulfide ion, S^2-, is actually too basic to exist in water. (It has a pKa > 14) So in water, it will react to form a less basic pair of one hydrosulfide ion and one hydroxide ion:
S^2- + H2O --> HS^- + OH^-

So your net reaction is:
Na₂S + H₂O--> 2Na⁺ + HS^- + OH^-

The electrochemical series doesn't come into this, because there's no redox reaction going on, just acid/base reactions.
So in this case, you reason from pKa values. (Or you could also use heats of formation to get the reaction energy. Again, this is equivalent, since pKa values and reaction energies are related)