What all factors decide which chemicals to use?

[jd12345]

I was studying what kind of chemicals are used in various processes like thermite welding , rocket propellant. On what basis are these chemicals used?
In welding Al + Fe2O3 is used. Why not Sodium and some weaker oxide as sodium is an alkali metal and highly reactive
In rocket propeller H2O2 is used, why not some stronger oxidising agent?

 

[Mike H]

For the welding example, any reactive metal - in principle - could do the job. Practically, aluminum has certain advantages in this case over something like sodium - its melting point is lower than sodium's but its boiling point is higher. It is also extremely inexpensive and easy to handle compared to sodium.

I don't happen to recall anything significant about rocket propellants at the moment, but I am sure that there are/were practical advantages to using hydrogen peroxide (doesn't require cryogenic storage, I imagine that on a small scale it's easy to clean up any spills with adequate water).

In general, these sorts of factors - cost, safety, specific physical/chemical properties relevant to its intended application - and others are what one needs to consider.