Posted by Raj on .
When given a chemical reaction, how would I know whether a reaction is endo or exothermic and whether work is done on the system or by the system. Without using any calculations.
So I have a couple reactions which I have to use chemical reasoning to to determine.
2 Cl(g) --> Cl2(g)
H2O(l) --> H2O(g)
2 H2(g) + O2(g) --> 2 H2O(l)
2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)
3 NaN3(s) --> Na3N(s) + 4 N2(g)
Lets use H2O(l) ==> H2O(g) as an example.
We know it takes energy to turn water into steam. We know that because we must heat it someway (electrically, gas burner, etc). So since we must ADD energy, it is an endothermic equation. We could write it this way.
H2O(l) + energy (or heat) ==> H2O(g) and we would write delta E = + some number.
When looking at the work part of it, compare delta n, the number of mols gas products vs mols gas reactants. In the case of the H2O conversion, we have 1 mol products and zero mol reactants. The equation for work is -p(V2-V1).
Larger n on products side (delta n is positive)(it expands) means work is negative that means work is done ON the surroundings BY the system (the reaction).
In general, going from s or l to g is endothermic. That answers 2-5. Now on 1, the Cl2 is a more stable, less energetic than Cl, so it must be a exothermic reaction.
These are general ideas, with lots of exceptions.
And what I wrote holds for constant P and T.
Oh, alrite so then heres what I get.
2 Cl(g) --> Cl2(g) exo-on
H2O(l) --> H2O(g) endo-by
2 H2(g) + O2(g) --> 2 H2O(l) exo-on
2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g) endo-by
3 NaN3(s) --> Na3N(s) + 4 N2(g)endo-by
I'm inclined to think Li and NaN3 reactions are exothermic but I don't have any data to prove that.
2 Li(s) + 2 H2O -> 2 LiOH (aq) + H2(g)
why is this exothermic?