Question

Which reaction of those listed is MOST exothermic?

`A.` `Cl_2rarr2dotCl;`
`B.` `H_2 + Cl_2rarr2HCl;`
`C.` `dotH + dotClrarrHCl;`
`D.` `H-O-O-Hrarr2dotH + stackrel(dot)O-stackrel(dot)O`

Answer 1

`"Option B"`...........

Explanation:

This is a poor question, tho perhaps it is a bit unfair to students WHO KNOW that the formation of `HCl` gives an exotherm. Anyway, they should also know the principle behind the thermodynamics.

`DeltaH_"rxn"=SigmaDeltaH_"Bonds formed"-SigmaDeltaH_"Bonds Broken"`

And if `DeltaH_"rxn"<0`, then the reaction as written is exothermic.

For `"A"` a `Cl-Cl` bond is broken to give `2xxdotCl`; the process is endothermic.

For `"C"` an `H-Cl` bond is FORMED; the process is exothermic.

For `"D"` `2xxH-O` bond are BROKEN; the process is endothermic.

But for `"B"` a `Cl-Cl` bond and a `H-H` are broken and `2xxH-Cl` bonds are FORMED.

We have no way of knowing a priori whether `H-Cl` bonds are stronger than the element-element bond. We might intuitively think so (and we would be RIGHT), but this is the province of experiment. And the question gives us no data with which to inform our supposition.