[mohemoto]

I am given these three reactions. How would I go about determining whether they are exothermic or endothermic? Also, I'm asked to state whether work is done on or by the system, or no work is done at all..how do I know?

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
N2(g) → N2(l)
HBr(aq) + KOH(aq) → KBr(aq) + H2O

[Fluorine]

N2(g) → N2(l)

This would be exothermic because nitrogen in it's gaseous state is at a higher energy level than nitrogen in it's liquid state. Recall the order decreasing in energy for state of matter goes; gas, liquid, solid. Thus if nitrogen is taking a lower state of energy - the molecule is slowing down to become a liquid - it must give off energy. The system is releasing it's energy to the surrounding. The reverse of this would be endothermic, taking in energy to reach a higher energy state.

I don't recall how to do it with reactions when enthalpy is not given, sorry.

[ooosh]

Neutralization always be exothermic.