posted by Fiona .
Which of the following reactions is exothermic?
1. Na(s) --> Na(g)
2. Na(g) --> Na+ + e-
3. Cl2(g) --> 2Cl(g)
4. Cl(g) + e- --> Cl-1
My guess is 4, but I'm not sure
Here is what you do.
For 1, do we think it will take energy to change Na as a solid to Na as a gas. If you don't know much about Na, think about ice(that's a solid). Do you think it takes energy to melt the ice from a solid to a liquid, raise the temperature to boiling, then boil all the water to steam. Of course it takes energy to do that. If we must add energy it is endothermic. So Na(s) ==> Na(g) is an endothermic reaction and you don't check that one. Use that same kind of reasoning for 2,3, and 4.
For 2 you are removing an electron. For 3 you are separating two Cl atoms. In 4 you are adding an electron to an almost filled shell.
exothermic reaction is one that releases heat. Adding an electron to a chlorine atom releases energy.
Cl(g) + e- Cl-(g)