Studying Thermochemistry, need help!?

Question

I'm having a real hard time understanding the thermochemistry parts of my study guide, Your help is appreciated!

18) All of the following have a standard enthalpy of formation value of zero at 25 degrees C except:
a) CO(g)
b) Fe(s)
c) C(s)
d) F2(g)
e) Ne(g)

answer is A, but how am I supposed to know that?

19) The equation for the standard enthalpy of formation of sodium bromide corresponds to which reaction?
a) Na(g) + 1/2 Br2(g) -> NaBr(g)
b) Na(s) + 1/2 Br2(s) -> NaBr(g)
c) Na(g) + Br(g) -> NaBr(s)
d) Na(s) + 1/2 Br2(I) -> NaBr(s)
e) Na(s) + 1/2 Br2(s) -> NaBr(s)

The answer is D, again I have no idea what this means!

20) Given the following data:
S(s) + O2(g) -> SO2(g) deltaH = -395 kJ / mol
S(g) + O2(g) -> SO2(g) deltaH = -618 kJ / mol

find the heat required for the reaction converting solid sulfur to gaseous sulfur at 298K and 1 atm pressure
a) -1013
b) +618
c) -618
d) -223
e) +223

answer is e
:( we're not allowed to use notes or book

Answer 1

18) All of them except for CO are in their natural elemental states at standard conditions. CO is a compound and not an element. F2 may appear to be a compund, but it diatomic by nature, so it is zero too. Any elemt in its natural form at stadard conditions has a enthalpy of formation of zero.

19)Once again for it to be "standard enthalpy of formation" all of the elemtns must be in their normal form at standard conditions. Na is a solid, Br is a diatomic liquid (if you have a periodic table in the room you can tell its a liquid because it is blue), and NaBr is a solid (but you didnt really need to know that, you could have narrowed it down just from the reactants).

(20)
S(s) + O2(g) -> SO2(g) deltaH = -395 kJ / mol
SO2(g) -> S(g) + O2(g) deltaH = 618 kJ / mol (reverse sign because flipped equation)

Now "add" them up and the O2 and SO2 cancel, leaving

S(s) -> S(g) at -395+618=223 kJ/mol

Answer 2

For the question 18, the answer is A because it's a composite of C and O. Only element have an enthalpie = 0.

19) you have to considerate the normal state of these element. then Na is usually solid and Br is normally liquid

20) You need to provide 395 KJ/mol to pass S to SO2 but for the second, what you do in really is to pass SO2 to S(g) then, you have to flip the equation, that mean you have to change sing of the enthalpy

Then you add -395 + 618

Answer 3

You can't study thermochemistry without the enthalpy formation table....with the table you are able to know the amount of heat produced in kJ per mol of a given substance

Answer 4

ew, thermo...

haha sorry, just had to say it