need help w/ GC, p l e a s e . . .

[RT0223]

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from kaplan GC subject test 1 #18.

n2 + 3H2 (arrow pointing to both directions) 2NH3 + 22kcal

if increase temp, will it shift to the L/R? endo/exo?

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my guess is shift to the L and it's endo rxn because heat is positive. however the key says it's endo instead. Why? Please help. Thanks a lot! 🙁

 

[dudewhensmydat]

from kaplan GC subject test 1 #18.

n2 + 3H2 (arrow pointing to both directions) 2NH3 + 22kcal

if increase temp, will it shift to the L/R? endo/exo?

-------------------------------------------------------------

my guess is shift to the L and it's endo rxn because heat is positive. however the key says it's endo instead. Why? Please help. Thanks a lot! 🙁

When heat (in the form of energy like kcal) is found in the product side of the equation, the reaction is exothermic. In exothermic reactions, heat is evolved. Reviewing Le Chatelier's principle will help you understand that a shift will occur away from a substance (in this case: heat) whose concentration is increased.

 

[RT0223]

ok, this one is from subject test 2 #29.

A + B (arrowS) C + D heat = -10 kcal/mol

the key says it's exo because of the negative H sign. So which is which? Is (+) = exo or endo? Man, it's soooooooooooooooooo confusing...

 

[hlsawdust]

ok, this one is from subject test 2 #29.

A + B (arrowS) C + D heat = -10 kcal/mol

the key says it's exo because of the negative H sign. So which is which? Is (+) = exo or endo? Man, it's soooooooooooooooooo confusing...

-Heat always exothermic. I think your confusion comes from the fact the 1st question included heat in the reaction, whereas the 2nd question it has deltaH = 10kcal/mol on the side.

If it is IN the reaction, then heat on the right (products) means exothermic and heat on the left (reactants) means endothermic.

If deltaH is simply mentioned on the side OUTSIDE the reaction, -deltaH is exothermic and +deltaH is endothermic.

Hope this helps!

 

[Uracil]

for the first problem that you posted the reaction is exothermic because heat is part of the product. Knowing this, you will then know that deltaH is negative!

The second problem you posted gives you dH and the sign is negative, so the reaction is exothermic.

Just be aware where heat is found as a reactant or as a product. As a reactant dH will be positive; as a product negative.