[sundrops]

Hess' Law

I'm having a lot of trouble with this question (Hess' Law)- its not that I
don't understand it - it's that I just can't get the right answer!! If you
could check over my work and let me know where I made my mistake that'd be awesome. I've already used up like 5 tries and I'm getting really
frusterated.

Thank you!


Given the following data:

C6H4(OH)2(aq) --> C6H4O2(aq) + H2(g) H= 177.4kJ
H2(g) + O2(g) --> H2O2(aq) H= -191.2kJ
H2(g) + 1/2O2(g) --> H2O h= -241.8kJ
H2O(g) --> H2O(l) h= -43.8kJ

Calculate H for the reaction:

C6H4(OH)2(aq) + H2O2(aq) --> C6H4O2(aq) + 2H2O(l)


So here's what I did:
C6H4(OH)2(aq) --> C6H4O2(aq) + H2(g) H= 177.4kJ
H2O2(aq) --> H2(g) + O2(g) H= +191.2kJ
2H2(g) + O2(g) --> 2H2O h= -483.6kJ
H2O(g) --> H2O(l) h= -43.8kJ

Then adding up the deltaH`s I get: 177.4kJ + 191.2kJ - 483.6kJ - 43.8kJ =
-158.8kJ
which is wrong. I hope that made sense - if anyone could shed some light on this problem that'd be awesome!

[Donaldson Tan]

u were right until the last part for enthalpy of vapourisation of water. u have 2moles of water formed per mole of reaction, so it's:

.. - 483.6 - 2 x 43.8 = ...