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Topic: Standard Enthalpy Change  (Read 6241 times)

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Offline xamberx101

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Standard Enthalpy Change
« on: April 30, 2010, 12:22:26 PM »
Here is the problem:

Calculate  ∆Hº for the reaction:
C4H4 (g) + 2H2 (g) [arrow] C4H8 (g)

C4H4 -2341 kJ/mol
H2 -286 kj/mol
C4H8 -2755 kJ/mol

It says the answer is -158, but why is it not positive? Any explanation would be helpful, thanks

Offline 2hotinhere

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Re: Standard Enthalpy Change
« Reply #1 on: May 01, 2010, 03:46:23 PM »
Could it be that they are asking about the bond energy that is being released?

Since if you do (change of H), you'll get +158.

However, if you were doing this rxn in perspective of bond energy with (change of H) , you'll have:

(change of H) = bond break - bond make
                      ( reactant) -( product)

Which will yield a -158

But anyway, have you try reading into the context of the problem? or any information they are giving to you before the question? Or if they told you if the rxn is exothermic or endothermic? Try looking for clues :)


I hope this helps.

Offline tamim83

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Re: Standard Enthalpy Change
« Reply #2 on: May 02, 2010, 03:44:39 PM »
Quote
H2 -286 kj/mol

This should be 0.  The standard enthalpy for any pure element in its standard state is 0.  Unless this is occurring under "non-standard" conditions. 


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