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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: xamberx101 on April 30, 2010, 12:22:26 PM

Title: Standard Enthalpy Change
Post by: xamberx101 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

Title: Re: Standard Enthalpy Change
Post by: 2hotinhere 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.

Title: Re: Standard Enthalpy Change
Post by: tamim83 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.