Chemistry Energetics question, help

[Ryltar]

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What is the change in enthalpy (delta H) for this reaction?

C4H4 (g) + 2(H2) (g) => C4H8 (g)

Given:
Enthalpies of combustion for:
C4H4 (g) = -2341 kJ/mol
H2 (g) = -286 kJ/mol
C4H8 (g) = -2775 kJ/mol

The right answer is supposed to be -158kJ/mol.
Please show me how to solve a problem like this.

How I tried to solve it:
Normally you solve these types of problems given the enthalpies of formation, as C4H8 is being formed.
Assuming the enthalpy of formation is the reverse of the enthalpy of combustion, so multiply combustions by -1 to get formations (I'm not sure if this is right).

delta H = sum enthalpies products - sum enthalpies reactants
= (2775) - [2(286) + 2341]
= -138 kJ/mol

Other way I tried to solve it. C4H4 is being combusted to CO2 and H2O, and 2H2 is being combusted to 4H. C4H8 is formed from the products of combustion, so

2775 - 2(286) - 2341 = -138 kJ/mol
^
Reverse for formation

 

[malibu06]

we just covered this in class last week. your first formula looks right... except that for any element the enthalpy of formation is zero

so in setting up your formula, it should be

delta H rxn = sum enthalpies products - sum enthalpies reactants
= [(C4H8)] - [2(H) + (C4H4)]
= [(-2775)] - [2(-0) + (-2341)]
= [-2775] - [-2341]
= - 2775 + 2341
= -435

i've been known to hit landmines in chem, so maybe I missed something here....

 

[Ryltar]

Thanks for your input. I'm not quite sure what you do when you you're given the enthalpy of combustion on a problem like this, where a compound is being formed. Normally, you're given the enthalpies of formation and not the enthalpies of combustion.

 

[Ryltar]

And I have no idea how ACS got -138kJ/mol for this problem. Ugh, it's frustrating. Maybe it's an error, but I thought ACS doesn't have errors.

 

[hellonurse]

And I have no idea how ACS got -138kJ/mol for this problem. Ugh, it's frustrating. Maybe it's an error, but I thought ACS doesn't have errors.

what page is this one in the acs gen chem book?

 

[Ryltar]

This is pg. 51, Q15.

 

[cjw7475]

And I have no idea how ACS got -138kJ/mol for this problem. Ugh, it's frustrating. Maybe it's an error, but I thought ACS doesn't have errors.

The enthalpy of combustion for C4H8 is 2755KJ not 2775KJ

If you didn't purchase it recently you might not have a yellow note that has revisions for typos

 

[bryantje]

You're not supposed to make delta H for H_2_ = 0. If you make it equal to the value they give you (multiplied by -1, that is), you get the answer they give. Believe me I'm no expert, but I can at least tell you how the book got what it got. Hope that helps