Error in SATII prep book?

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Hi all,

I'm currently reading through The Princeton Review's Cracking the SATII Chemistry (2003-2004) and I came across this problem on page 80:

Quote:

Suppose we need to estimate Delta H for the reaction Hsub2 + Brsub2 -> 2HBr, given the following bond energies:
H-H bond: 436kJ/mol
Br-Br bond: 193kJ/mol
H-Br bond: 366kJ/mol

Now, Hsub2 + Brsub2 requires (436+193 -> ) 629kJ of energy to be broken into the product, 2HBr, which has a bond energy of 2x366 = 732kJ.

According to the definition of Delta H (change of enthalpy), it is calculated as:
Delta H (reaction) = Delta H (product) - Delta H (reactants)

Therefore, to solve the problem above, I did:
Delta H (reaction) = 732 - 629 -> 103

However, the book did just the opposite. It did:
Delta H (reaction) = 629 - 732 -> -103
and called the reaction exothermic since the reaction released 103 kJ of energy.

To me, the reaction seems to be endothermic. 103kJ of energy needs to be added for Hsub2 and Brsub2 to form 2HBr. It makes sense since the entropy of 2HBr is less than the two individual molcules.

The final question is:
Am I wrong or is the book wrong? (and Why?)

Thanks for your time and help.

mikeXstudios

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Bonds release energy when formed; conversely you have to add energy to a system to break bonds. When energy goes into a system, deltaE is positive, and when energy leaves the system deltaE is negative. We agree on this, right?

Ok, then you have to add 436 kJ/mol to break the H2 and 193 kJ/mol to break the Br2.
Now remember that energy is released when bonds form, so to form 2 moles of HBr 732 kJ of energy must be released. So the sum is:
+436
+193
-732
-------
-103

So in all, 103 kJ of energy is RELEASED. (The reaction is exothermic.) This is how they get the equation deltaH(reaction)=deltaH(reactants)-deltaH(products)
which, yes, is the opposite of some of the other stuff you've learned. It just has to do with the convention that energy going into a system gets a positive sign while energy leaving a system gets a negative sign.

Hope that helps!

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Thanks for the explanation.

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Error in SATII prep book?

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