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I get everything up until the very end. We need to identify the bond enthalpies first: \begin{align} \ce{C=C} &= 612\\ \ce{O=O} &= 248\\ \text{SUM of these}&= 860\\ \end{align}

\begin{align} \ce{C-C} &= 348\\ 2\cdot(\ce{C-O}) &= 720\\ \text{SUM of these} &= 1068\\ \end{align} We know that enthalpy change is = products - reactants, which is $1068 - 860 = 208$, but the problem comes here: I know a minus sign is supposed to go there, but how do we know that? Exothermic reaction? Is it because of the oxygen, suggesting it's a combustion reaction or not?

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When you use bond enthalpies to calculate $\Delta H$, you need to subtract the sum of the bond enthalpies for the products from the sum of the bond enthalpies for the reactants. In other words "reactants - products". This is because all bond enthalpies are positive, since energy must be put into the bond to break it. Bond formation releases energy; the minus sign compensates for this. In short, bonds of reactant molecule must break, and new are formed to form product molecules. So, your answer should be negative.

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  • $\begingroup$ Thank you but does this mean that there is supposed to be a negative sign whenever I see that the enthalpy for products is > enthalpy for reactants? I am sorry, but I really need to understand this, because I always mess up questions as such, due to dumb errors. $\endgroup$ – Molly Mar 11 '15 at 21:12
  • $\begingroup$ In other words: why is it "reactants - products". I have always thought it was "products - reactants". Also, how can I see that the bond is broken or not.. is it because the enthalpy is greater for products.. meaning since greater energy is needed to break them, but less to form bonds, which is what we do here, we then have an exothermic reaction? $\endgroup$ – Molly Mar 11 '15 at 21:58
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There is one mistake in your calculation: oxirane is not just two standard C-O bonds plus a standard C-C bond. Due to ring strain there is some extra energy in the three-member ring.

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  • $\begingroup$ But I'm not sure if the data sheet had the additional info for the extra energy due to the ring strain. If it did not have the data, how can we use non-existent data to do any calculation? $\endgroup$ – Gaurang Tandon Mar 1 '18 at 15:31

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