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Kerolos_4875
#1 Posted : Saturday, July 25, 2020 6:48:55 AM
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Hi,

For Chem ICE 4 Q70, I tried using the approach given on page 143 -- Delta H (products) - Delta H (reactants). However, that does not arrive at the right answer. After looking at the answer key, it uses another equation: Sum bonds broken minus sum bonds formed. This appears no where in the CC or EK - how are we supposed to know this? Or am I using the first question I mentioned incorrectly?

Thank you.
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INSTR_Katerina_102
#2 Posted : Saturday, July 25, 2020 5:34:31 PM
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Hi Kerolos,

Can you elaborate on your method? If you write it out I can provide feedback on where you might have gone wrong.

As to why in this question they used bonds broken and bonds formed to calculated dH, it is because as a state function, you can essentially use any differences in dH corresponding to any reaction, as long as the overall change adds up to the reaction you are trying to calculate a dH for.

In this case, because you are given the dHs for bonds broken and formed in a table, you assume that all the bonds are broken and then reformed in this reaction, and use the sum of these changes to get the overall reaction.

Again, I am not sure how you did the approach on page 143 (is that 143 on the CC or on EK?), but if you elaborate on what you did I might be able to provide insight into that.

I hope this helps,

Katt
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Kerolos_4875
#3 Posted : Saturday, July 25, 2020 8:00:55 PM
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Hi Katt,

Thanks so much. On page 143 of EK it shows dH products minus dH reactants for an example they did. However, that doesn't work for the ICE question and the answer in the back shows they used a different method of "bonds broken minus bonds formed" but I do not recall learning this at all. Is it just an adaptation of the dH products - dH reactants?
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INSTR_Katerina_102
#4 Posted : Sunday, July 26, 2020 3:03:55 PM
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Hi Kerelos,

No worries - yeah, it's an adaptation of dH products - dH reactants. You can take the sum of any enthalpy difference, as long as it adds up to the overall reaction. Usually the way you do this is by taking dH products - dH reactants, but you can also do this by taking the dH of all the bonds broken - dH of all the bonds formed.

This is Hess's law - it doesn't matter what enthalpy changes that you choose, as long as they all add up to the overall reaction, the answer should always be the same. It's a very powerful property of a state function.

The only way you can be off is if you don't add up all of the processes correctly.

Let me know if this is unclear!

Katt
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Kerolos_4875
#5 Posted : Sunday, July 26, 2020 11:49:55 PM
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Makes sense, thank you! I guess I wasn't assigning the positives and negatives properly :)
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