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# Help with Hess Cycle question (Enthalpy changes)

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1. Help with Hess Cycle question (Enthalpy changes)
Hi everyone.

These questions are usually quite straight forward, but I came across this one today (see attached) and the arrows seem wrong to me, even though they are part of the exam question and haven't been drawn on. Usually you can just say Delta Hr = Delta H3 - Delta H2 or something along those lines and work it out from there, but I have no idea with this one.

I am trying to work out the enthalpy change of reaction given the other two enthalpy changes in the hess cycle.

Thanks for any help!
Attached Thumbnails

2. Re: Help with Hess Cycle question (Enthalpy changes)
Hi,
if it's not a mistake on the paper I think that you just have to reverse the processes (and the arrows) to which the enthalpy is given, so that the -82 becomes +82 and so on, and then add the values to calculate the reaction enthalpy and answer the question.
3. Re: Help with Hess Cycle question (Enthalpy changes)
(Original post by ERdoctor)
Hi everyone.

These questions are usually quite straight forward, but I came across this one today (see attached) and the arrows seem wrong to me, even though they are part of the exam question and haven't been drawn on. Usually you can just say Delta Hr = Delta H3 - Delta H2 or something along those lines and work it out from there, but I have no idea with this one.

I am trying to work out the enthalpy change of reaction given the other two enthalpy changes in the hess cycle.

Thanks for any help!
Hess law allows you to go via any other route.

Start at the top left and go down to the bottom and then up to the top right.

This requires you to go in the opposite direction to the arrows shown, so you simply reverse the signs of the energy changes before adding them together.
Last edited by charco; 16-04-2012 at 22:41.
4. Re: Help with Hess Cycle question (Enthalpy changes)
I see! Okay, thanks

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Last updated: April 16, 2012
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