# gchem Destroyer

Discussion in 'DAT Discussions' started by Doctor PJ, Jun 5, 2008.

1. ### Doctor PJ Member

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On number 155 to calculate the enthalpy of the reaction, you can use Hess Law correct? I've never really done it with Products minus the Reactants. I just made sure to switch the sign if its being formed or not and added the enthalpies and have been getting them all right in kaplan subject tests except for this problem. Any chance there is an error. I get -3920kj as my answer, anyone else?

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I am getting -60 using the Hess law...I don't know how you can calculate just by adding the enthalpies are you multiplying by the corresponding coefficient?

4. ### Doctor PJ Member

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here is my work:

2(-404)+3(-394)+3(-220)+-1270

comes out to -3920kj.

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well i don't know how that works. I wouldn't risk not following the formula when I am taking the real thing.

6. ### arpitpatel86

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whats the difference in these two problems how come in 121 u do reactants minus products....but in 155 u do products minus reactants?

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For #121 the reaction is broken down into 2 series of steps and their enthalpies are given. you are not subtracting products from reactants. You are just making some changes to the reactions and adding the enthalpies. Like for example for the first reaction we need only 1 Rh on the reactant side so you divide the whole reaction, and hence the enthalpy, by 2. For the second reaction we need only 1 RhO on the product side so we reverse the reaction (change the sign to opposite) and divide it by 2. Then you add the enthalpies and that is your answer.

For # 155 you are looking for enthalpy change of the reaction and you are given enthalpy of individual compound not the reaction.

8. ### Doctor PJ Member

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okay I found out what I was doing wrong. Problem 155 you DO NOT use Hess Law. Products minus the reactants is for finding the standard heat of the reaction. In this problem you are correct it gives just the individual heat for each compound and not the reaction. If reactions were seen here then Hess Law would apply.