Hess Law Destroyer vs Actual DAT

[roquer2]

So going thru my 2nd round of Destroyer and I come to this question again:

I learned the concept/rules of Hess' Law from Chad and he didn't mention anything of this sort of question. His rule was a simple, "Concentrate on the main reaction. Flip around the other equations and the respected enthalpy values. Plug in coefficents as well for those respected equations/enthalpies. Done". I tried doing that, but the solution for the equation said something along the lines of 'divide by 2', etc. I guess those diatomic oxygens are throwing me off. Applying the aforementioned concept/rule, I got -2200 kJ. Clearly, it's not the answer and the actual answer was A, -550 kJ.

My question is, when it comes to Hess' Law on the DAT, do I have to worry about this type of problem, or will Chad's explanation be sufficient?

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[babowc]

even going by the Destroyer concept, I think it was fairly easy.
You just divide everything by 2, since you want coefficient of 1 on everything. then add it up.

Chad's idea may have been simplified, which is what I find a lot, compared to the Destroyer.

*edit, sorry I was a ****** for saying "flip the equation" when the product was on the proper side of the reaction.
I was thinking enthalpies of formation when I said "products - reactants"

 

[jnines]

So going thru my 2nd round of Destroyer and I come to this question again:

I learned the concept/rules of Hess' Law from Chad and he didn't mention anything of this sort of question. His rule was a simple, "Concentrate on the main reaction. Flip around the other equations and the respected enthalpy values. Plug in coefficents as well for those respected equations/enthalpies. Done". I tried doing that, but the solution for the equation said something along the lines of 'divide by 2', etc. I guess those diatomic oxygens are throwing me off. Applying the aforementioned concept/rule, I got -2200 kJ. Clearly, it's not the answer and the actual answer was A, -550 kJ.

My question is, when it comes to Hess' Law on the DAT, do I have to worry about this type of problem, or will Chad's explanation be sufficient?

this is fairly simple. don't overthink it.
We want Rh(s) + O2(g) -> RhO2(s)

From the half reactions, we would have to divide the 1st & 2nd reaction by 2 which will give you:
Rh(s) + 1/2 O2 -> RhO(s) H= -400 kJ
RhO(s) + 1/2 O2 -> RhO2(s) H= -150kJ

We add both half-reactions (add all reactants together, add all products together) we get...
Rh(s) + 1 O2 + RhO(s) -> RhO(s) + RhO2(s)

Now we can subtract RhO(s) to both sides and we get the final product (the original reaction)

Rh(s) + 1 O2 -> RhO2(s)

So the final delta H = -400 + -150 = -550kJ, which is your answer A.

Hope this helps.

 

[roquer2]

Thanks guys. It wasn't that I was overthinking it, it was more of me applying such a simplified concept to a question like this (kinda like fitting a circle block into an oval peg).

It is rather simple when you divide by 2 and finish out the problem. So my understanding is you are just getting to the overall rxn presented, so you divide by 2?

Rather, let me change it up and say those coefficients weren't 2, but were 3 instead. Would you divide everything by 3?

I hope what babo said is correct. Chad's stuff was so simple and really hope his style of questions for gen chem are what shows up in the actual DAT, rather than Destroyer

 

[babowc]

Sorry, I edited my post.
I started typing without thinking... Hess law isn't "products - reactants", rather its adding up all the deltaH.

If the coefficients were 3 instead of 2, you couldn't just divide everything by 3.
If you did, you'd get a yield of:
1/3 O2 + 1/3 O2, which wouldn't equate to 1 O2.

If it were 3.. the problem would be either a royal PITA to solve, or not possible.

 

[bluemoon27]

Ok this prob is not too bad. I think you may be over-thinking it.. before looking at your answer I figured it out and here is what I did.

First, you add both of those rxns (including their delta H). You will notice that RhO occurs as a product in one rxn and as a reactant in the next, so that cancels these terms out. Sum up the rest of the stuff, including the delta H and it will you 2Rh + 2O2 --> 2RhO2 ; delta H= -1100...technically, you can treat delta H as a reactant or product, depending on the sign. here it will be on the product side since the rxn is exothermic and the sign is negative...when its exothermic, then you can put it on the product side as i did below. note that you incorporate as a product, you will just keep the magnitude (aka the value, not its sign). the sign is actually taken care of because of its placement on the product side.

so u can also write this summed result as this as well

2Rh + 2O2 --> 2RhO2 + ( delta H= 1100)


notice thats the same reaction as they wanted you to solve, except its multiplied by two. also since the delta H is negative, you can also put it as a product. now you divide this whole equation by two and this will get you -550, which is -1100/2.

showing the steps, it looks like:

[Rh + O2 --> RhO2 + ( delta H= 550)]

since delta H on the product side, remember it will have a negative sign attached to it, so answer is -550.

hope this helps!

So going thru my 2nd round of Destroyer and I come to this question again:

I learned the concept/rules of Hess' Law from Chad and he didn't mention anything of this sort of question. His rule was a simple, "Concentrate on the main reaction. Flip around the other equations and the respected enthalpy values. Plug in coefficents as well for those respected equations/enthalpies. Done". I tried doing that, but the solution for the equation said something along the lines of 'divide by 2', etc. I guess those diatomic oxygens are throwing me off. Applying the aforementioned concept/rule, I got -2200 kJ. Clearly, it's not the answer and the actual answer was A, -550 kJ.

My question is, when it comes to Hess' Law on the DAT, do I have to worry about this type of problem, or will Chad's explanation be sufficient?

 

[BrotherAli]

Thanks guys. It wasn't that I was overthinking it, it was more of me applying such a simplified concept to a question like this (kinda like fitting a circle block into an oval peg).

It is rather simple when you divide by 2 and finish out the problem. So my understanding is you are just getting to the overall rxn presented, so you divide by 2?

Rather, let me change it up and say those coefficients weren't 2, but were 3 instead. Would you divide everything by 3?

I hope what babo said is correct. Chad's stuff was so simple and really hope his style of questions for gen chem are what shows up in the actual DAT, rather than Destroyer

Yes you want to get the overall rxn....So you make sure everything cancels out in order to get the overall reaction if for reaction 1 you have A on the products and then reaction 2 has A on the reactants then you can cancel them out. If A is double for reaction 1 then you can divide the whole reaction 1 by 2.

All you need to do is look up the rules for K, G, S, and H to know what rule to apply if you are dealing with eq constant, gibbs, entropy, and enthalpy.

I think when you divide by two you divide the H,G,or S value by two but K (equilibrium) is alittle different so that might be the only trick that could come up for hess law (think you take 1/Kvalue in that case instead of dividing by two). You should also know the rules if you have to multiply one of the reactions by 2

Sorry I cant explain better my pchem class was a blur.

Here is an example problem that might help:

http://answers.yahoo.com/question/index?qid=20080325194538AAedE6l

Let me know if this helps, I havent reviewed this section yet but im pretty sure im right lol