gen chem question

[polarmolar]

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I am confused between:

ΔHrxn = (sum of ΔHf of products) - (sum of ΔHf of reactants)

&

ΔHrxn = (ΔH of bonds broken) - (ΔH of bonds formed)

how is it the same if its reversed?

(Kaplan book pg 865 bottom)

 

[doc3232]

I am confused between:

ΔHrxn = (sum of ΔHf of products) - (sum of ΔHf of reactants)

&

ΔHrxn = (ΔH of bonds broken) - (ΔH of bonds formed)

how is it the same if its reversed?

(Kaplan book pg 865 bottom)

So, I think these are two different formulas. The H's are different in each one. You can only apply one to a question, not both.
But they are similar in a way. Hf of products is is less than Hf of reactants if energy is released (100--> 80 + 20, the 20 is in the form of heat).
Similarly H of bonds broken is energy - H of bonds made is..................

What I am writing doesn't make sense, but eh...

 

[yakuza]

It gives you the same thing but like he said its just two different equations. Just that someone decided to wana make a new way to calculate it. Kinda like in physics where you can use 50 equations to get the same answer.

 

[polarmolar]

I'm looking at the signs and it wouldn't give you the same thing...right?

 

[yakuza]

I'm pretty sure, I remember watching a lecture video over this and the guy said so. I could be wrong though but I'm pretty sure its right

 

[doc3232]

I am confused between:

ΔHrxn = (sum of ΔHf of products) - (sum of ΔHf of reactants)

&

ΔHrxn = (ΔH of bonds broken) - (ΔH of bonds formed)

how is it the same if its reversed?

(Kaplan book pg 865 bottom)

The signs do make sense.
First one: lets say you are burning gasoline so exergonic. Like my old example 100-->80+20, woth the 20 in the ofrm of heat
You get deltaH = -20

Second equation, you get break a few bonds and make strong ones, it is again negative.

 

[yakuza]

HMMM..

I think I told you wrong. I think its the bond breaking one thats the same as the one where you have a list of equations and you flip/divide/add/subtract.

While theres Enthalpies of Formation (deltaHf' =prod' - react') which is only for one mole, one specific reaction, and at standard conditions 1atm and 25C.

 

[polarmolar]

now you've got my head spinning lol

 

[ongmeister]

I am confused between:

ΔHrxn = (sum of ΔHf of products) - (sum of ΔHf of reactants)

&

ΔHrxn = (ΔH of bonds broken) - (ΔH of bonds formed)

how is it the same if its reversed?

(Kaplan book pg 865 bottom)

For the first one, its basically a state function, so its always product - reactants from all gchem classes you should already remember.

The second one is a little more confusing, think about it. I'll use an example to explain.
C2H4 + O2 ---> CO2 + H2O

You have to break the C2H4 bond to make the carbon dioxide right?
then when u form the CO2 you have energy. But the equation states that its the energy difference, so you have to put energy to break the bond to create the bond. Thats how it works, lol, I don't really know how to explain it better than that. Hope that helped.