Hess's Law

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Hess's Law

Postby Megan_Ervin_1F » Wed Jan 30, 2019 1:37 pm

Is there a certain order for approaching a Hess's Law problem that anyone finds useful? For example, is there a certain order in which you choose to balance and compute all the different equations?

Sophia Ding 1B
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Re: Hess's Law

Postby Sophia Ding 1B » Wed Jan 30, 2019 1:58 pm

I usually like to start with noticing where the products and reactants are from the final equation into the ones you're given; regardless of the coefficients it's important to first get them on the proper side. From there, often times things will begin to cancel out, but if not then I think that at least it's easier from that point on to find the coefficients needed to multiply to get that final equation and thus your reaction enthalpy.

Gillian Murphy 2C
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Joined: Fri Sep 28, 2018 12:22 am

Re: Hess's Law

Postby Gillian Murphy 2C » Wed Jan 30, 2019 2:28 pm

I find that it's easiest to start off with the thing that only exists in one place. For example, in my discussion we did problem 4D.19:
find the reaction enthalpy for the reaction [H2 + Br2 -> 2HBr]
NH3 + HBr -> Nh4Br
N2 + 3H2 -> 2NH3
N2 + 4H2 + Br2 -> 2NH4Br

We know that we need 2HBr for the overall reaction, and HBr only exists in the first equation, whereas the H2 and the BR2 are found in more than one. If we start with manipulating that one to have 2 HBr in the reactant by flipping and doubling it, it becomes easier to see where the rest of the reactions should fall into place to make the reactants and products cancel out until we are only left with the original reaction. After changing the first equation, we would then flip the second equation so that the NH3 canceled out, and leave the last equation the same so that the N2, 3 of the H2's, and NH4Br cancels out and we are left with the only equation. Whatever you do to an equation, you have to do to the enthalpy given for that reaction (negate it if you reverse a reaction, double it if you double the reaction). It's kinda like doing a puzzle and requires a lot of trial and error but starting like that and getting the pieces to fall into place it what's worked for me so far!

Mona El Masri 1F
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Joined: Fri Sep 28, 2018 12:20 am

Re: Hess's Law

Postby Mona El Masri 1F » Wed Jan 30, 2019 4:27 pm

I would start with one reactant, then product. Combine both those equations and cancel stuff out. Then do the third reactant and everything should cancel. Sometimes you have to do - deltaH to get a product/reactant one the correct side.

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Joined: Thu Jan 10, 2019 12:17 am

Re: Hess's Law

Postby JacobHershenhouse3G » Wed Jan 30, 2019 6:43 pm

The thing that works best for me with solving problems like this is to think about it like solving a puzzle. Each equation that is given gives you one piece of what you need to find your answer (or else it wouldnt have been a given). So, looking at each one, find the part thats important and move to the next until you have everything you need and use the equations to solve for the specific value they're looking for. Hope this helps!

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