Emupa
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Hey
We've recently done Hess's law in AS chemistry, including the diagrams showing (and in working out) enthalpy changes. I understand that in a combustion reaction all arrows with face "downward", but aside from that, I'm not sure how to figure out with direction the arrows point? Does anyone know a general rule concerning that? Thanks so much
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redwhiteandbrit
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(Original post by Emupa)
Hey
We've recently done Hess's law in AS chemistry, including the diagrams showing (and in working out) enthalpy changes. I understand that in a combustion reaction all arrows with face "downward", but aside from that, I'm not sure how to figure out with direction the arrows point? Does anyone know a general rule concerning that? Thanks so much
Hi Emupa

This may have been the way you have been taught, but with Hess' Law it is not the direction of the arrow which is important, but rather where they start and where they lead to.

For instance, the cycle can have products, intermediates, and reactants at any position, but the crucial part is where the arrows go. Reactants ultimately form products, but the arrows dictate the route the reaction takes.

I hope this helps!!

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Mimi97
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An easy way I learned to remember how to work out Enthalpy changes is by learning these generic equations:

Enthalpy of Combustion= Sum of reactants - Sum of products
(CRP or as my chemistry teacher says 'the naughty C word...crap')

Enthalpy of Formation= Sum of Products - Sum of Reactants
(Just swap around from Enthalpy of Combustion)

Mean bond Enthalpy= Sum of bonds broken - Sum of bonds made
(BBBM)

I found learning arrow directions confusing, if you know these equations, you can easily draw a Hess' Law cycle and put the arrows in correctly.

P.s I missed out all the standard condition requirement in the equations, but you should assume they're there anyway.
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Emupa
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Thanks I know that where the arrows go to can change the sign of the given bond enthalpy, but do you know of a way to determine where the arrow starts and ends up? I'm so sorry!

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Emupa
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That's awesome, thanks so much - mocks are soon and I was freaking out XD


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username986184
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(Original post by Emupa)
That's awesome, thanks so much - mocks are soon and I was freaking out XD


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I do not teach my students arrows down or up for formation or combustion because this just attempts to avoid understanding of what is going on.

Combustion reactions the arrow goes from the reactants (that are being burned) to the products.

Formation reactions the arrow goes from the elements (the reactants) to the products (the compounds).

Once the cycle is set up, all the arrows (and associated values) going in one direction (say clockwise) equal the arrows (and associated values) going in the othe direction (anti-clockwise). Write an equation and rearrange to find the unknown.
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username986184
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(Original post by Emupa)
That's awesome, thanks so much - mocks are soon and I was freaking out XD


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For example
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username986184
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(Original post by Emupa)
That's awesome, thanks so much - mocks are soon and I was freaking out XD


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Here's another. Note the arrows are not where you might have been taught they 'need' to go.
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