Do we need to know how to draw Hess's law diagrams?

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For as chem aqa do we need to know how to draw out hess's law diagrams for the exam?

Posted Fri 25th April, 2014 @ 12:59 by :) PurpleJaguar (: - Team GR

3 Answers

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I think so, I'm not 100% sure but we've been practising them a lot in class and in revision so it's probably a good thing to make sure you know how to do them just incase you need them in the exam, that's what i'm going to do! :') 

Answered Sun 27th April, 2014 @ 14:19 by Beckzwe
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Okay, thankyou!

Answered Tue 29th April, 2014 @ 18:38 by :) PurpleJaguar (: - Team GR
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You can write out the full cycles, but it's not a requirement until A2 - at least that's what I was told last year. But, if you don't use the full Hess Cycle, you still need to write out an equation, because I seem to remember it being worth at least one mark. Sorry, but this might be a little rambling.

So, for enthalpies of formation that equation could be:

Enthalpy Change = Σ Enthalpies of formation of Products - Σ Enthalpies of formation of Reactants

Where: Sigma (the unusual spiky thing) means the 'sum of'

Personally, I'd shorten that to:

ΔHf = Σ (ΔHf Products) - Σ (ΔHf Reactants)

To remember which way round they go, I always used to think of the phrase 'Prof. Carp'

'Prof.' Has an 'f' in and so refers to the enthalpy of formation, and in the word 'p' comes before 'r', so it's Products minus Reactants (as above).

'Carp' has a 'c' in and so is enthalpy of combustion; but 'r' comes before 'p' in that word, so it's the opposite way around: Reactants minus Products

Hope that helps... Not the clearest it could be, but I can clarify anything if you think it would be useful.

Answered Fri 30th May, 2014 @ 22:02 by DJKOWALSKI