I think I missed the lesson we did on this, and Tom tried explaining, but I'm still more or less lost as to what it actually is?
I know to change the arrow bit around then if its +'s they become -'s... blah blah I know that bit.
But.. what IS Hess's Law?
I tried google but it was distinctly unhelpful on the subject :(
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I'm delighted that you're using the blog to ask for help, Lucinda!
In answer to your question...
The definition of Hess's Law is this:
"The overall energy change to get from REACTANTS to PRODUCTS is the same no matter which route is taken."
This means that you could go directly from A to B but you might also be able to go from A to C to B. The sum of the energy changes for each route should be the same so if you don't know one of the energy changes in the cycle you should be able to use the others to work it out.
It's difficult explaining this completely in writing rather than in person so I hope it makes a bit of sense! This page of the Salter’s revision website might be helpful: www.4college.co.uk/as/index.php
(Any pages from ‘Elements for Life’ and ‘Developing Fuels’ on this website will help you with the rest of your revision for this exam.)
Make sure that you also know full definitions of things like:
STANDARD ENTHALPY CHANGE OF FORMATION (ΔH˚f) - The total energy given out or taken in when forming 1 mole of a compound from its basic elements in their standard states under standard conditions.
STANDARD ENTHALPY CHANGE OF COMBUSTION (ΔH˚c) - The total energy given out when burning 1 mole of an element or compound completely in oxygen under standard conditions.
STANDARD CONDITIONS - Temperature = 298K (25˚C). Pressure = 1atm (100kPa). Concentrations = 1mol/dm3.
EXOTHERMIC - Energy given out. -ΔH, +ΔT.
ENDOTHERMIC - Energy taken in. +ΔH, -ΔT.
Also remember from GCSE that you can calculate the overall energy change for a reaction by adding up ΔH for all the bonds broken, then for all the bonds made, followed by doing BONDS BROKEN - BONDS MADE.
(Always draw out the full structures of the compounds involved in the reaction so that you know exactly how many of each type of bond there are.)
I hope the rest of your revision is going well. Please let me know if there's anything else that I can help with. :)
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