OCR Chemistry F322~ 4th June 2013~ AS Chemistry

that's right but you need 240 for an A not 230

In cracking when you break the c-c bond is this homolytic or heterolytic?

neither? I don't think we have to know how it breaks

Actually dreading this exam. Not because I don't enjoy chemistry but because I have unit 2 biology and psychology and left revision to last minute. Do we have to learn the Hess law pathways for enthalpy change of combustion and formation? Or can we use a formula?

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I hope that's clear enough. Basically you draw lines coming from the reactants and products. Carbon has an enthalpy change of formation of 0 so u just kinda ignore it, thats why i put it in brackets. So to make CO, it takes -111, which is the enthalpy change from C+0.5o2..then to make CO + N2 it takes -193, and this can only be made from n20 + carbon. It doesnt look like a normal hess's law cycle because you havent got individual compounds, ur given whole equations so you have to use this kind of cycle. then to work out the enthalpy change you make the arrows in one direction = to the arrow(s) in the other direction and rearrange.
a question similar to this came up before, but it had already drawn the cycle for u and u had to add in the values for the enthalpy changes from given equations. my explanation sucks but I hope you kinda get it now.

Hi, sorry I'm a bit confused why the arrows go down when I've seen the arrows going up for formation. Can you explain when the arrows go up or down?
Thanks

Im not really sure what you mean about hess' law pathways, just remember that if you are dealing with values for enthalpy of formation, write the reaction across top, and then draw an 'elements' box below, draw arrows from this box to both the reactants and products, the arrows will represent the enthalpy of formation of whatever the arrow is pointing to.
e.g take
drawing arrows from the elements to reactants and products, if its combustion values its the opposite, and the arrows face downwards towards the combustion products.
if you don't get my explanation then ocr purple book page 201 has a little blue box explaining it

hi it's fine lol, my explanation was rubbish! it's not a normal hess's law cycle so ignore the rule that when you've got formation values the arrows go up and when you have combustion values the arrows go down

do you mean for combustion values the arrows go downwards towards the elements?

For combustion arrows go down to combustion products not elements, usually co2 + h2o.

Someone please correct me if I'm wrong

I mean they just go in a downwards direction generally

Anybody have a definitions list ??? would be very useful!

The first part is brilliant - thanks for sharing


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I just cannot get my head around hess's law....is there a formula for this?

Ditto 😫and its a major component of the exam


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