heat of formation equation of CaCO3 (2009 test, #67)

[super112]

I chose Ca^(2) + CO3^(-2) ---> CaCO3 but the correct answer was broken down further,

Ca + C(graphite) + 3/2 O2 ---> CaCO3

I'm pretty sure I've seen some practice questions where they just list the heat of formation for products and reactants (even for similar things to this product like CO), but I've never seen them break it down further to C and 1/2 O2 for example. On the test should I just choose the most "broken down" form? Thanks

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[cc609]

I think formation reactions all have to be in their elemental form, and produce 1 mol of the product. As you can see Co3 is not an element (element as in O2, C etc).
When I did practice tests all of them seemed to break down Carbon for C(graphite)! That's the first thing I look for. Hope this helps

 

[super112]

Yeah it must be the elemental form requirement because for example here is a table of heat of formation which includes CO2, etc
http://wps.prenhall.com/wps/media/objects/3311/3390593/imag0507/TB05_003.GIF

 

[DentalGirl5]

Chad mentions that graphite is the standard state of carbon 🙂