Posted by Heena on Friday, March 16, 2012 at 5:13pm.
Which of the following substances has a ∆H°f equal to zero?
I2(g)
Tin(s, gray)
F2(g)
NaCl(s)
Tin(s, white)
Thank you

Chemistry  DrBob222, Friday, March 16, 2012 at 5:30pm
All elements are zero. A few elements (carbon, tin and a few others) occur as allotropes; therefore, some arbitrary decisions have been made about which one will be called the "standard." In the case carbon (diamonds, graphite, amorphous carbon, nanofoam, etc) graphite is the one. In the case of Sn, it is the white form. Other elements are in this list for you to name.
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