Question 13

[admin]

First let's begin with the definition of the Standard Heat of Formation: it is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (ie. the most stable form of the element at 101.3 kPa of pressure and the temperature, usually 298 K or 25 degrees Celsius).

Yes, the diatomic molecules of H2 (g) would have a Hf of 0. But it is not because they are diatomic, it is both because the diatomic form is most stable at the conditions described above AND because the gaseous form is most stable in the described conditions. Thus, as you can see in the table provided by Wikipedia below, I2 solid has a Hf of 0 but the gaseous form does not (since it requires energy to create at standard conditions).

http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_%28data_table%29

[jsfkt78927]

So, in terms of the MCAT is there a list of molecules that we should memorize as having a zero heat of formation, because unlike people with your fortunate plight of having wikipedia at your fingertips, the other lot of us have no such advantage.

[moeenimran6713]

lol ^^ good one

[sanji2896161]

I guess from common knowledge you should think mercury is liquid at room temp, Hg in thermometers