Simple qestion on enthalpy?
1/4 P4 (s) ---> P (g) (where the 4 in P4 is subscript)
and not P(s) --> P (g)
Answer:
Phosphorous does not exist naturally as monotomic particles, but forms a tetrahedral P4 structure.
The enthalpy of atomisation is the change in enthalpy when 1 mole of gaseous atoms are formed from 1 mole of the element in its standard state.
So to form 1 mole of P(g) you need ¼ of P4(s) as phosphorous exists naturally as P4 in its standard state.
It is the same reason that for oxygen: ½O2(g) --> O(g), oxygen exists naturally as diatomic molecules.
Elemental phosphorus as prepared artificially exists in several allotropes, most commonly white, red and black.
White phosphorous is the most common being P4 which is where you got your equation for the enthalpy of the reaction.
The balanced equation is: P4(s) --> 4P(g) (as you have it)
ref: http://en.wikipedia.org/wiki/phosphorus...
Allotropes are elements which can exist bonded to itself in two or more forms as in carbon (coal and diamond is a good example).
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