Standard Enthalpy Formation
http://dbhs.wvusd.k12.ca.us/webdocs/Thermochem/StandardEnthalpyFormation.html
The standard enthalpy of formation for an element in its standard state is zero.
First two definitions of chemistry words with very specific meanings:
(1) Standard - this means a very specific temperature and pressure: one atmosphere and 25 °C (or 298
K). If a solution is being discussed, then everything in solution will be at a 1.00-molar concentration.
(2) Formation - this word means a substance, written as the product of a chemical equation, is formed
DIRECTLY from the elements involved. The substance in question is always written with a coefficient
of one. Here are some examples:
C (s) + O2 (g) ---> CO2 (g)
C (s) + (1/2) O2 (g) ---> CO (g)
H2 (g) + O2 (g) --> H2O2 (l)
H2 (g) + (1/2) O2 (g) ---> H2O (l)
C (s) + 2 H2 (g) + (1/2) O2 (g) ---> CH3OH (l)
(1) Every substance is shown in its standard state. This is the physical state (solid, liquid, or gas) that a
substance would be in under standard conditions. Thus, the standard state for carbon is solid, for water is
liquid and for hydrogen is gas. Why? Because at 1.00 atm. and 25 °C, these substances are in the physical
state specified.
You need to know the specifics of a substance's standard state, but it is not something that gets taught.
You just sorta absorb it. For example, the standard state for carbon is graphite (remember, a solid), not
diamond!!! The standard state for the element bromine (Br2) is liquid and iodine (I2) is solid. You will
need to pick up these facts on your own in most classes. One hint: there will be a table of standard
enthalipess of formation somewhere in your text. Examine the various compounds and elements for (s),
(l), and (g) markings, as well as (aq).
(2) There is never a compound on the reactant side, only elements. What is being written is a formation
reaction. Look again at the definition of formation. Here is an example of a chemical reaction that IS
NOT a formation reaction:
6 CO2 + 6 H2O ---> C6H12O6 + 6 O2
Here is the formation reaction for C6H12O6:
6 C (s) + 6 H2 (g) + 3 O2 (g) ---> C6H12O6 (s)
Once again, a formation reaction involves making a substance from its elements and ONLY the elements.
(3) Formation reactions sometimes are "fake" reactions, in that they cannot possibly happen as written.
For example:
H2 (g) + O2 (g) --> H2O2 (l)
is simply not possible. You can't make hydrogen peroxide by reacting hydrogen and oxygen directly.
When you do that, you get water. Hydrogen peroxide is made a different way, but you can still write the
formation reaction. And it is still useful, as you will see.
The symbol for the standard enthalpy of formation is:
H°f
All chemical reactions involve a change in enthalpy (defined as the heat produced or absorbed during a
reaction at constant pressure). The symbol for the change is H. The subscripted "f" is taken to mean
formation when used in the thermochemistry area. (There are other uses of a subscripted "f," however the
differences in context will be obvious.) The symbol "°" is taken to mean "standard conditions." (Yes, I
know that symbol is also used for degrees Celsius. Context is all important!!!!)
A typical source for these values is in an appendix to a textbook. However, when you examine one, you
will see that only the substance and the value are listed. For example, the Ag2S (s) value in one textbook
is -32 kJ/mol. Two points:
(1) The equation is never listed, since "formation" means something very specific: making the substance
directly from the elements. This would be the formation reaction for Ag2S (s):
2 Ag (s) + (1/8) S8 (s) ---> Ag2S (s)
The standard enthalpy of formation for an element in its standard state is zero! Elements in their standard
state are not formed, they just are. So, H°f for C (s, graphite) is zero, but the H°f for C (s, diamond) is 2
kJ/mol. That is because graphite is the standard state for carbon, not diamond.