Thermochemistry
Properties
of a substance that don't depend on its History are called State Functions.
One of the most important State functions for a Chemical system is the
Enthalpy, because it tells us the ability to produce Heat, a form of Energy.
The problem is that we can only measure CHANGES in the Enthalpy of the
system, and have no way to determine the ABSOLUTE Enthalpy. We have
to define a convenient ZERO of Enthalpy for all the chemical compounds that
we can make. We do this by defining the enthalpy of the Standard
State of the Elements, which is only about
100 definitions and should cover all possible matter since all matter is
made of elements.
The Enthalpy of an Element in its
Standard State at 298K is Defined a Zero
The Standard State of an element is defined as pure, at
1 atmosphere pressure, and in the phase it would normally occur at 298K.
Thus, the standard state of Argon is Ar(gas), Oxygen is O2
(gas), Carbon is C(solid, graphite), and Bromine is
Br2 (liquid).
We can now define the Enthalpy of any molecule in any
phase at 298 K through its Standard Heat of Formation, DHfo.
The Heat of Formation, DHfo,
is the heat evolved from the synthesis reaction of one mole of the substance
from the Standard State of its' constituent elements. The DHfo(Diamond)
is not zero because it is not the Standard State of the element Carbon.
The following reaction defines the heat of formation of
methane:
C(s, graphite) + 2 H2(gas)
= CH4(gas) ;
DHfo = qp
=
DHrxno
The superscript o means that all pressures
are 1 atmosphere and all concentrations (for solutions) are 1 molar.
The symbol qp is the heat of the reaction carried out at constant
pressure. Can you write the Standard Formation reaction for Calcium
Carbonate? answer
As you recall, the enthalpy of a substance is a measure of
how much internal energy it has, or its' ability to produce heat when reacted.
Substances with a positive Heat of Formation are less stable energetically
than the elements from which they are formed. When a reaction proceeds
from reactants with large enthalpy to products of low enthalpy, the reaction
releases Heat, so reactants with high Enthalpy are good fuels or explosives.
How much Heat is released depends on the Enthalpy of the fuel (and oxidant)
relative to that of the Enthalpy of the final products. Hydrocarbon
fuels burn in oxygen to produce carbon dioxide and water and release heat.
When you have to carry fuel, you want to know how much energy
(heat) you can get per unit mass of the substance. Chemists are more
interested in how much energy is released per molecule, so Standard Heats
of Formation are quoted in kJ/mol. Remember, Enthalpy is an Extensive
property, so you must know the quantity of a substance to determine its
energy content. Using the table above, calculate what the heat evolved
would be when using one mole of Hydrogen, H2 (gas), as Fuel
would be (in kJ)? answer
In 1995, the United States consumed 9.2 x 1019J
of energy, most of it by the archaic process of burning things
Remember, we can calculate the heat of any reaction, even
a reaction that we do not now how to perform from the heats of formation
of the products minus the heats of formation of the reactants of the given
reaction. Enthalpy, like all forms of energy, is an extensive property.
Wouldn't it be nice if you could make a diamond from
burning methane in oxygen to make carbon and water. The reaction
would look like this:
CH4 (gas) + O2 (gas) = C(diamond) + 2 H2O(l)
Will heat be given off or consumed in this transformation?
How much heat will be evolved to make a 1.00 gram diamond?
We can use Thermochemistry to determine the molar heat
of reaction. We can relate all the reactants an products back to
the standard state of the elements from which they are formed.
This results in the generally useful equation
where the {ci} are the stoichiometric coefficients
in the balanced chemical reaction for which you wish to derive the heat
of reaction.
So the heat of reaction as written above can be derived
from the table of heats of formation above:
1.88 kJ/mol + 2(-285.8 kJ/mol) - {(-74.85 kJ/mol) + (0)} = -494.9 kJ/mol
Note that this is the heat of forming one mole of diamond.
One gram of diamond is (1.00/12.011) = 8.33 x 10-2 moles, so
the heat evolved to make one gram of diamond is:
Qp = -494.9kJ/mol ( 8.33 x 10-2 moles) = -41.2 kJ
The heat is negative, which means that it is given off from
the system to the environment (surroundings) This would be great:
we could use methane to heat our houses and cook our food, and collect
diamonds as waste! Why doesn't this work?
How much more heat do you get by burning methane
to produce 1.00 gram of graphite (and water)?
CH4 (gas) + O2 (gas) = C(graphite) + 2 H2O(l)
We as living things need fuel,
and our waste products are not diamonds, graphite,
or CO2 and H2O but a more complicate brew.
Nonetheless, different foods provide different amounts of energy
per unit mass to our bodies; It is clear from the following table
why the body stores large quantites of excess energy in fat.
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PJ Brucat // University of Florida