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Course: AP®︎/College Chemistry > Unit 9
Lesson 2: Gibbs free energy and thermodynamic favorabilityFree energy of formation
The standard free energy of formation of a substance is the free energy change that occurs when 1 mole of the substance is formed from its constituent elements in their standard states. A pure element in its standard state has a standard free energy of formation of zero. For any chemical reaction, the standard free energy change is the sum of the standard free energies of formation of the products minus the sum of the standard free energies of formation of the reactants. Created by Jay.
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What is the difference between formation energy, free energy of formation and heat of formation?(2 votes)
Formation energy is the general energy change when a certain amount of a chemical (usually 1 mole) is created. These changes can be either positive or negative values depending whether energy is absorbed or released by the reaction. There’s not a single measurement of formation energy, instead we use either enthalpy of formation or free energy of formation. Additionally these values are recorded with the chemicals in their standard states so it should really be standard enthalpy of formation and standard free energy of formation. Standard state for a gas means the pressure is exactly 1 bar, liquids and solids are pure substances in their most stable form at a pressure of 1 bar and a temperature of interest (usually 25°C or 298 K), and the concentrations of solutes in solution are 1 M.
Standard enthalpy of formation, also known as standard heat of formation, is the change in enthalpy when all reactants and products are in their standard states.
Standard free energy of formation is the change in free energy when 1 mol of a compound in its standard state forms from its constituent elements in their standard states.
In both instances the heats of formation for pure elements are 0.
Hope that helps.(4 votes)
Video transcript
- [Instructor] Free
energy is symbolized by G. And the change in free energy is delta G. When there's a subscript of F, this is talking about the change in the free energy of formation. Which refers to the
change in the free energy for the formation of
one mole of a substance from its elements in
their standard states. These superscript naught
refers to the fact that the substances are
in their standard states. By convention, the standard change in free energy of formation for an element and its standard state is equal to zero. Therefore, if we think about
forming one mole of oxygen gas, we would be making it
from elemental oxygen which is also oxygen gas. So forming one mole of
oxygen gas from one mole of oxygen gas means
there will be no change and therefore the standard change in free energy of
formation is equal to zero. Textbooks often tabulate
data for standard change in free energy of formation of substances at 25 degrees Celsius. However, it doesn't have
to be that temperature. The units are in kilojoules per mole. We've already seen that
the standard change in free energy of formation for an element is equal to zero. Therefore for elemental
oxygen, O2 delta G naught is equal to zero. And for carbon in the solid
state in the form of graphite, delta G naught is also equal to zero. The value for carbon monoxide
is naught equal to zero. Delta G naught is equal to negative 137.2 kilojoules per mole. So if we think about forming
one mole of carbon monoxide from its elements, carbon monoxide is composed
of carbon and oxygen. And the elemental form of oxygen is O2 gas and the elements of form of carbon is the solid form in graphite. So this value for the standard change in free energy of formation
of carbon monoxide is talking about the change in free
energy for the formation of one mole of carbon
monoxide from its elements in their standard states. Standard changes in free energy
of formation of substances are useful because they
can be used to calculate the standard change in free
energy for a chemical reaction. The standard change in free
energy of a chemical reaction is equal to the sum of the standard change in free energy of formation
of the products and from that you subtract the sum
of the standard change in free energy of
formation of the reactants. For our reaction, let's look at these
synthesis of ammonia gas from nitrogen gas and hydrogen gas. Our goal is to calculate the
standard change in free energy for this reaction at 25 degrees Celsius. First, we need to think
about our products. For this reaction, we have only one product
and that's ammonia. Notice how there's a two as a coefficient in front of ammonia and
the balanced equation. So we would need to look
up the standard change in free energy of formation of ammonia at 25 degrees Celsius. And because this value
is per mole of ammonia, we would need to multiply it by two because we have two moles
in our balanced equation. Next, we think about our reactants. And we have two reactants,
nitrogen and hydrogen. And we need the sum of
their standard changes in free energy of formation. So we would write the standard change in free energy of formation of nitrogen. And because there's a one
in our balanced equation, we multiply that value by
one and two that we add the standard change in
free energy of formation of hydrogen gas. And since we have a three
in our balanced equation, we would multiply that by three. So going back to ammonia, the standard change in
free energy of formation is equal to negative
16.5 kilojoules per mole, multiply that by two moles
and the moles will cancel out. For nitrogen and hydrogen, those are both elements
in their standard states and therefore their standard
free energy of formations are equal to zero. So this is just zero plus zero. So the standard change in
free energy for this reaction is equal to negative 33.0
kilojoules at 25 degrees Celsius. Standard state for a gas
refers to the pure gas at a pressure of one atmosphere. So we think about all three of our gases at a partial pressure of one atmosphere at a temperature of 25 degrees Celsius. The standard change and free
energy for this reaction is negative 33.0 kilojoules. Since the change in
free energy is negative, that tells us the reaction as
thermodynamically favorable in the forward direction. So nitrogen and hydrogen
will react together to form more ammonia. Textbooks often give
the units as kilojoules for the standard change and free energy of a chemical reaction. However, sometimes you also see kilojoules per mole of reaction. Per mole of reaction simply refers to how the balanced equation is written. And here we have the formation
of two moles of ammonia. So we can write a conversion
factor of two moles of ammonia per one mole of reaction. For standard free energies of formation, the units are kilojoules
per mole of the substance. So in this case, it's negative 16.5 kilojoules
per mole of ammonia. So moles of ammonia would cancel out, which gives kilojoules per
mole of reaction for the units.