Questions from 9/20/11 Lecture
Pressures in Equilibrium Constant of Gibbs Free Energy
During class, there was a question about the specifics of and the use of pressure, along with molar
concentration in the equilibrium constant of the Gibbs free energy calculation. The pressure provided
was the concentration of the degradation product (CO2) of glucose. Because CO2 is a gas, this
concentration was reported as a partial pressure. This pressure is not equal to atmospheric pressure or
the concentration of dissolved CO2 from the atmosphere. However, this pressure is normalized by the
standard atmospheric pressure of 1 atm (calculation not shown in most examples), and thus becomes
unitless in the equilibrium constant. Similarly, in the Gibbs free energy, the aqueous concentrations of
the other reactants and products are actually activities. Thus, taking what we learned in aquatic
chemistry, we know that the activity is equal to your activity coefficient times your concentration. For
most purposes and for the examples in class, the activity coefficient is considered 1 L/mole, which will
cancel the concentration units out. Thus, the entire equilibrium constant of is unitless. If you have
further questions, you can read Aquatic Chemistry by Strum and Morgan, page 37-38.
Balancing reactions
Many of you had concerns about balancing reactions. I have outlined the general steps that can be used
to balance most half-reactions, yet, if you are still having troubles, you may want to consult Dr. Gunsch
or your text.
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Identify products and reactants
Balance atoms like (C, N, P, and S)
Balance Oxygen with H2O
Balance hydrogen with H+
Balance charge with eNormalize equation by number of electrons (electron equivalency) or common denominator of
the number of moles for products and reactants
The Redox Tower – Page 119 of your text
The redox tower’s main importance is as a means of conceptualizing electron transfers through
reduction-oxidation (redox) reactions common in the environment. Remember that redox reactions are
the cell’s means of passing the “currency” of electrons from one reaction (oxidation) to another
(reduction), so that both reactions can occur. Because redox potential is used to indicate the tendency
of a compound to transfer electrons, the electron tower shows the range of typical potentials in nature.
As stated in class, reduced substances at the top of the tower (on the right side of the pair) are typically
the electron donators and the oxidized substances (on the left side of the pair) at the bottom of the
tower are typically the electron acceptors. As an electron “falls” from a donor at the top (in actuality it
is being supplied through the oxidation reaction of the donor) and is used in the reduction reaction of
the acceptor, the further the gap between them (or the further down the electron makes it) the greater
the amount of energy released. For example, electrons donated from glucose and accepted by O2 will
release much more energy than H2 donating electrons to fumarate. Furthermore, redox pairs in the
middle of the tower can serve as electron donors or acceptors depending on the pair they are reacting
with and can be determined by examining the sign of the redox potential of the net equation(>0 is
favorable). Overall, a general rule is that the reduced substrate in the pair with the more negative
reduction potential (or least positive) donates the electrons, while the oxidized form of the pair whose
potential is more positive is the acceptor.
Electron Equivalent
Common practice when working with redox reactions is to normalize the moles of the products and
reactants by the number of electrons being transferred. This is done to make balancing half-reactions
and comparing net reactions easier.
Example 1 – Not shown in class (See attached sheet, page 3)
Example 2 – Propionate example shown in class (See attached sheet, page 4)
Enzymes
Several of you expressed concern on how much you should know about enzymes. To this point in the
class you are only expected to be familiar with what was covered during in-class lectures and in your
readings. These points include, but may not be limited to:
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What is activation energy and why it is important
How catalysts (enzymes) affect the activation energy
That enzymes are highly specific
That enzymes are continuously reused
Understand and briefly explain the general equation for an enzymatic reaction (MichaelisMenton equation)