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What is the relationship between ΔE 0 ' And ΔG

Remember for a reaction to be exergonic the reaction needs to have a negative change in free energy or -ΔG , this will correspond to a positive ΔE 0 ' . In other words, when electrons flow "downhill" in a redox reaction from a compoubnd with a higher (more positive) reduction potential to a second compound with a lower (less positive) reduction potential, they release free energy. The greater the difference between the redox potentials of two substances ( ΔE 0 ' ), the greater the vigor with which electrons will flow spontaneously from the less positive to the more positive (more electronegative) substance. The greater the voltage, ΔE 0 ' , between the two components, the greater the energy available when electron flow occurs. It is, in fact, possible to quantify the amount of free energy available. The relationship is:

ΔG = − n F (kJ/V) ΔE (V)

    Where:

  • n is the number of moles of electrons transferred
  • F is the Faraday constant of 96.485 kJ/V. Sometimes it is given in units of kcal/V which is 23.062 kcal/V, which is the amount of energy (in kJ or kcal) released when one mole of electrons passes through a potential drop of 1 volt

What you should notice is that ΔG and ΔE have different signage: When ΔG is positive, ΔE is negative and when ΔG is negative ΔE is positive. For a review see Red/Ox discussion in the Bis2A Discussion Manual.

The electron tower: a tool to be used for understanding red/ox

As you may have figured out, all kinds of compounds can be used in red/ox reactions in the cell. Making sense of all of this information and ranking potential red/ox pairs can be confusing. A tool has been developed to rate red/ox half reactions based on their E 0 ' values. Whether a particular compound can act as an electron donor (reductant) or electron acceptor (oxidant) sill depend on what other compound it is interacting with. The electron tower ranks a variety of common compounds (their half reactions) from most negative E 0 ' , compounds that readily get rid of electrons, to the most positive E 0 ' , compound most likely to accept electrons. The tower organizes these half reactions based on the ability of electrons to accept electrons, with the most electronegative at the bottom of the tower. So the most negative E 0 ' values are at the top. In addition each half reaction is written by convention with the oxidized form on the left/followed by the reduced form. For example the half reaction for the reduction of NAD + to NADH is written: NAD + /NADH + 2e - . An electron tower is shown in figure 2 below.

Common Red/ox tower used in Bis2A

Video on electron tower

For a short video on how to use the electron tower in red/ox problems click here . This video was made by Dr. Easlon for Bis2A students.

The right and left sides of the chemical reactions in the redox tower are separated by a "/". The form of the compound on the left of the slash is____________, and the form of the compound on the right of the slash is______________.

  1. oxidized, reduced
  2. reduced, oxidized
  3. oxidized at the top of the tower, reduced at the bottom of the tower
  4. reduced at the top of the tower, oxidized at the bottom of the tower

a

Remember, by convention the tower half reactions are written with the oxidized form of the compound on the left and the reduced form on the right. Compounds that make excellent electron donors, remember that first class of compounds we discussed earlier, are found at the top of the tower. Compounds such as Glucose and Hydrogen gas are excellent electron donors. Notice, that they are found on the right hand side of the red/ox pair half reactions. At the other end of the tower lies compounds that make excellent terminal electron acceptors, such as Oxygen and Nitrite, these compounds are found on the left side of the red/ox pair and have a positive E 0 ' value.

The tower is a tool to help determine whether a compound can act as an electron donor or an electron acceptor. Here lies the beauty of the that third class of compounds discussed above. These intermediate carriers, such as cytochromes or quinones, can act as either acceptor or donor depending upon their red/ox state and whether the other component in the red/ox reaction has a higher or lower E 0 ' value.

An example

Let's look at metaquinone ox/red , it sits in the middle of the electron tower with an E 0 ' value of -0.074 eV. Metaquinone ox can accept electrons from compounds that sit higher (above it) in the electron tower. In other words any reduced compound that has a lower E 0 ' value can donate electrons to metaquinone ox to form metaquinone red and the oxidized form of the original electron acceptor. Examples of compounds that could act as electron donors include FADH 2 , an E 0 ' value of -0.22, or NADH, with an E 0 ' value of -0.32 eV. Remember the reduced forms are on the right hand side of the red/ox pair.

Once metaquinone has been reduced, it can now act as an electron donor to any compound that sits lower (below it) on the electron tower: any compound that has a higher E 0 ' value. Possible electron acceptors include cytochrome b ox with an E 0 ' value of 0.035 eV; or ubiquinone ox with an E 0 ' of 0.11 eV. Remember that the oxidized forms lie on the left side of the half reaction.

Which of the following could be used as an electron acceptor for Ubiquinone red

  1. FAD
  2. NADH
  3. cytrochrome a ox
  4. cytochrome c red

C

Summary

Red/Ox reactions involve the movement of electrons from one compound to another. As electrons move from, the energy released can be captured by the cell to do work, such as synthesize ATP. Every red/ox reaction can be thought of as 2 half reactions, in one reaction a compound looses electrons and in the second reaction a different compound gains electrons. The amount of potential energy released is the difference in each half reactions reduction potential, E 0 ' . The electron tower is a tool that ranks different common half reactions (and therefore various compounds) based on how likely they are to donate or accept electrons. The lower, more negative, the electrochemical potential for each half reaction, the higher it sits in the electron tower. Reduced compounds can donate electrons to oxidized compounds that are below it on the electron tower. Oxidized compounds can accept electrons from any compound that are above it in the electron tower. The use of the electron tower will be more evident as we discuss electron transport chains in a few modules.

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Source:  OpenStax, Ucd bis2a intro to biology v1.2. OpenStax CNX. Sep 22, 2015 Download for free at https://legacy.cnx.org/content/col11890/1.1
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