Energy And Redox Reactions

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Oxidization Reduction reaction (Redox) occur when one species losing electrons (called oxidized) and another species gaining electrons (called reduced). One uses the Oxidation Number in the procedures outlined in Notes on Oxidation-Reduction Reactions and Direction to Determine Redox Reactions to determine which species in the reaction has their electron moving.

There is another way of explaining the species losing and gaining electrons. They are called Half cells. So in the net ion reaction:
1 Mg0(s) + 1 Cu+2 --> 1 Cu0(s) + 1 Mg+2

the two half cells are:

Reduction Half cell : 1 Cu+2 + 2 e- ---> 1 Cu0(s) (E0 = +0.34V)

Oxidation Half cell : 1 Mg0(s) ---> 1 Mg+2 + 2 e- (E0 = - (-2.37V) = +2.37V )

Since 2 electrons are lost and 2 electrons are gained, then electrons conservation (and all other atoms) is upheld.

So, we have accounted for Conservation of Mass, but what about Energy in the reaction.

If you look at either of the above Half cell, there is chemical energy (we called Gibbs Free Energy) in the reactant that is different from the products. Therefore, there exists an Energy difference (chemical potential energy difference caused by the Gibbs Free energy difference) in each Half cell. In Redox chemistry (called Electrochemistry), this Energy potential difference is called Volts (V). Technically, Volt = work or energy (J) per Coulomb of electrical charge (C). So each Half cell has its own Energy potential difference or Volt. Since this is about Energy, then this information is explained by Thermodynamics. With most all Thermodynamic principles, it a difference between an initial condition and a final condition so there needs to be a basis or standard for the difference to apply to. For volts from Half cells, we use the formation of Hydrogen gas as the 0.0 V and then compare all other Half cell volts to hydrogen to determine the stated volts.

There is a table of these Half cells and associated volts called Standard Reduction Potential Table, (see link below to see table),


As the name of the table indicates, all the Half cell in the table are Reduction Half cell. To create an Oxidation Half cell, you reverse the Half cell reaction and the volt sign changes (from positive to negative or visa versa). This is how the two half cell or the overall Redox reaction are created. Now, the combined voltage of the reactants and products produce a combined difference that is called the Cell potential, Ecell. This is the Free Energy Difference between the reactants and the products so it is the Energy that "makes the reaction go" or in a Redox reaction, it is the energy difference that moves the electrons.



Application of Electrochemistry/Redox Reactions - Voltaic/Daniel Cells & Electrolysis/Electroplating


Voltaic/Daniel Cells

Here, the Cell potential needs to be a positive voltage (+V) after the Reduction and Oxidation Half cells are added together. This is the condition that corresponds to a negative Gibbs Free Energy Difference that indicates that the Redox reaction will occur spontaneously (remember that means without energy help from outside the system/reaction). The difference (i.e. voltage) will decrease as the reaction occurs until there is no energy difference ( 0.0V). The most common application of this system we call Batteries. The cell (or battery) is converting chemical energy to electrical energy (electron movement). All batteries work in this manner. So now one know what a "dead" battery really means, that the cell voltage is "0.0V" because all the Gibbs Free Energy difference has been used up and there is no Gibbs Free Energy difference between the reactants and products anymore (so the battery can not convert chemical energy into electrical energy). Another common application of these cells is corrosion of metals (i.e. iron corrosion or rusting).

Electrolysis or Electroplating
In both Electrolysis and Electroplating, electrical energy is being converted into chemical energy. Therefore, it is the reverse of Voltaic/Daniel Cell and will NOT be spontaneous and will have a negative Cell potential. The difference between Electrolysis and Electroplating is that Electrolysis separates compounds into their elements whereas Electroplating reduces metal ions into element metals (usually as solids). The most common application of Electrolysis is making hydrogen gas and oxygen gas from water. The most common application of Electroplating is plating metals such as in jewelry making or copper deposit to produce "motherboards" in electronic device like computers.


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Last edited March 17, 2019 9:33 pm (diff)
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