Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 9 December

© R. Paselk 2005
 
     
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Oxidation/Reduction Reactions

In these reactions we see a transfer of electrons from one atom or molecule to another. First let's look at some terms.

CH4 + 2 O2 right arrow CO2 + 2 H2O

Notice that the methane is oxidized by the oxygen. We say that the carbon and hydrogen are both oxidized to give the new covalent products, water and carbon dioxide.

Examples:

H+(aq) + Cl-(aq) + Ca0(s) right arrow H2 (g) + Ca2+(aq) + Cl-(aq)

Balancing: 2 H+ + Ca0(s) right arrow H2 (g) + Ca2+

Ag+ + NO3- + Cu0 right arrowAg0 + NO3- + Cu2+

Balancing: 2Ag+ + Cu0 right arrow 2Ag0 + Cu2+

Cu2+ + SO42- + Fe0 right arrow Cu0 + Fe3+ + SO42-

Balancing: 3 Cu2+ + 2 Fe0 right arrow 3 Cu0 + 2 Fe3+

As another example we can look at a key oxidation reaction in glycolysis, the central pathway of metabolism. Don't worry about these reactions - they will not be on an exam. They are presented for your interest.

    • In this case we use a biological oxidizing agent, NAD+ to take electrons (in the form of a hydride ion, H:-)

chemical equation for the oxidation of Ga-3-P to 1,3-bisPGA by NAD+

    • Frequently organisms need to operate this reaction under anaerobic (oxygen free) conditions. For example the maximum power (energy/sec) you can get from your muscles is anaerobically. For this to occur you need to regenerate the oxidizer (NAD+) without the presence of oxygen. For this to occur we use another reaction:

chemical equation for the reduction of Pyruvate to L-lactate by NADH

The mechanism for this electron transfer is shown below:

simplified mechanism for the elctron transfer used in lactate dehydrogenase

Balancing Redox Equations

There are two common methods for balancing redox reactions: the oxidation number method and the half-reaction method. The half-reaction method works very well for ionic reactions, it is relatively easy to give partial credit, and it is the only method I will use in this class. If you know how to do the other method you are welcome to do so, but be careful to make sure you show your work or I won't be able to give partial credit!

The Half-Reaction Method

In the half-reaction method what we do is first break an equation into two parts and then balance the parts individually. Presented stepwise:

Example. Balance the following equation as it occurs in acid solution:

MnO4- + Cl- right arrow Mn2+ + Cl2

 


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