Oxidation-reduction (redox) reactions are chemical reactions in which there is a transfer of one or more electrons between atoms. These reactions often produce energy and are the primary pathway in which molecules such as sugars, fats, and proteins are broken down in the body. One of the early scientists who studied redox reactions was the German chemist, Walther Nerst (1864-1941).
In a redox reaction, electrons are transferred from one reactant to another. Oxidation is the process by which one substance loses electrons. Reduction is the process by which a substance gains electrons. The term reduction refers to the fact that the amount of positive charge on the substance is reduced by the additional electrons. A simple example of a redox reaction is the one which creates sodium chloride (2NaCl) from elemental sodium (Na) and chlorine (Cl2. <bu>Na + Cl2 <rarr> 2NaCl In this reaction, the sodium is oxidized and chlorine is reduced. Since the sodium is the substance that is reducing chlorine, it is called the reducing agent. Chlorine, which is the substance that is oxidizing sodium, is known as the oxidizing agent. Since this type of electron transfer always requires a reducing and oxidizing substance, the process is called oxidation-reduction.
In the reaction of sodium with chlorine, there is a complete transfer of electrons between the two substances. However, this does not occur in all redox reactions. Some redox reactions change the amount of electron sharing that goes on in covalent bonds. An example of this is the combustion of methane in which it reacts with oxygen to form carbon dioxide and water. At the start of this reaction, electrons are shared equally between carbon and hydrogen in covalent bonds. When methane reacts with oxygen, a covalent bond is formed between the oxygen and carbon. Since oxygen is more electronegative than carbon, the electrons are shifted away from the carbon. This shifting of electrons releases potential energy that can be utilized by the cell.
The amount of energy that is released during a redox reaction is dependant on the relative electronegativities of the reactants. The electronegativity of an element is determined by its oxidation state or oxidation number. This value is determined by a set of rules which describe how electrons are positioned around an atom or molecule. Typically, it reflects the number of electrons an atom will accept or donate. For example, oxygen has a oxidation number of -2 because it has a tendency to accept two electrons.
Redox reactions are important in living systems because they allow energy in food molecules to be released and transferred to make adenosine triphosphate (ATP). This process takes place during cellular respiration. In this reaction, glucose (or other food molecules) is combined with oxygen, or oxidized, to produce carbon dioxide, water, and energy. The energy is derived from the potential energy that is released as electrons are transferred to oxygen. The oxidation of glucose is done in a step-wise fashion by a series of enzymes which make up the Krebs cycle and the electron transport chain. One of the key enzymes in this process is nicotinamide adenine dinucleotide (NAD). The energy is released slowly because the complete release of all its stored energy would not be efficient.
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