Oxidation-reduction, or redox, reactions change the oxidation states of atoms via the transfer of electrons from one atom, the reducing agent, to another atom that receives the electron, the oxidizing agent. Here, the atom that donates electrons is oxidized—it loses electrons—and the atom that accepts electrons is reduced—it has a less positive charge because it gains electrons. The movement of energy in redox reactions is dependent on the potential of the atoms to attract bonding electrons—their electronegativity. If the oxidizing agent is more electronegative than the reducing agent, then energy is released. However, if the oxidizing agent is less electronegative than the reducing agent, the input of energy is required.
Is oxidation a loss or gain of electrons? The terminology can be confusing. The acronym OIL RIG is commonly used to remember. It stands for oxidation is loss; reduction is gain. So, if an atom is oxidized, then it loses electrons. As a reducing agent, the oxidized atom transfers electrons to another atom, causing it to be reduced. With OIL RIG in mind, most questions about the members of a redox reaction can be answered.
Redox reactions either produce or require energy. If an atom loses an electron to a more electronegative atom, then it is an energetically favorable reaction, and energy is released. This is actually very logical—similar to a strong man winning a tug-of-war with a weaker man—because a more electronegative atom has a greater capacity to pull electrons to itself. A biological example of this type of reaction is cellular respiration, in which energy is released and used to create ATP, a form of energy that cells can easily use.
Other redox reactions require, rather than release, energy. If an electron moves from a more electronegative atom to a less electronegative atom, energy must be used. This is like a weaker man winning the tug-of-war with a stronger man—it requires energy from an outside source. A biological example is photosynthesis, in which electrons are transferred from water to carbon dioxide with the help of energy in the form of light.
A redox reaction can occur not only when an electron is transferred but also when there is a change in the sharing of an electron in a covalent bond! For example, when methane and oxygen react, they yield carbon dioxide and water. In this case, the carbon in methane is oxidized. This is because the electrons in methane are shared equally between carbon and hydrogen, whereas the carbon in carbon dioxide is partially positive since oxygen attracts electrons more than carbon does.
