Electrochemistry
Electrochemistry involves oxidation-reduction reactions that can be brought about by electricity or used to produce electricity.
 
Oxidation and reduction, which will be considered here as the loss and gain of electrons, occur in many chemical systems. The rusting of iron, the photosynthesis that takes place in the leaves of green plants, and the conversion of foods to energy in the body are all examples of chemical changes that involve the transfer of electrons from one chemical species to another. When such reactions can be made to cause electrons to flow through a wire or when a flow of electrons makes a redox reaction happen, the processes are referred to as electrochemical changes. The study of these changes is called electrochemistry.
 
The applications of electrochemistry are widespread. Batteries, which produce electrical energy by means of chemical reactions are in almost anything portable and electronic. In the laboratory, electrical measurements enable us to monitor chemical reactions of all sorts, even those in systems as tiny as a living cell. In industry, many important chemical - including liquid bleach (sodium hypochlorite) and lye (sodium hydroxide) are manufactured by electrochemical means. If it weren't for electrochemistry the important structural metals of aluminum and magnesium would only be laboratory curiosities and most people would see them only in museums.
 
Oxidation-Reduction Reactions
Oxidation-reduction reactions or redox reactions involve the transfer of electron density from one atom to another. Two example are the reaction of sodium with chlorine and the reaction of hydrogen with oxygen.
 
2 Nao + Cl2 ------> 2 NaCl

2 H2 + O2 --------> 2 H2O
 

At first glance these reaction appear to be very different from each other. In NaCl ions have been formed, and that has certainly involved the transfer of electrons. The electron is transferred completely from a sodium to a chlorine atom as the Na+ and Cl- ions are created.   But how about the reaction that produces water? Here we have the formation of a molecule held together by covalent bonds - bonds in which electrons are shared. How does this reaction involve a transfer of electrons? To answer this question, we have to look closely at the bonds in both the reactants and products.
 
Hydrogen and oxygen molecules are non-polar. This is because both atoms in an H2 or O2 molecule are the same, so the electronegativity difference between them is zero. In a non- polar molecule, the electron pair in the bond is shared equally, and neither atom carries a partial charge. Stated another way, each atom in an H2 or O2 molecule is electrically neutral. Now let's look at the product, water. The electronegativities of hydrogen and oxygen are quite different, oxygen being more electronegative than hydrogen. this means that the O-H bonds are quite polar, with the hydrogen carrying a substantial positive partial charge and the oxygen carrying a substantial negative partial charge.
 
Now we can look at what happens to the electrons around an atom during the reaction. A hydrogen atom begins with a zero positive charge in H2 and finishes with a partial positive charge in H2O. Similarly, oxygen begins with a zero partial charge in O2 and finishes with a partial negative charge in H2O. Thus, during the reaction there is a shift of electron density from a hydrogen atom to an oxygen atom, and it is in this sense that the reaction of H2 and O2 is similar to the reaction of Na with Cl2.
 
Many chemical reactions involve (or at least appear to involve) a shift of electron density by one atom to another. Collectively, such reactions are called oxidation-reduction reactions, or simply redox reaction. The term oxidation refers to the loss of electrons by one reactant, and reduction refers to the gain of electrons by another. For example, the reaction between sodium and chlorine involves a loss of electrons by sodium (oxidation of sodium) and a gain of electrons of chlorine (reduction of chlorine).
 
Nao ------> Na+ + e- (oxidation)

Cl2 + 2 e- -------> 2 Cl- (reduction)

we say that the sodium is oxidized and the chlorine is reduced.
 

Oxidation and reduction always occur together. No substance is ever oxidized unless something else is reduced. Otherwise, electrons would appear as a product of the reaction, and this is never observed. During a redox reaction, then, some substance must accept the electrons that another substance loses. this electron-accepting substance is called the oxidizing agent because it helps something else to be oxidized. The substance that supplies the electrons is called the reducing agent because it helps something else be reduced. Sodium is a reducing agent, for example, when it supplies electrons to chlorine. In the process, sodium is oxidized. Chlorine is an oxidizing agent when it accepts electrons from the sodium, and when that happens, chlorine is reduced to chloride ion. One way to remember is:
 
The substance that is oxidized is the reducing agent.
 
The substance that is reduced is the oxidizing agent.
 
Redox reactions are very common. Whenever you use a battery, a redox reaction occurs. The metabolism of foods, which supplies our bodies with energy, also occurs by a series of redox reactions that use oxygen to convert carbohydrates and fats to carbon dioxide and water. Ordinary household bleach works by oxidizing substances that stain fabrics, making them easier to remove from the fabric or rendering them colourless.

Liquid bleach contains hypochlorite OCl-, as the oxidizing agent.