Key Information & Summary
- The oxidation state (or oxidation number) of an atom in a substance is defined as the difference between the number of valence electrons and the number of electrons left after having assigned all the binding electrons.
- A chemical element undergoes oxidation when an electron is subtracted, which translates into an increase in its oxidation number.
- A chemical element undergoes reduction when an electron is added, which translates into a decrease in its oxidation number.
- The oxidation and reduction reactions are two semi-reactions of the electron-exchange process, which takes the name of redox or
- There are some basic rules to balance a redox reaction.
The oxidation state (or oxidation number) of an atom in a substance is defined as the difference between the number of valence electrons and the number of electrons left after having assigned all the binding electrons. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. It is the hypothetical charge that an atom would have if all bonds to atoms of different elements were ionic.
The oxidation state expresses the "degree of oxidation" of an atom, taking as reference the 0, corresponding to the oxidation state of the atom in a simple substance.
A chemical element undergoes oxidation when an electron is subtracted, which translates into an increase in its oxidation number. This electron subtraction can be caused by another element, which undergoes the complementary reduction process.
Let’s take the example of sulfuric acid (H2SO4). After considering the number of bonds established by the atoms and knowing that oxygen is more electronegative than sulfur and hydrogen, that the oxygen has 6 valence electrons, sulfur has 6 valence electrons and hydrogen has a valence electron, the oxidation numbers of the atoms are:
for each hydrogen atom 1 - 0 = 1;
for each oxygen atom 6 - 8 = -2;
for the sulfur atom 6 - 0 = 6.
The substances that have the ability to oxidize other substances are known as oxidizing agents. They subtract electrons from other substances and, since they accept electrons, they are also called electron acceptors.
Oxidants are generally chemical substances that possess elements with a high number of oxidation, for example hydrogen peroxide, permanganate or chromic anhydride, or highly electronegative substances, such as oxygen, fluorine, chlorine or bromine, capable to subtract one or more electrons from other substances.
In chemistry, the reduction is the acquisition of one or more electrons by a chemical species.
Each reduction occurs at the same time as an oxidation, which consists in the loss of electrons by another chemical species, in such a way that the electrons are exchanged by the two chemical species.
The oxidation and reduction reactions are therefore two semi-reactions that are part of this process of electron exchange, which takes the name of redox.
In chemistry, with the term oxidoreduction or redox we mean all those chemical reactions in which the number of oxidation of atoms changes, i,e. in which there is an exchange of electrons from a species chemistry to another.
This type of reaction can vary from a simple redox process (such as carbon oxidation, that generates carbon dioxide or carbon reduction from hydrogen, which produces methane) to the more complex oxidation of sugars in the human body, through a series of complicated electron transfer processes.
A redox reaction can be seen as the simultaneous unfolding of two distinct "semi reactions":
oxidation: the chemical specie that undergo oxidation loses electrons, so its number of oxidation increases. The chemical species which loses electrons is called "reducing".
reduction: a decrease in the number of oxidation of a chemical species, generally due to an acquisition of electrons by the species.
oxidizing species + n e- → reduced species
reducing species → oxidized species + n e-
being n the number (or moles) of electrons exchanged during the reaction, equal in both half-reactions.
The global redox reaction can therefore be represented as:
oxidizing species + reducing species → oxidized species + reduced species
In electrochemistry, the chemical species directly involved in the electron transfer process are called "electroactive species", to distinguish them from any other chemical species present in the system that do not undergo reduction or oxidation.
Balancing of redox reactions
Chemical redox reactions can be balanced by various methods, including the ion-electronic method. This method consists of the following steps:
1- Write the oxidation numbers of the chemical species to identify the oxidant and the reducing agent;
2- Dissociate the electrolyte and take them into consideration as they are in solution;
3- Balance the value of the element by adding electrons;
4- Balance the charge of the ion by adding ions H + (if you are in acidic environment) or of OH- (if you are in a basic environment);
5 -Balance the mass of the hydrogens with the addition of H2O;
6- Multiply the oxidation reactions by the number of electrons of the reduction and vice versa;
7- Add the two semi-reactions making the least common multiple of the electrons so that they can be simplified by the calculation;
8- If necessary, balance the oxygens.