Knowledge database: Chemical reactions: Redox reactions

Redox reactions are reactions in which changes in oxidation states of elements occur. This happens because of the transfer of electrons during the reaction.

When an atom of an element releases the electrons, we say that it is oxidized. By releasing the electrons there is an excess of positively charged protons, and because of that, the new oxidation state is more positive than before. On the other hand, when an atom recieves electrons, we say that it is reduced. Its oxidation state gets a more negative value after receiving an electron, bacuse it gets more negatively charged.

In addition to the basic rules of oxidation states that are listed in a separate chapter, one should also know a few more rules for easier understanding of redox reactions:

The value of the oxidation number of elementary substances (e.g. Fe, O₂, P₄) equals zero.

The sum of all oxidation numbers of elements in a molecule is always zero. In case of NaCl, if one knows that the oxidation number of sodium is equal to +1, then one can easily conclude that the chlorine must have the oxidation number of -1.

The sum of all oxidation numbers of elements in an ion are equal to the charge number of that same ion. For example, in the ion NO₃^1-, we have a nitrogen atom with the oxidation number 5 and three oxygen atoms, each of which has the oxidation number -2. Thus the oxidation number of the ion is: 5 + (-2*3) = -1.

When solving redox equations, one should first determine the oxidation numbers of each element in all the ions and compounds. Then one must look for the particles that are being reduced and oxidized. After that, the number of released and received electrons is equalized. When one does this, the ratio of stoichiometric coefficients of particles in which there have been changes of oxidation numbers, must remain the same during the further equalization of the equation.

The following reaction is listed as an example:

The oxidation number of the chlorine molecule is 0; because of that, it must receive 2 electrons (one for each atom in the molecule of chlorine) in order to pass into two negatively charged chloride ions. The atoms in the chlorine molecule will receive these two electrons from two iodide ions, that, in the presence of elemental chlorine, release electrons (each ion releases 1 electron, so there is a total of 2 electrons, exactly as much as the chlorine needs). By doing this, the iodide ions turn into one molecule of iodine with the oxidation number equal to 0. Now one can see that the number of received electrons equals the number of released electrons. It is also clear that the reaction is properly balanced on both sides (equal number of atoms and sum of charge), which means that the redox equation is successfully solved.

Beginners will probably find redox equations complicated, but eventually one can solve even more complex cases without problems, but in order to get better at solving them, one should practise frequently.