Oxidation and Reduction
Redox Reactions Useful to Understand Batteries and Metal Extraction
A transfer of electrons happens in many important inorganic reactions, such as electrochemistry and extraction of metals from ores
One of the most fundamental concepts needed to understand much of inorganic chemistry is called redox. This is a word made out of two words, reduction and oxidation. Many reactions that occur in this branch of chemistry are redox reactions. If we understand the concept we will understand how electrochemistry, batteries and iron-smelting happen.
Oxidation States
In every compound, the elements involved have an oxidation state. This is a number assigned to that part of the molecule which describes the number of bonds it can form with other atoms. For example, oxygen usually has an oxidation state of -2 and hydrogen of +1. In a molecule the sum of the oxidation states of all the constituents must be zero, so a water molecule has one oxygen and two hydrogen atoms.
Ionic Compounds
It is easy to see the oxidation state in simple ionic compounds, because it is equal to the charge on the ion. In Sodium Chloride (table salt) the sodium ion has a single positive charge, so its oxidation state is +1. The chloride ion has a single negative charge so its oxidation state is -1. Some ions are more complicated. A nitrate ion has one nitrogen atom and three oxygen atoms and it has a charge of -1. Remembering that an oxygen atom usually has a charge of -2, the oxidation state of nitrogen in nitrate must be +5.
Covalent Compounds
Molecules with covalent bonds are not as obvious but it is usually possible to work out oxidation states knowing those of common elements. The example of water demonstrates this and the knowledge that carbon is usually +4 helps even more.
Oxidation and Reduction
Look at reactions between molecules it can be seen that in some reactions the oxidation state of some atoms changes. This can be seen most clearly when an element reacts with a compound to form another compound. In the element the oxidation state is always zero. So in the reaction between copper metal and magnesium oxide (MgO) in which magnesium and copper oxide (CuO) is formed, a change of oxidation state of both the magnesium and copper has occurred.
The oxidation state of copper has gone from zero to +2 (oxygen has remained at -2). The oxidation state of magnesium has gone from +2 to zero. When an oxidation state increases we call it oxidation, so copper has been oxidised. When an oxidation state decreases we call it reduction, so magnesium has been reduced.
Iron production
Another example is demonstrated in one of the reactions which happen when iron metal is formed from iron ore (iron oxide). Iron ore contains iron (III) oxide (the Roman numeral in brackets is used to show the oxidation state of that element). The oxide reacts with carbon monoxide (the carbon has an oxidation state of +2 in CO) to form iron metal and carbon dioxide. The oxidation state of iron has gone from +3 to zero, it has been reduced. The oxidation state of carbon has gone from +2 in carbon monoxide to +4 in carbon dioxide, it has been oxidised.
A comprehension of this concept will help to understand various reactions which will come up on some of the everyday chemistry articles.
