Acidic Redox Reactions
Redox reactions involve the transfer of electrons between atoms or changes in oxidation state. They are nearly ubiquitous. Many of the important reactions in human cells, for example, are redox reactions, as is combustion or the rusting of exposed steel on a car frame. Some acids, such as nitric acid and sulfuric acid, are strong oxidizing agents and can oxidize other substances as part of a redox reaction.
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Oxidation Numbers
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Chemists keep track of electrons in reactions using oxidation numbers---numbers assigned to each element in the reaction---using seven simple rules:
Any element by itself has an oxidation number of zero;
A monoatomic ion, such as a calcium ion, has an oxidation number equal to its charge;
The sum of the oxidation numbers in a compound is always equal to the net charge of the compound. CO2, for example, has no net charge, so the sum of the oxidation numbers for all three atoms must be zero;
Hydrogen has an oxidation state of +1 when combined with a nonmetal and -1 when combined with a metal;
Oxygen, when combined with elements other than fluorine, always has an oxidation number of -2;
Halogens---elements from group 17 of the periodic table---always have an oxidation number of -1 unless combined with a halogen higher up in the group or with oxygen; and
Elements in group 1 of the periodic table have an oxidation number of +1 when they form compounds, while group 2 elements have an oxidation number of +2.
Oxidation Number Examples
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The compound FeCl2 contains one iron atom and two chlorine atoms. Chlorine is a group 17 element(a halogen). In the rules above we see that each chlorine atom has an oxidation number of -1. Since the compound does not have a net charge, iron must have an oxidation number of +2, so the sum of all oxidation numbers is zero.
The phosphate ion has a chemical formula of PO4 and a charge of -3. Because it has a net charge, the sum of all the oxidation numbers must equal the net charge. As noted in Section 1, oxygen has an oxidation number of -2. There are four oxygens, so 8 - 3 = +5, so the oxidation number of phosphorus in this ion is +5. -
Significance
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Even though oxidation numbers are assigned to elements in a compound as if they had charges, the oxidation number doesn't actually mean the compound is made up of ions; the oxidation number is more like an apparent charge that helps determine which atoms in a reaction gain or lose electrons during reactions. If the oxidation number of an element decreases during a reaction, the element is said to be reduced. If, on the other hand, the oxidation number increases or becomes more positive, the element is oxidized. A reaction involving oxidation/reduction is a redox reaction.
Acids
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There are several different ways to define an acid. Some definitions are more inclusive than others. The most common definition classifies any substance that donates hydrogen ions as acids. Hydrochloric acid, for example, has the chemical formula HCl. In water it typically donates the hydrogen ion to a water molecule to form H3O.
Acids in Redox Reactions
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Acids take part in a variety of important redox reactions. In many cases strong acids added to pure metal oxidize the metal. If sulfuric acid is added to iron, for example, the following reaction occurs:
H2SO4(aq) + Fe(s) ' H2(g) + FeSO4(aq)
The iron had an oxidation number of 0 before the reaction and -2 afterwards, so this is a redox reaction in which iron is oxidized by sulfuric acid.
Another example is the reaction between nitric acid and copper:
Cu(s) + 4HNO3(aq) ' Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)
In this case, the copper had an oxidation number of 0 before and +2 afterwards, so the copper was oxidized. The byproduct is nitrogen dioxide, a poisonous gas. Both nitric acid and sulfuric acid are strong oxidizing agents---one of the reasons to exercise precaution when handling them.
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