5.5: Oxidation and Reduction Reactions

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The oxidation number of an element represents the total number of electrons which have been removed (a positive oxidation state) or added (a negative oxidation state) to get the element into its present state. The term oxidation describes the loss of electrons by an element and an increase in oxidation state; the term reduction describes the gain of electrons and a decrease in oxidation state. Oxidation numbers for elements in compounds can be calculated using a simple set of rules, which are reproduced below in Table 5.3.

Table 5.3 Rules for Assigning Oxidation Numbers
Table 5.3 Rules for Assigning Oxidation Numbers
1. The oxidation number of an element in the free state is zero.
2. A monoatomic ion will have an oxidation number that is equal to its charge.
3. In compounds with metals, hydrogen will be –1, otherwise it will always be +1.
4. Oxygen, within a compound, will generally have an oxidation number of –2.
5. Halogens will be –1, except in compounds with oxygen.
6. Sulfur will generally be –2, except in compounds with oxygen.
7. In a molecular compound, the most electronegative element is assigned a negative oxidation number

In many chemical reactions, the oxidation number of elements change. Consider the synthesis reaction shown below. In the reactants, carbon and oxygen are both elements and their oxidation numbers are zero (Rule 1). In the product, oxygen will have an oxidation number of –2 (Rule 4), therefore, carbon in CO₂ must have an oxidation number of +4 in order to balance the four negative charges on the oxygens. During this reaction, the oxidation number of carbon has changed from zero in the reactants to +4 in the products and the oxidation number of oxygen has changed from zero to –2. This is an example of a redox reaction; a chemical reaction in which the oxidation numbers of elements change on going from reactants to products.

C (s) + O₂ (g) → CO₂ (g)

In a redox reaction, the element that “loses electrons” is said to be oxidized and will have an increase in its oxidation number. In the example above, the oxidation number of carbon increases from zero to +4; it has “lost electrons” and has been oxidized. The element that “gains electrons” in a redox reaction is said to be reduced and will have a decrease in its oxidation number. In the reaction above, the oxidation number of oxygen has decreased from zero to –2; it has “gained electrons” and has been reduced.

Exercise \(\PageIndex{1}\)

Arsonic and nitric acids react to form nitrogen monoxide, arsenic acid and water according to the equation shown below. Is this an example of a redox reaction?
2 HNO₃ (aq) + 3 H₃AsO₃ (aq) → 2 NO (g) + 3 H₃AsO₄ (aq) + H₂O (l)

Exercise \(\PageIndex{1}\)

For each of the reactions given below, calculate the oxidation number of each of the elements in the reactants and the products and determine if the reaction involves oxidation-reduction. If it is a redox reaction, identify the elements that have been oxidized and reduced.

Cu₂S → 2 Cu + S
Reactants: Cu _____ S _____
Products: Cu _____ S _____
Element oxidized: __________ Element Reduced __________
CaCO₃ → CaO + CO₂
Reactants: Ca _____ C _____ O _____
Products: Ca _____ C _____ O _____
Element oxidized: __________ Element Reduced __________
Fe₂O₃ + 3 H₂ → 2 Fe + 3 H₂O
Reactants: Fe _____ O _____ H _____
Products: Fe _____ O _____ H _____
Element oxidized: __________ Element Reduced __________
AgNO₃ + NaCl → AgCl (s) + NaNO₃
Reactants: Ag _____ N _____ O _____ Na _____ Cl _____
Products: Ag _____ N _____ O _____ Na _____ Cl _____
Element oxidized: __________ Element Reduced __________