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9.2 Oxidation Numbers 377
Phosphates, like ammonium phosphate, are important components of fertilizers used to stimulate the growth of agricultural crops and to make our gardens green. Their commercial synthesis requires elemental phosphorus, which can be acquired by heating phosphate rock (containing calcium phosphate) with sand (containing silicon dioxide) and coke (a carbon‑rich mixture produced by heating coal). This method for isolating phosphorus, called the furnace process , is summarized in the first equation below. The other equations show how phosphorus can be converted into ammonium phosphate.
2Ca3(PO4) 2 + 6SiO 2 + 10C → P 4 + 10CO + 6CaSiO 3 P4 + 5O 2 + 6H2O → 4H3PO 4 H3PO 4 + NH 3 → (NH4)3PO 4
Are these reactions oxidation‑reduction reactions? Are electrons transferred? Simply reading a chemical equation does not always tell us whether oxidation and reduction have occurred, so chemists have developed a numerical system to help identify a reaction as redox. For redox reactions, this system also shows us which element is oxidized, which is reduced, what the oxidizing agent is, and what the reducing agent is. The first step in this system is to assign an oxidation number to each atom in the reaction equation. As you become better acquainted with the procedure, you will gain a better understanding of what the numbers signify, but for now, just think of them as tools for keeping track of the flow of electrons in redox reactions. Oxidation numbers are also called oxidation states. If any element undergoes a change of oxidation number in the course of a reaction, the reaction is a redox reaction. If an element’s oxidation number increases in a reaction, that element is oxidized. If an element’s oxidation number decreases in a reaction, that element is reduced. The reactant containing the element that is oxidized is the reducing agent. The reactant containing the element that is reduced is the oxidizing agent (Table 9.1).
Table 9. Questions Answered by the Determination of Oxidation Numbers
Question Answer Is the reaction redox? If any atoms change their oxidation number, the reaction is redox. Which element is oxidized? The element that increases its oxidation number is oxidized. Which element is reduced? The element that decreases its oxidation number is reduced. What’s the reducing agent? The reactant that contains the element that is oxidized is the reducing agent. What’s the oxidizing agent? The reactant that contains the element that is reduced is the oxidizing agent.
9.2 Oxidation Numbers
ObjeCtive 4
ObjeCtive 6
ObjeCtive 5
ObjeCtive 4 ObjeCtive 5 ObjeCtive 6
Study Sheet 9.1 on the next page describes how you can assign oxidation numbers to individual atoms.
378 Chapter 9 Oxidation-Reduction Reactions
Tip-off You are asked to determine the oxidation number of an atom, or you need to assign oxidation numbers to atoms to determine whether a reaction is a redox reaction, and if it is, to identify which element is oxidized, which is reduced, what the oxidizing agent is, and what the reducing agent is. General STepS Use the following guidelines to assign oxidation numbers to as many atoms as you can. (Table 9.2 provides a summary of these guidelines with examples.) The oxidation number for each atom in a pure element is zero. The oxidation number of a monatomic ion is equal to its charge. When fluorine atoms are combined with atoms of other elements, their oxidation number is ‒1. When oxygen atoms are combined with atoms of other elements, their oxidation number is ‒2, except in peroxides, such as hydrogen peroxide, H2O2, where their oxidation number is ‒1. ( There are other exceptions that you will not see in this text .) The oxidation number for each hydrogen atom in a molecular compound or a polyatomic ion is +1. If a compound’s formula contains one element for which you cannot assign an oxidation number using the guidelines listed above, calculate the oxidation number according to the following rules. The sum of the oxidation numbers for the atoms in an uncharged formula is equal to zero. The sum of the oxidation numbers for the atoms in a polyatomic ion is equal to the overall charge on the ion. example See Example 9.1.
ObjeCtive 3
Sample Study
Sheet 9.
Assignment
of Oxidation
Numbers
Table 9. Guidelines for Assigning Oxidation Numbers
Oxidation number
Examples Exceptions
Pure element 0 The oxidation number for each atom in Zn, H2, and S 8 is zero.
none
Monatomic ions charge on ion
Cd in CdCl 2 is +2. Cl in CdCl 2 is ‒1. H in LiH is ‒1.
none
Fluorine in the combined form
‒1 F in AlF 3 is ‒1. F in CF 4 is ‒1.
none
Oxygen in the combined form
‒2 O in ZnO is ‒2. O in H2O is ‒2.
O is ‒1 in peroxides, such as H2O Hydrogen in the combined form
- 1 H in H2O is +1. H is ‒1 when combined with a metal.
380 Chapter 9 Oxidation-Reduction Reactions
The oxidation number for the phosphorus atom in PO 4 3‒^ is always the same, no matter what the cation is that balances its charge. Thus we could also have determined the oxidation number of each phosphorus atom by considering the phosphate ion separately from the calcium ion. (ox # P) + 4(ox # O) = −
(ox # P) + 4( −2) = −
(ox # P) = + 5 The silicon atoms in CaSiO 3 must have an oxidation number of +4. (ox # Ca) + (ox # Si) + 3(ox # O) = 0 (+2) + (ox # Si) + 3(−2) = 0 (ox # Si) = + 4 The oxidation numbers for the individual atoms in the first reaction are below.
Oxidation number increases, oxidized
Oxidation number decreases, reduced
- 5 − 2 + 2 + 4 − 2 2Ca3(PO4) 2 + 6SiO 2 + 10C P 4 + 10CO + 6CaSiO 3
Phosphorus atoms and carbon atoms change their oxidation numbers, so the reaction is redox. Each phosphorus atom changes its oxidation number from +5 to zero, so the phosphorus atoms in Ca3(PO 4 ) 2 are reduced, and Ca 3 (PO 4 ) 2 is the oxidizing agent. Each carbon atom changes its oxidation number from zero to +2, so the carbon atoms are oxidized, and carbon is the reducing agent. b. Now, let’s consider the second reaction.
P 4 + 5O2 + 6H2O 4H3PO 4
Hydrogen in a molecular compound, oxidation number + 1
Pure elements, oxidation number 0
Combined oxygen, oxidation number − 2
The following shows how we can determine the oxidation number of the phosphorus atom in H3PO4: 3(ox # H) + (ox # P) + 4(ox # O) = 0 3(+1) + (ox # P) + 4(−2) = 0 (ox # P) = + 5
9.2 Oxidation Numbers 381
The oxidation numbers for the individual atoms in the second reaction are below. Oxidation number increases, oxidized
0 0 + 1 −^2 + 1 + 5 − 2
Oxidation number decreases, reduced
P 4 + 5O 2 + 6H2O 4H3PO 4
Phosphorus atoms and oxygen atoms change their oxidation numbers, so the reaction is redox. Each phosphorus atom changes its oxidation number from zero to +5, so the phosphorus atoms in P 4 are oxidized, and P 4 is the reducing agent. Each oxygen atom in O 2 changes its oxidation number from zero to −2, so the oxygen atoms in O 2 are reduced, and O 2 is the oxidizing agent. c. Finally, let’s consider the third reaction.
H3PO 4 + NH 3 (NH4)3PO 4
Combined oxygen, oxidation number − 2
Hydrogen in a molecule, oxidation number + 1
Hydrogen in a polyatomic ion, oxidation number + 1
We know from part (b) that the oxidation number of the phosphorus atoms in H3PO 4 is +5. The oxidation number of the nitrogen atom in NH 3 is calculated below. (ox # N) + 3(ox # H) = 0 (ox # N) + 3(+1) = 0 (ox # N) = − We can determine the oxidation number of each nitrogen atom in (NH (^) 4)3PO 4 in two ways, either from the whole formula or from the formula for the ammonium ion alone. 3(ox # N) + 12(ox # H) + (ox # P) + 4(ox # O) = 0 3(ox # N) + 12(+1) + (+5) + 4(−2) = 0 (ox # N) = − or (ox # N) + 4(ox # H) = + 1 (ox # N) + 4(+1) = + 1 (ox # N) = − The oxidation numbers for the individual atoms in this reaction are below.
- 1 + 5 − 2 − 3 + 1 − 3 + 1 + 5 − 2 H3PO 4 + NH 3 (NH4)3PO 4
None of the atoms change their oxidation number, so the reaction is not redox.
