4.11
Oxidation Numbers
In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the oxid…
Redox reactions between metals and nonmetals typically involve a complete transfer of electrons to form ionic compounds; hence, they are easy to identify. However, redox reactions involving only nonmetals with a partial transfer of electrons are not as easily identifiable.
Redox reactions are characterized by changes in the oxidation states of the atoms, which indicates electron movement between the atoms.
The oxidation state, or oxidation number, of an atom in a compound is the charge it would have if the shared electrons in each heteronuclear bond were completely transferred to the more electronegative atom. Homonuclear bonds are divided equally.
For instance, in gaseous hydrogen chloride, chlorine is more electronegative. If hydrogen’s electron is transferred completely to chlorine, chlorine gets a 1− charge, corresponding to the −1 oxidation state, and hydrogen gets a 1+ charge, corresponding to the +1 oxidation state.
Oxidation states can be assigned to atoms in elemental form and in most ions and compounds using specific rules. The first three rules are always followed. The remaining rules are applied one by one until the first three rules are satisfied.
These rules will now be applied to identify whether the formation reactions of sulfur dioxide and calcium carbonate are redox reactions.
According to rule number 1, elements in the free state have an oxidation number of zero, so elemental sulfur and oxygen are both assigned the oxidation number zero.
According to rule number 3, the sum of the oxidation numbers in a neutral compound is zero so the oxidation numbers of sulfur and oxygen in SO2 must sum to zero.
In accordance with rule number 6, the oxidation number of each oxygen is −2 in SO2. Two oxygen atoms sum to −4. The oxidation number of sulfur is, therefore, +4.
The oxidation number of sulfur increases from zero to +4, so it is oxidized, while the oxidation number of oxygen decreases from zero to −2, so it is reduced. Thus, this is a redox reaction.
In the case of calcium carbonate, the oxidation number of oxygen is −2 in all three compounds, and calcium is +2 in calcium oxide and calcium carbonate. According to rule 3, carbon must be +4 in carbon dioxide and calcium carbonate.
Since there is no change in the oxidation numbers of the atoms during the reaction, this is not a redox reaction.
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Q1: What is an oxidation number and how does it relate to electron transfer?
An oxidation number is a hypothetical charge assigned to an atom in a compound that represents the effective charge if shared electrons in heteronuclear bonds were completely transferred to the more electronegative atom. Oxidation numbers track electron movement in oxidation reduction reactions electron transfer reactions, helping identify which atoms are oxidized or reduced during chemical reactions.
Q2: How do you assign oxidation numbers to elements in their free state?
According to the first rule for assigning oxidation numbers, all free elements have an oxidation number of zero, regardless of whether they are monoatomic, diatomic, or polyatomic. This applies to elemental forms like O₂, S₈, or individual atoms before they form compounds.
Q3: What are the key differences between oxidation numbers in ionic and covalent compounds?
In ionic compounds, oxidation numbers are assigned based on the number of electrons transferred between reacting species. In covalent compounds, electrons are shared rather than transferred, so oxidation numbers are assigned hypothetically by assuming complete transfer to the more electronegative atom. Both allow identification of redox reactions.
Q4: Why is oxygen typically assigned an oxidation number of −2?
Oxygen has an oxidation number of −2 in most compounds because it is highly electronegative and typically gains two electrons when bonding. The exception is in peroxides, where oxygen has an oxidation number of −1 due to the O-O bond structure.
Q5: How can you determine if a reaction is a redox reaction using oxidation numbers?
A reaction is a redox reaction if the oxidation numbers of atoms change during the reaction. If oxidation numbers remain unchanged, it is not a redox reaction. For example, in the formation of SO₂, sulfur's oxidation number increases from zero to +4 while oxygen decreases from zero to −2, indicating electron transfer and confirming it as a redox reaction.
Q6: What is the relationship between oxidation numbers and the charge of polyatomic ions?
For polyatomic ions, the sum of oxidation numbers equals the charge on the ion. For neutral compounds, the sum of oxidation numbers equals zero. This rule helps determine unknown oxidation numbers when other atoms' oxidation states are known in complex compounds.
Q7: How does electronegativity influence the assignment of oxidation numbers in covalent bonds?
In covalent bonds, the more electronegative atom is assigned the electrons in the bond when calculating oxidation numbers. For example, in hydrogen chloride, chlorine is more electronegative, so it receives hydrogen's electron, giving chlorine an oxidation number of −1 and hydrogen +1, even though the bond is covalent.