4.10
Oxidation-Reduction Reactions
Oxidation–Reduction Reactions
Earth’s atmosphere contains about 20% molecular oxygen, O2, a chemically reactive gas that plays an essential role in th…
Certain processes that are vital to life, including photosynthesis, combustion, and corrosion, fall into the class of reactions called oxidation–reduction, or redox, reactions.
Redox reactions consist of two simultaneous processes: oxidation and reduction.
The term oxidation means an increase in oxidation number, which corresponds to the loss of electrons, while reduction means a decrease in oxidation number, which corresponds to the gain of electrons. To remember the role of electrons, use the acronym OIL RIG, which stands for: “oxidation is losing, reduction is gaining.”
Oxidation and reduction are complementary processes. In a redox reaction between two reactants, one reactant loses electrons and is oxidized, while the other reactant gains electrons and is reduced.
Consider the oxidation–reduction reaction between potassium — an alkali metal — and chlorine, a nonmetal.
The neutral potassium atom loses an electron to become a potassium ion. Potassium is oxidized, and its charge increases from zero in the neutral atom to one-plus in the cation.
The neutral chlorine atom gains an electron and becomes a chloride ion. Chlorine is reduced, and its charge decreases from zero in the neutral atom to one-minus in the anion.
Since potassium donates an electron, it is the reducing agent, or reductant. Chlorine accepts the electron, so it is the oxidizing agent, or an oxidant. The redox process leads to the formation of potassium chloride.
In general, in redox reactions between alkali or alkaline earth metals and nonmetals, the metal is oxidized and the nonmetal is reduced to form an ionic compound through complete electron transfer. This is often true for reactions between other metals or metalloids and nonmetals as well, but not always.
Another example of a redox process is the formation of gaseous hydrogen chloride. Here, both reactants — hydrogen and chlorine — are nonmetals, so there is no complete transfer of electrons. Instead, hydrogen shares an electron with chlorine in a partial, or formal, electron transfer.
Thus, in the formation of hydrogen chloride, hydrogen is oxidized and acquires a partial positive charge, while chlorine is reduced and acquires a partial negative charge. Since both oxidation and reduction processes occur, this is a redox reaction.
In general, redox reactions between nonmetals involve partial electron transfer between elements to form a covalent compound.
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Q1: What is the difference between oxidation and reduction in redox reactions?
Oxidation is an increase in oxidation number corresponding to electron loss, while reduction is a decrease in oxidation number corresponding to electron gain. These complementary processes occur simultaneously in redox reactions. The acronym OIL RIG helps remember this: oxidation is losing, reduction is gaining electrons.
Q2: What are reducing and oxidizing agents in a redox reaction?
A reducing agent is the species that is oxidized and loses electrons, while an oxidizing agent is the species that is reduced and gains electrons. In the reaction between potassium and chlorine, potassium acts as the reducing agent by donating an electron, and chlorine acts as the oxidizing agent by accepting it.
Q3: How do redox reactions between metals and nonmetals differ from those between two nonmetals?
Redox reactions between alkali or alkaline earth metals and nonmetals involve complete electron transfer, forming ionic compounds. In contrast, redox reactions between two nonmetals involve partial electron transfer, forming covalent compounds. For example, hydrogen and chlorine share electrons partially to form hydrogen chloride gas.
Q4: Why is oxidation number important in identifying redox reactions?
Oxidation number clarifies which species are oxidized and reduced, even in reactions not involving explicit electron transfer. This property permits an unambiguous definition of redox reactions across different types of chemical processes, including those forming covalent compounds where electron transfer is partial rather than complete.
Q5: What happens to the charge of a reducing agent during a redox reaction?
The charge of a reducing agent becomes more positive during a redox reaction because it loses electrons. For instance, when potassium loses an electron, its charge increases from zero in the neutral atom to one-plus in the potassium cation, making it the reducing agent in the reaction with chlorine.
Q6: How can you identify which reactant is oxidized and which is reduced?
Track the change in oxidation number for each reactant. The reactant whose oxidation number increases is oxidized and loses electrons; the reactant whose oxidation number decreases is reduced and gains electrons. Using half-reactions helps visualize the fate of each reactant separately in the overall redox process.
Q7: Why are redox reactions important in biological and environmental processes?
Redox reactions are vital to life processes including photosynthesis and cellular respiration, and to environmental processes like corrosion. Molecular oxygen in Earth's atmosphere plays an essential role in the metabolism of aerobic organisms and participates in many redox reactions that shape the world.