23 4.4 Oxidation-Reduction Reactions

Redox Reactions

Earth’s atmosphere contains about 20% molecular oxygen, O₂, a chemically reactive gas that plays an essential role in the metabolism of aerobic organisms and in many environmental processes that shape the world. The term oxidation was originally used to describe chemical reactions involving O₂, but its meaning has evolved to refer to a broad and important reaction class known as oxidation-reduction (redox) reactions.

The most easily identified redox reactions involve the exchange of electrons between reactants. A reactant that loses, or gives up electrons during such a reaction is said to be oxidized as it is converted to product. A reactant that gains, or accepts electrons during the reaction is said to be reduced during the reaction.

What strange, counter-intuitive language this is! It may be helpful to think about the charge on a species (atom, ion or formula) being reduced during the reaction, as electrons with their negative charges are accepted.

Some redox reactions involve the transfer of electrons between reactant species to yield ionic products, such as the reaction between sodium and chlorine to yield sodium chloride:

What happens in such a reaction can be made more clear by considering each individual reactant in the form of an equation called a half-reaction:

These equations show that Na atoms lose electrons while Cl atoms (in the Cl₂ molecule) gain electrons, the “s” subscripts for the resulting ions signifying they are present in the form of a solid ionic compound. For redox reactions of this sort, the loss and gain of electrons define the complementary processes that occur:

In this reaction, then, sodium is oxidized and chlorine undergoes reduction.

Some redox processes, however, do not involve the actual transfer of electrons. Consider, for example, a reaction similar to the one yielding NaCl:

1. The oxidation number of an atom in an elemental substance is zero.
2. The oxidation number of a monatomic ion is equal to the ion’s charge.
3. Oxidation numbers for common non-metals are usually assigned as follows:
   • Hydrogen: +1 when combined with nonmetals, −1 when combined with metals
   • Oxygen: −2 in most compounds, sometimes −1 (so-called peroxides, O₂²⁻), very rarely [latex]-\frac{1}{2}[/latex] (so-called superoxides, O₂⁻), positive values when combined with F (values vary)
   • Halogens: −1 for F always, −1 for other halogens except when combined with oxygen or other halogens (positive oxidation numbers in these cases, varying values)
4. The sum of oxidation numbers for all atoms in a molecule or polyatomic ion equals the charge on the molecule or ion.

Using the oxidation number concept, an all-inclusive definition of redox reaction has been established. Oxidation-reduction (redox) reactions are those in which one or more elements involved undergo a change in oxidation number.

Returning to the reactions used to introduce this topic, they may now both be identified as redox processes. In the reaction between sodium and chlorine to yield sodium chloride, sodium is oxidized (its oxidation number increases from 0 in Na to +1 in NaCl) and chlorine is reduced (its oxidation number decreases from 0 in Cl₂ to −1 in NaCl). In the reaction between molecular hydrogen and chlorine, hydrogen is oxidized (its oxidation number increases from 0 in H₂ to +1 in HCl) and chlorine is reduced (its oxidation number decreases from 0 in Cl₂ to −1 in HCl).

Some Important Types of Redox Reactions

1 – A composition reaction (sometimes also called a combination reaction or a synthesis reaction) produces a single substance from multiple reactants. A single substance as a product is the key characteristic of the composition reaction. There may be a coefficient other than one for the substance, but if the reaction has only a single substance as a product, it can be called a composition reaction. In the reaction

2 H₂(g) + O₂(g) [latex]\longrightarrow[/latex] 2 H₂O(ℓ)

water is produced from hydrogen and oxygen. Although there are two molecules of water being produced, there is only one substance—water—as a product. So this is a composition reaction.

2 – A decomposition reaction starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),

2 NaHCO₃(s) [latex]\longrightarrow[/latex] Na₂CO₃(s) + CO₂(g) + H₂O(ℓ)

sodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.

Composition and decomposition reactions are difficult to predict; however, they should be easy to recognize.

3 – Combustion reactions in which the reductant, also called a fuel, and oxidant, molecular oxygen, react vigorously and produce significant amounts of heat, and often light, in the form of a flame.  Combustion reactions produce oxides of all other elements as products; any nitrogen in the reactant is converted to elemental nitrogen, N₂. Many reactants, called fuels, contain mostly carbon and hydrogen atoms, reacting with oxygen to produce CO₂ and H₂O. For example, the balanced chemical equation for the combustion of methane, CH₄, is as follows:

CH₄ + 2 O₂ [latex]\longrightarrow[/latex] CO₂ + 2 H₂O

Kerosene can be approximated with the formula C₁₂H₂₆, and its combustion equation is

2 C₁₂H₂₆ + 37 O₂ [latex]\longrightarrow[/latex] 24 CO₂ + 26 H₂O

Sometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C₂H₅OH, whose combustion equation is

C₂H₅OH + 3 O₂ [latex]\longrightarrow[/latex] 2 CO₂ + 3 H₂O

If nitrogen is present in the original fuel, it is converted to N₂, not to a nitrogen-oxygen compound. Thus, for the combustion of the fuel dinitroethylene, whose formula is C₂H₂N₂O₄, we have

2 C₂H₂N₂O₄ + O₂ [latex]\longrightarrow[/latex] 4 CO₂ + 2 H₂O + 2 N₂

Example 4

Complete and balance each combustion equation.

a) the combustion of propane, C₃H₈

b) the combustion of ammonia, NH₃

 

 

Solution

a) The products of the reaction are CO₂ and H₂O, so our unbalanced equation is

C₃H₈ + O₂ [latex]\longrightarrow[/latex] CO₂ + H₂O

Balancing (and you may have to go back and forth a few times to balance this), we get

C₃H₈ + 5 O₂ [latex]\longrightarrow[/latex] 3 CO₂ + 4 H₂O

b) The nitrogen atoms in ammonia will react to make N₂, while the hydrogen atoms will react with O₂ to make H₂O:

NH₃ + O₂ [latex]\longrightarrow[/latex] N₂ + H₂O

To balance this equation without fractions (which is the convention), we get

4 NH₃ + 3 O₂ [latex]\longrightarrow[/latex] 2 N₂ + 6 H₂O

 

Test Yourself

Complete and balance the combustion equation for cyclopropanol, C₃H₆O.

 

Answer

C₃H₆O + 4 O₂ [latex]\longrightarrow[/latex] 3 CO₂ + 3 H₂O

4 – Single-displacement (replacement) reactions are redox reactions in which an ion in solution is displaced (or replaced) via the oxidation of a metallic element. One common example of this type of reaction is the acid oxidation of certain metals:

Metallic elements may also be oxidized by solutions of other metal salts; for example:

This reaction may be observed by placing copper wire in a solution containing a dissolved silver salt. Silver ions in solution are reduced to elemental silver at the surface of the copper wire, and the resulting Cu²⁺ ions dissolve in the solution to yield a characteristic blue color (Figure 2).