5.5 Oxidation-Reduction (Redox) Reactions

Learning Objective

  1. Identify a chemical reaction as an oxidation-reduction reaction.
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Figure 5.4 Zinc Metal plus Hydrochloric Acid.

 

It is fairly obvious that zinc metal reacts with aqueous hydrochloric acid! The bubbles are hydrogen gas. Source: Photo courtesy of Chemicalinterest,http://commons.wikimedia.org/wiki/File:Zn_reaction_with_HCl.JPG.

When zinc metal is submerged into a quantity of aqueous HCl, the following reaction occurs (Figure 5.4 “Zinc Metal plus Hydrochloric Acid”):

Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq)

This is one example of what is sometimes called a single replacement reaction because Zn replaces H in combination with Cl.  In a single replacement reaction, a more active metal replaces a less active metal in a compound.

Because some of the substances in this reaction are aqueous, we can separate them into ions – when ionic compounds dissolve in water, they separate into their component ions:

Zn(s) + 2H+(aq) + 2Cl−(aq) → H2(g) + Zn2+(aq) + 2Cl−(aq)

Viewed this way, the net reaction seems to be a charge transfer between zinc and hydrogen atoms. (There is no net change experienced by the chloride ion.) In fact, electrons are being transferred from the zinc atoms to the hydrogen atoms (which ultimately make a molecule of diatomic hydrogen), changing the charges on both elements.

To understand electron-transfer reactions like the one between zinc metal and hydrogen ions, chemists separate them into two parts: one part focuses on the loss of electrons, and one part focuses on the gain of electrons. The loss of electrons is called oxidation. The gain of electrons is called reduction.  Loss of electrons by one substance must be accompanied by a gain in electrons by another substance, so oxidation and reduction always occur together. As such, electron-transfer reactions are also called oxidation-reduction reactions or simply redox reactions. The atom that loses electrons is oxidized, and the atom that gains electrons is reduced. Also, because we can think of the species being oxidized as causing the reduction, the species being oxidized is called the reducing agent, and the species being reduced is called the oxidizing agent.

History Note

The terms oxidation and reduction can be traced to the fact that, when ores are processed to obtain metals, the mass of the metal is always less than the mass of the original ore as impurities are removed by physical and chemical means.  Thus the pure metal was a reduction of the ore.  If the pure metal tarnished or rusted, often by reacting with oxygen, the mass went up, and the process was called oxidation.  Later, the science of chemistry caught up, showing that reduction of ores involved gain of electrons by metal ions in the ores, producing the pure metal atoms.  And in oxidation of pure metals, metal atoms lost electrons and became ions in the tarnish or rust compounds.

Although the two reactions occur together, it can be helpful to write the oxidation and reduction reactions separately as half reaction. In half reactions, we include only the reactant being oxidized or reduced, the corresponding product species, any other species needed to balance the half reaction, and the electrons being transferred. Electrons that are lost are written as products; electrons that are gained are written as reactants. For example, in our earlier equation, now written without the chloride ions,

Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)

zinc atoms are oxidized to Zn2+. The half reaction for the oxidation reaction, omitting phase labels, is as follows:

Zn → Zn2+ + 2e−

This half reaction is balanced in terms of the number of zinc atoms, and it also shows the two electrons that are needed as products to account for the zinc atom losing two negative charges to become a 2+ ion. With half reactions, there is one more item to balance: the overall charge on each side of the reaction. If you check each side of this reaction, you will note that both sides have a zero net charge.

Hydrogen is reduced in the reaction. The balanced reduction half reaction is as follows:

2H+ + 2e− → H2

There are two hydrogen atoms on each side, and the two electrons written as reactants serve to neutralize the 2+ charge on the reactant hydrogen ions. Again, the overall charge on both sides is zero.

The overall reaction is simply the combination of the two half reactions and is shown by adding them together.

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Because we have two electrons on each side of the equation, they can be canceled. This is the key criterion for a balanced redox reaction: the electrons have to cancel exactly. If we check the charge on both sides of the equation, we see they are the same—2+. (In reality, this positive charge is balanced by the negative charges of the chloride ions, which are not included in this reaction because chlorine does not participate in the charge transfer.)

Redox reactions are often balanced by balancing each individual half reaction and then combining the two balanced half reactions. Sometimes a half reaction must have all of its coefficients multiplied by some integer for all the electrons to cancel. The following example demonstrates this process.

Example 5

Write and balance the redox reaction that has silver ions and aluminum metal as reactants and silver metal and aluminum ions as products.

Solution

Skill-Building Exercise

  1. Write and balance the redox reaction that has calcium ions and potassium metal as reactants and calcium metal and potassium ions as products.

Note

Potassium has been used as a reducing agent to obtain various metals in their elemental form.

To Your Health: Redox Reactions and Pacemaker Batteries

All batteries use redox reactions to supply electricity because electricity is basically a stream of electrons being transferred from one substance to another. Pacemakers—surgically implanted devices for regulating a person’s heartbeat—are powered by tiny batteries, so the proper operation of a pacemaker depends on a redox reaction.

Pacemakers used to be powered by NiCad batteries, in which nickel and cadmium (hence the name of the battery) react with water according to this redox reaction:

Cd(s) + 2NiOOH(s) + 2H2O(ℓ) → Cd(OH)2(s) + 2Ni(OH)2(s)

The cadmium is oxidized, while the nickel atoms in NiOOH are reduced. Except for the water, all the substances in this reaction are solids, allowing NiCad batteries to be recharged hundreds of times before they stop operating. Unfortunately, NiCad batteries are fairly heavy batteries to be carrying around in a pacemaker. Today, the lighter lithium/iodine battery is used instead. The iodine is dissolved in a solid polymer support, and the overall redox reaction is as follows:

2Li(s) + I2(s) → 2LiI(s)

Lithium is oxidized, and iodine is reduced. Although the lithium/iodine battery cannot be recharged, one of its advantages is that it lasts up to 10 years. Thus, a person with a pacemaker does not have to worry about periodic recharging; about once per decade a person requires minor surgery to replace the pacemaker/battery unit. Lithium/iodine batteries are also used to power calculators and watches.

Oxidation and reduction can also be defined in terms of changes in composition. The original meaning of oxidation was “adding oxygen,” so when oxygen is added to a molecule, the molecule is being oxidized. The reverse is true for reduction: if a molecule loses oxygen atoms, the molecule is being reduced. For example, the acetaldehyde (CH3CHO) molecule takes on an oxygen atom to become acetic acid (CH3COOH).

2CH3CHO + O2 → 2CH3COOH

Thus, acetaldehyde is being oxidized.

Similarly, oxidation and reduction can be defined in terms of the gain or loss of hydrogen atoms. If a molecule adds hydrogen atoms, it is being reduced. If a molecule loses hydrogen atoms, the molecule is being oxidized. For example, in the conversion of acetaldehyde into ethanol (CH3CH2OH), hydrogen atoms are added to acetaldehyde, so the acetaldehyde is being reduced:

CH3CHO + H2 → CH3CH2OH

For organic molecules, it is much easier to recognize oxidation as an increase in the number of bonds to oxygen atoms and/or decrease in the number of bonds to hydrogen atoms within a molecule than to see electron loss.  Conversely, it is easier to recognize reduction as a decrease in the number of bonds to oxygen atoms and/or an increase in the number of bonds to hydrogen atoms within a molecule than to see electron gain.

Example 6

In each conversion, indicate whether oxidation or reduction is occurring based on addition or loss of oxygen or hydrogen.

  1. N2 → NH3
  2. CH3CH2OHCH3 → CH3COCH3
  3. HCHO → HCOOH

Solution

  1. Hydrogen is being added to the original reactant molecule, so reduction is occurring.
  2. Hydrogen is being removed from the original reactant molecule, so oxidation is occurring.
  3. Oxygen is being added to the original reactant molecule, so oxidation is occurring.

Skill-Building Exercise

In each conversion, indicate whether oxidation or reduction is occurring.

  1. CH4 → CO2 + H2O

  2. NO2 → N2

  3. CH2=CH2 → CH3CH3

 

Concept Review Exercises

  1. Give two different definitions for oxidation and reduction.

  2. Give an example of each definition of oxidation and reduction.

Answers

Key Takeaways

  • Chemical reactions in which electrons are transferred are called oxidation-reduction, or redox, reactions.
  • Oxidation is the loss of electrons.
  • Reduction is the gain of electrons.
  • Oxidation and reduction always occur together, even though they can be written as separate chemical equations.

Exercises

  1. Which reactions are redox reactions? Explain.

    1. NaOH + HCl → H2O + NaCl
    2. 3Mg + 2AlCl3 → 2Al + 3MgCl2
    3. H2O2 + H2 → 2H2O
    4. KCl + AgNO3 → AgCl + KNO3

  2. Which reactions are redox reactions? Explain.

    1. 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O
    2. 2C2H6 + 7O2 → 4CO2 + 6H2O
    3. 2NaHCO3 → Na2CO3 + CO2 + H2O
    4. 2K + 2H2O → 2KOH + H2

  3. Balance each redox reaction by writing appropriate half reactions and combining them to cancel the electrons.

    1. Ca(s) + H+(aq) → Ca2+(aq) + H2(g)
    2. I−(aq) + Br2(ℓ) → Br−(aq) + I2(s)

  4. Balance each redox reaction by writing appropriate half reactions and combining them to cancel the electrons.

    1. Fe(s) + Sn4+(aq) → Fe3+(aq) + Sn2+(aq)
    2. Pb(s) + Pb4+(aq) → Pb2+(aq) (Hint: both half reactions will start with the same reactant.)

Answers