5.5 Oxidation-Reduction (Redox) Reactions
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.
Because some of the substances in this reaction are aqueous, we can separate them into 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 oxidationThe loss of electrons by an atom.. The gain of electrons is called reductionThe gain of electrons by an atom.. Because any loss of electrons by one substance must be accompanied by a gain in electrons by something else, oxidation and reduction always occur together. As such, electron-transfer reactions are also called oxidation-reduction reactionsA chemical reaction in which electrons are transferred from one atom to another. Also called a redox reaction., 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 agentA species that causes reduction, which is itself oxidized., and the species being reduced is called the oxidizing agentA species that causes oxidation, which is itself reduced..
Although the two reactions occur together, it can be helpful to write the oxidation and reduction reactions separately as half reactionsA chemical reaction that shows only oxidation or reduction.. 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.
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.
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