1. Redox: the two inseparable processes of reduction and oxidation
We give ship names to couples we adore, like Brangelina for Brad Pitt and Angelina Jolie, and Songsong for Song Joong-Ki and Song Hye-Kyo. In Chemistry, we too have a ship name: redox for reduction and oxidation. It describes the inseparable, intertwined relationship between these two chemical processes. While couples may split, reduction and oxidation always happen together.
We classify a reaction as a redox reaction when it involves a:
- Transfer of oxygen from one substance to another, or
- Transfer of hydrogen from one substance to another, or
- Transfer of electrons from one substance to another, or
- Change in the oxidation states of the elements
2. Reading from chemical equation: which reactant is oxidised and which is reduced?
Gain and Loss of Oxygen
The first way to think about a redox reaction is to follow the transfer of oxygen from one reactant to another.
Here, the reactant carbon gains oxygen to form carbon dioxide. So we say that carbon is oxidised. As this oxygen comes from the other reactant copper(II) oxide, copper(II) oxide loses oxygen and is hence reduced.
In other words, carbon is the reducing agent that reduces copper(II) oxide to copper metal.
Gain and Loss of Hydrogen
Another way to analyse reactants is to trace the journey of hydrogen by looking at the chemical equation.
In the formation of ammonia, hydrogen “moves” from hydrogen molecule to nitrogen molecule. As nitrogen molecule gains hydrogen here, we say that it is reduced. On the other hand, the other reactant hydrogen molecule is oxidised.
| Substance Oxidised (Reducing Agent) |
Substance Reduced (Oxidising Agent) |
|---|---|
| Gain oxygen | Lose oxygen |
| Lose hydrogen | Gain oxygen |
3. Redox reaction as the transfer of electrons
In the equation above, we can see that sodium metal is oxidised as it gains oxygen to form sodium oxide. The contrary is true for oxygen molecules: oxygen molecules are reduced.
However, there is something more profound going on during the reaction. Underlying the transfer of oxygen is the gain and loss of electrons. For sodium metal to become sodium ions in sodium oxide, it must give its valence electron to oxygen. This means that the oxidation of sodium is associated with the loss of electrons.
Meanwhile, the oxygen atoms in oxygen molecules receive the electrons to become oxide ions. Thus, we can think about the reduction of oxygen as the gain of electrons.
| Substance Oxidised (Reducing Agent) |
Substance Reduced (Oxidising Agent) |
|---|---|
| Lose electrons | Gain electrons |
4. Redox reactions that do not involve oxygen (or hydrogen)
The expanded definition of redox as electron transfer has an important consequence. We can now call many reactions that do not involve oxygen (or hydrogen) redox, so long as they involve the transfer of electrons.
5. Half equations focus on either the gain or loss of electrons during a redox reaction
We can divide the chemical equation showing the redox reaction between sodium and oxygen into two half equations.
Oxidation Half Equation
[O]: 2I– ⟶ I2 + 2e–
The oxidation half-equation shows the loss of electrons. When iodide is oxidised, it loses the electron it previously gained to become neutral iodine molecule. We show the electron that is lost as the product.
Reduction Half Equation
[R]: Cl2 + 2e– ⟶ 2Cl–
Conversely, we highlight the gain of electrons in the reduction half-equation. Here, electrons are the reactants. They combine with neutral chlorine molecule to form negatively charged chloride ions.
6. Oxidation state: tracking the gain and loss of electrons
To describe how oxidised an element in a substance is, chemists use oxidation state. It tells us the number of electrons an element has gained or lost to have formed the substance.
The reference point is an atom in its elemental form, like Na, O2 and Br2. They are like the default state, with an oxidation state of zero.
Oxidation State of Ions in Ionic Compounds
When an atom in its elemental form has gained or lost electrons to form ions, the oxidation state is equivalent to the charge. For example, as copper(II) ions has a charge of 2+, the element copper in copper(II) oxide has an oxidation state of +2.
Unlike how we write the charge of an ion, however, we write the plus or minus sign before the number for oxidation states.
Oxidation State of Atoms in Covalent Compounds
While atoms in covalent compounds like hydrogen chloride do not have complete charges, their oxidation state is not zero. This is because the shared pair of electrons is unevenly distributed. They are more strongly attracted to chlorine atom.
Therefore, we can approximate the uneven distribution of electrons towards chlorine atom as a “gain” of electron, and so we give it an oxidation state of -1. On the other hand, we shall pretend that hydrogen has “lost” its electron in this uneven distribution, and so it has an oxidation state of +1.
The same uneven distribution of electrons happens to all covalent compounds. The oxidation state of elements in covalent compounds are usually as follow:
| Element | Oxidation State in Substances |
|---|---|
| Hydrogen | +1 when bonded to non-metals, like H2O -1 when bonded to metals, like NaH |
| Oxygen | -2 in most compounds -1 in peroxides, like H2O2 |
| Halogen | -1 when bonded to metals, like HCl Variable when bonded to oxygen, like KClO |
For the other non-metallic elements, their oxidation state changes and depends on what substances they are found in. For example, the oxidation state of nitrogen is +4 in NO2, +2 in NO but -3 in NH3.
7. All redox reactions involve changes in oxidation state
As all redox reactions involve the transfer of electrons, this will be accompanied by changes in oxidation state. Monitoring these changes is the best way of identifying the reactants that are oxidised and reduced respectively.
Given that (1) oxidation is the loss of electrons, and (2) losing electrons increases the charge and hence oxidation state of an element, oxidation involves an increase in oxidation state. The opposite is true: reduction decreases the oxidation state of an element.
| Substance Oxidised (Reducing Agent) |
Substance Reduced (Oxidising Agent) |
|---|---|
| Increase in oxidation state | Decrease in oxidation state |
For example, in the above halogen displacement reaction, iodide is oxidised as the oxidation state of the element iodine increases from -1 in iodide to 0 in elemental iodine molecule. Conversely, chlorine is reduced as its oxidation state decreases from 0 in elemental chlorine molecule to -1 in chloride.
