Oxidation and Reduction

Oxidation and reduction are processes involving the transfer of electrons between substances. In a reaction, oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. These processes always occur simultaneously in what is called a redox reaction.

Oxidation and Reduction in Terms of Electrons

Oxidation: Loss of electrons.
Reduction: Gain of electrons. A simple mnemonic to remember this is:

OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Rules for Writing Oxidation and Reduction Equations

1. Identify the species oxidised and reduced.

Oxidation → element loses electrons → oxidation number increases.

Reduction → element gains electrons → oxidation number decreases.

2. Write half-equations for oxidation and reduction separately.

3. Balance each half-equation:

Balance atoms (except O and H).
Balance O with H₂O.
Balance H with H⁺ (in acidic solutions).
Balance charge by adding electrons.

4. Combine half-equations, making sure the electrons cancel out.

Examples of Oxidation and Reduction

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Zinc ( $Zn$ ) and Copper ( $Cu^{2+}$ ): Reaction: Zinc displaces copper from a copper sulphate solution.

Oxidation: Zinc loses two electrons to form $\text{Zn}^{2+}$

\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-

Reduction: Copper ions ( $Cu^{2+}$ ) gain two electrons to form copper metal.

\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}

Overall Reaction:

\text{Zn} + \text{CuSO}\_4 \rightarrow \text{ZnSO}\_4 + \text{Cu}

Zinc is oxidised, and copper ions are reduced.

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Example: Magnesium reacts with oxygen in the air forming magnesium oxides and loses its shine. The reaction of magnesium and oxygen is shown:

Mg+1/2O\_2\space🡪\space MgO

In this reaction the magnesium atom gives 2 electrons to an oxygen atom.

Therefore, magnesium is oxidised as it loses electrons and oxygen is reduced as it gains electrons.

The magnesium Atom becomes the magnesium ion ( $Mg^{2+}$ ), and the oxygen atom becomes the oxygen ion ( $O^{2-}$ ).

It is important to know that whenever oxidation occurs Reduction must also occur.

Reactions where both oxidation and reduction both occur are called redox reactions.

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Everyday example of oxidation and reduction Rusting of iron is an everyday example of oxidation and reduction.

The iron reacts with oxygen in the air according to the equation:

2Fe + 1½ O\_2 🡪 Fe\_2O\_3 (Iron (III) oxide)

The iron atoms (Fe) lost three electrons each to become $Fe^{3+}$ ions and are therefore oxidised.

The oxygen atoms (O) gain 2 electrons each and become $O^{2-}$ ions and are therefore reduced.

Oxidising and Reducing Agents

An oxidising agent causes oxidation by accepting electrons (itself reduced).

A reducing agent causes reduction by donating electrons (itself oxidised).

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Example In the reaction of sodium metal with chlorine, sodium chloride is formed

Na + ½ $Cl\_2$ 🡪 NaCl

The sodium Atom donates one electron to the chlorine Atom.

Therefore, sodium is oxidised as it loses an electron and chlorine is reduced as it gains an electron.

The sodium Atom is the reducing agent as it reduces chlorine.

Note that the reducing agent is itself oxidised.

The chlorine Atom is the oxidising agent and is itself reduced.

Everyday examples of oxidising and reducing agents

Bleach can act as either oxidising or reducing agents.

Household bleach contains sodium hypochlorite (NaOCl) which produces the ion OCl- which is an oxidising agent. Sodium hypo chloride is also used in the treatment of swimming pool water as it kills harmful microorganisms by oxidising them.
Bleach containing the reducing agent sulfur dioxide is used to bleach paper a yellow colour.

The Electrochemical Series

The electrochemical series is a list of metals arranged in order of their ability to be oxidised, which means their tendency to lose electrons and form positive ions. Metals that lose electrons more easily are higher in the series, while those that are less likely to lose electrons are lower down.

Metals at the top of the series are the most reactive and are easily oxidised, meaning they have a high tendency to lose electrons.
Metals at the bottom are the least reactive and resist oxidation, meaning they do not easily lose electrons.

Key Points about the Electrochemical Series:

Highly Reactive Metals: Metals like potassium ( $K$ ) and sodium ( $Na$ ) lose electrons easily and are at the top of the electrochemical series.
Less Reactive Metals: Metals like copper ( $Cu$ ) and silver ( $Ag$ ) are at the bottom of the series because they are less likely to lose electrons.
Hydrogen ( $H$ ) is included in the series even though it's not a metal. It is used as a reference point for the reactivity of metals.

Example of the Electrochemical Series (from most to least easily oxidised):

Potassium ( $K$ )
Calcium ( $Ca$ )
Sodium ( $Na$ )
Magnesium ( $Mg$ )
Aluminium ( $Al$ )
Zinc ( $Zn$ )
Iron ( $Fe$ )
Copper ( $Cu$ )
Silver ( $Ag$ )
Gold ( $Au$ )

Importance of the Electrochemical Series:

The position of a metal in the electrochemical series indicates how easily it can be oxidised.
Metals higher in the series can displace metals lower in the series from their compounds in solution (displacement reactions, though not required here).
Metals at the top, like potassium and sodium, are more reactive and readily lose electrons, while metals at the bottom, like gold and silver, are less reactive and resist oxidation.

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Summary

Oxidation is the loss of electrons in a reaction.
Reduction is the gain of electrons in a reaction.
Redox reactions involve both oxidation and reduction occurring simultaneously.
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Oxidising agents cause other substances to be oxidised and are themselves reduced (e.g., chlorine in $NaCl$ formation).
Reducing agents cause other substances to be reduced and are themselves oxidised (e.g., sodium in $NaCl$ formation).
The Electrochemical series ranks metals based on their ability to be oxidised, with highly reactive metals (e.g., potassium, sodium) at the top and less reactive metals (e.g., copper, silver) at the bottom.

Oxidation and Reduction Simplified Revision Notes for Leaving Cert Chemistry

Oxidation and Reduction