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In the introduction to Electrolysis , we stated that oxidation and reduction occur simultaneously hence the reason why it is also known as a redox reaction. The word redox comprises two terms reduction and oxidation and these two words have different meanings.

An Oxidation-reduction(redox) reaction involves the transfer of electrons from one reactant to another. In such a reaction, one reactant is an oxidant while the other is a reductant. We would cover redox reactions used to generate electricity and these reactions only occur in aqueous solutions.

Definition Of Oxidation & Reduction

Oxidation and Reduction can be defined in terms of electron-transfer and change in oxidation number. In terms of electron transfer, oxidation is the process of electron-loss, while reduction is the process of electron gain. In terms of oxidation number, oxidation leads to an increase in oxidation number while reduction leads to a decrease in the oxidation number of an atom.

Example Of Oxidation Reactions

Mg(s) + 1/2O2(g) MgO(s)

0 0 +2 -2 [Oxidation Numbers]

In the reaction above, the Magnesium atom, Mg is oxidized to Mg2+ by an increase in oxidation number from 0 to +2 while the oxygen atom is reduced to the oxide, O2- by a decrease in oxidation number from 0 to -2.

When greenish-yellow chlorine gas is bubbled into a green solution of Iron(II) chloride, a brown solution of iron(III) chloride is obtained:

FeCl2(aq) + 1/2Cl2(g) FeCl3(aq)

+2 -1 0 +3 -1 [Oxidation Number]

In the reaction above, iron(II) in FeCl2 is oxidized to iron(III) in FeCl3 by an increase in oxidation number from +2 to +3, while the Chlorine atom in Chlorine gas is reduced to Chloride ion, Cl in FeCl3 by a decrease in oxidation number from 0 to -1.

When defining redox reaction, we mentioned oxidizing agents and reducing agents also. An oxidizing agent(oxidant or oxidizer) is reduced to a lower oxidation number and in the examples above, Oxygen and chlorine gas are the oxidizing agents.

A reducing agent(reductant or reducer) is oxidized to a higher oxidation number and from the examples above, Fe2+ in FeCl2( is oxidized to Fe3+ hence, FeCl2 is the reducing agent.

In the laboratory, an oxidizing agent is used to test for a reducing agent while a reducing agent is used to test for an oxidizing agent.

Balancing Simple Redox Equations

In order to obtain the equation of a redox reaction, an oxidation half-reaction is paired with a reduction half-reaction. In order to balance such a reaction, the number of electrons lost must be equal to the number of electrons gained, and the two methods via which equations can be balanced are outlined below:

  • Ion-electron Method
  • Oxidation number method

Ion Electron Method

Simple ionic equations contain simple ions, e.g When chlorine gas is bubbled into aqueous iron(II) chloride, aqueous iron(III) chloride is obtained:

Fe2+(aq) + Cl2(g) Fe3+(aq) + Cl(aq)

Ion-electron method involves splitting the redox reaction into oxidation and reduction halves. The first step to follow is to assign oxidation numbers to each atom or element that has changed in the equation.

Fe2++ Cl2 Fe3++ Cl

+2 0 +3 -1

Separate the reaction into oxidation and reduction half

Oxidation half: Fe2+ Fe3+ [Increase in Oxidation Number]
Reduction Half: Cl2 Cl [Decrease in Oxidation Number]

Balance each half-reaction as follows:

Oxidation half: Fe2+ Fe3+ [Atoms Balanced]
Reduction Half: Cl2 Cl [Atoms Not Balanced because a molecule of Cl2 produces two chloride ions]

Cl2 2Cl [Atom Balanced]

Add the appropriate number of electrons to account for the number of electrons lost or gained to balance the charge.

Oxidation half: Fe2+ Fe3+

change in Oxidation number of Fe = (+3) – (+2) = +1 corresponds to a loss of 1 electron. Add one electron to the right-hand side.

Fe2+ Fe3+ + e [Charges Balanced]

Reduction Half: Cl2 2Cl [Gain of electrons]

Change in oxidation numbers -1-0 = -1 which corresponds to a gain of 1 electron. Cl2 gains 2e. Add 2e to the left-hand side.

Cl2 + 2e 2Cl [Charges Balanced]

Multiply each half-reaction by the appropriate coefficients, to balance electron loss and electron gain.

Fe2+ Fe3+ + e * 2 = 2Fe2+ 2Fe3+ + 2e
Cl2 + 2e 2Cl

Add the two half-reactions, canceling the common terms 2e :

Net equation: 2Fe2+ + Cl2 2Fe3+ + 2Cl

Oxidation Number Method

The oxidation method is used without splitting the redox reaction.

Fe2+(aq) + Cl2(g) Fe3+(aq) + Cl(aq)

+2 0 +3 -1

The first step is to balance the chlorine atoms on the right

Fe2+(aq) + Cl2(g) Fe3+(aq) + 2Cl(aq)

Change in oxidation number of Chlorine = -1-0 = -1[Gain of 1e]

Change in oxidation number of Fe = (+3) -(+2) = +1[Loss of 1e]

For each Fe2+ oxidized, one Cl atom is reduced. Since there are 2 Cl atoms, multiply Fe atoms by 2

Net Equation: 2Fe2+ + Cl2 2Fe3+ + 2Cl

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