Chapter 3: Further Redox Chemistry

Oxidation Numbers

Oxidation number is a number representing the number of electrons lost or gained by an atom in a compound.

Oxidation is a loss of electrons during a reaction or an increase in oxidation number.

Reduction is a gain of electrons during a reaction or a decrease in oxidation number.

Redox Titration

Redox titration is the process used to determine the concentration of a solution of a reducing agent or an oxidizing agent.

Redox Equations

A redox reaction can be constructed from two half equations; one representing an oxidation process and the other a reduction process.

To construct a full equation from half equations:

  1. Balance the electrons
  2. Combine the equations
  3. Cancel the electrons
  4. Check the change balance and stoichiometry

For example;

  1. Mg  →Mg2+ + 2e

Cu2+ + 2e→Cu

  1. Mg + Cu2+ + 2e →Mg2+ + 2e + Cu
  2. Mg + Cu2+ →Mg2+ + Cu

For balancing half equation:

  1. Balance all the atoms except for oxygen and hydrogen
  2. Add H2O to balance oxygens (if needed)
  3. Add H+ ions to balance hydrogens (If needed)
  4. Add e to balance out charges

Electrochemical Cells

Electrochemical cells consists of two half cells. At one oxidation occurs, at the other reduction. Electrons flow between the two cells, driving the redox reaction.

An equilibrium is reached

  • Each one of these beaker is a half cell. A solution in a standard half-cell will have a concentration of 1.00 mol dm-3.
  • In half cells with both oxidation states present as aqueous ions.
  • The standard potential is voltage measured under standard condition.
  • The voltage measured is also known as the electromotive force of the cell (EMF).

An electrochemical series is a list of standard electrode potential of all the possible half-cell.

 

The more negative the electrode potential, the more the oxidation reaction is favored.

Batteries

Electrochemical cells can be used as a commercial source of electrical energy. In a rechargeable battery, when the chemicals have reached fully, a potential difference can be applied to cell in opposite direction. Lithium-ion batteries are rechargeable.

At positive electrode

Li+ + CoO2 + e→Li+[CoO2]–              Eθ= +0.56 V

At negative electrode

Li→Li+ + e                                       Eθ= -3.04 V

 Some cells are non-rechargeable and disposed of when the chemicals have fully reacted. Example is an alkaline hydrogen cell,

Rusting

It is the process due to which the iron metal is oxidized to iron ions at the center of the water drop where oxygen concentration is low, and the electrons released reduce the oxygen at the surface of the drop where oxygen concentration is high. It only occurs at the presence of water and oxygen.

Fe(s) + ½ O2 (g) + H2O (I)→Fe2+ (aq) + 2OH (aq)

The process of protection from rusting include:

  • Panting
  • Galvanization
  • Sacrificial painting