CHEMISTRY HOME
BLACKBOARD LOGIN

LAB MANUAL HOME

SYLLABUS

Oxidation and reduction are the terms used to describe the transfer of electrons in a chemical reaction.  Atoms or ions that accept electrons in a reaction are said to be reduced and atoms or ions that lose electrons are said to be oxidized.  The atom or ion that is being reduced is often referred to as the “oxidizing agent” since it is taking electrons away from another atom or ion. Conversely, the atom or ion that is being oxidized is often referred to as the “reducing agent” because it is donating electrons to another atom or ion. The oxidation or reduction of an atom or ion can be followed by observation of the change in the atom or ion’s oxidation number from the reactant to product side of a reaction.  The rules for assigning oxidation numbers are as follows:
Be sure to apply these rules in the order given.

1. Elements are assigned an oxidation number of zero. Ex: Li, Cu, O₂, S₈, P₄, Cl₂
2. Monoatomic ions (ions consisting of a single atom) have an oxidation number equal to the charge of the ion. Ex: Cl^-, Al⁺³, O^-2.
3. In compounds: Group IA metals are assigned an oxidation number of +1, Group IIA metals are +2 , Group IIIA are +3
4. In compounds, fluorine is assigned an oxidation number of -1.
5. In compounds, hydrogen has an oxidation number of +1. Exceptions: In metal hydrides like NaH and CaH₂, hydrogen has an oxidation number of -1 (Rule 3 takes priority.)
6. In compounds oxygen is assigned an oxidation number of -2. Exceptions: Peroxides: H₂O₂, Na₂O₂ – oxygen = -1 (Rules 3&5 have priority.) Superoxides: KO2, CsO2 – oxygen = -½ (Rule 3 has priority.) Others: OF2 – oxygen = +2 (Rule 4 has priority.)
7. The sum of the oxidation numbers is equal to the overall charge of the species. For neutral compounds, the total of the oxidation numbers is zero.

The transfer of electrons from one place to another is also commonly known of as electricity.  The electricity that you use to power your radio is simply the flow of electrons down a metal wire.  In a voltaic cell, electricity is produced by a spontaneous redox (oxidation-reduction) reaction. The oxidation and reduction processes are separated, and therefore, the transfer of electrons occurs through an external wire. The separated parts of the cell are called half-cells (oxidation half and reduction half).
For Example:    A voltaic cellcan be constructed taking advantage of the spontaneous nature of the redox reaction,
Zn(s) + Cu²⁺(aq) Zn²⁺ (aq) + Cu(s)

A 1.0 M solution of CuSO₄ is placed in a container. A piece of copper metal (copper electrode) is placed in this solution.  ZnSO₄ solution is then placed in another container and a piece of zinc metal (zinc electrode) is placed in the ZnSO₄ solution. In this construction zinc is the negative electrode (anode) and copper the positive electrode (cathode). Both solutions are then connected to a third solution that acts as a salt bridge.  When the two electrodes are connected by means of an external metal wire (most commonly a voltmeter) completing the circuit, a current of electrons (electric current) will flow from the zinc electrode to the copper electrode. The direction of the flow of electrons depends on the potential of the metal ion. The potential is a measure of the driving force behind an electrochemical reaction that is reported in units of volts. In this case, the electrons flow away from zinc towards copper.  This means that copper is being reduced and is therefore the stronger oxidizing agent and that zinc is being oxidized and is therefore the stronger reducing agent. Oxidation always occurs at the anode and reduction always occurs at the cathode of an electrochemical cell.  The flow of electrons should therefore always have a positive potential value from anode (-) to cathode (+).  A negative potential value on your voltmeter indicates that the electrons are flowing in the opposite direction and that you need to switch the electrode connections on the voltmeter.
For the zinc – copper cell the reactions taking place at the individual electrodes are the following:

Reaction at the negative electrode (anode):
Zn(s) Zn²⁺(aq) + 2e^-
Reaction at the positive electrode (cathode):
Cu²⁺(aq) + 2e^- Cu(s)
The overall reaction is
  Zn(s) +     Cu²⁺(aq)Zn²⁺ (aq) + Cu(s)

The potential (electromotive force) of this cell is 1.1 V and is measured by a voltmeter.