A redox reaction is a reaction which involves a change in oxidation state of one or more elements. When a substance loses its electron, its oxidation state increases, thus it is oxidized. When a substance gains an electron, its oxidation state decreases thus being reduced.  For example, for the redox reaction: 

H2+F2?2HFRewrite it as

Oxidation: H2?2H++2e?

Reduction: F2+2e??2F?

Overall Reaction : H2+F2?2H++2F?

Oxidation reaction lose electrons thus they are being oxidized, while Reduction is gaining electrons thus they are being reduced. In this case, H₂ is being oxidized, while F₂ is being reduced.

OIL RIG

Oxidation Is Losing electrons Reduction Is Gaining electrons or

LEO the lion goes GER

Lose Electrons Oxidation, Gain Electrons Reduction. 

For a full description of RedOx reactions see Chapter 11.

Using chemical reactions to produce electricity is now a priority for many researchers. Being able to adequately use chemical reactions as a source of power would greatly help our environmental pollution problems. In this section of electrochemistry, we will be learning how to use chemical reactions to produce this clean electricity and even use electricity to generate chemical reactions. In order to induce a flow of electric charges, we place a strip of metal (the electrode) in a solution containing the same metal, which is in aqueous state. The combination of an electrode and its solution is called a half cell. Within the half cell, metals ions from the solution could gain electrons from the electrode and become metal atoms;or the metal atoms from the electrode  could lose electrons and become metals ions in the solution.

Introduction to Cells

We use two different half cells to measure how readily electrons can flow from one electrode to another, and the device used for measurement is called a voltmeter. The voltmeter measures the cell potential, denoted by E_cell, (in units of Volts, 1V=1J/C), which is the potential difference between two half cells. The salt bridge  allows the ions to flow from one half cell to another but prevents the flow of solutions. 

As indicated in the diagram, the anode  is the electrode whre oxidation occurs; Cu loses two electrons to form Cu⁺². The cathode is the electrode where reduction occurs; Ag⁺ (aq) gains electron to become Ag(s). As a convenient substitution for the drawing, we use a cell diagram to show the parts of an electrochemical cell. For example above, the cell diagram is :  

Zn(s) | Zn²⁺ (aq) || Cu²⁺ (aq)| Cu(s)

oxidation- (half-cell) (salt bridge) (half-cell)-reduction

Where we place the anode on the left and cathode on the right,  "|" represents the boundary between the two phases, and  "||" represents the salt bridge. There are two types of electrochemical cells:

A Galvanic Cell (aka Voltaic Cell) induces a spontaneous redox reaction to create a flow of electrical charges, or electricity. Non-rechargeable batteries are examples of Galvanic cells.

• A Reaction is spontaneous when the change in Gibb’s energy, ?G is < 0.
• Electrons flow from the anode(negative since electrons are built up here) to the cathode (positive since it is gaining electrons). 

An Electrolytic cell is one kind of battery that requires an outside electrical source to drive the non-spontaneous redox reaction. Rechargeable batteries act as Electrolytic cells when they are being recharged.

• A reaction is non-spontaneous when ?G is > 0.
• Must supply electrons to the cathode to drive the reduction, so cathode is negative.
• Must remove electrons from the anode to drive the oxidation, so anode is positive.           

Both Galvanic and Electrolytic cells contain:

• Two electrodes: the Anode and the Cathode  (NOTE: Cathode does not mean +, and Anode does not mean -)
• Volt meter: measures the electric current. In Galvanic cells, this shows how much current is produced; in Electrolytic cells, this shows how much current is charging the system.
• Electrolyte
  • conducting medium
  • has contact with electrodes
  • usually in aqueous solution of ionic compounds
• Salt Bridge
  • joins the two halves of the electrochemical cell
  • filled with a salt solution or gel
  • keeps the solution separate
  • Completes the circuit 

Basic Terminology

• Electrochemical cells use a vast amount of terminology. Here is a brief definition of some of the more common terms:
• Voltage- The potential difference between two half cells, also the amount of energy that drives a reaction. Voltage is an intensive property (amount of voltage does matter).
• Current-The flow of electric charges (in units of electrons per second). It is an extensive property (amount of current does matter). NOTE: High voltage does not mean high current. 
• Primary Battery- non-rechargeable batteries. AA, AAA, etc.
• Secondary Battery- Rechargeable batteries. Lithium, cell phone batteries, etc.
• Tertiary Battery- Fuel cells. Although not always considered as batteries, these often require a constant flow of reactants.