Electrochemistry

• E^o is a type of energy per electron so it remains unchanged even if we double the numbers of reactants and products in the reaction

• If an equation is reversed (so that the reactants become the products), the sign of E^o is also reversed

• The cell's emf (electromotive force, often referred to as the cell voltage), is calculated by adding together the E^o values for each half reaction:
  E^o_cell = E^o_reduction + E^o_oxidation

• The reaction is spontaneous in the direction as written if
  E^o_cell > 0
  (E^o_cell positive)

• The reaction is spontaneous in the reverse direction to that written if
  E^o_cell < 0
  (E^o_cell negative)

• A galvanic cell (voltaic cell) produces electricity so the overall cell reaction must have a positive E^o_cell value
  (E^o_cell > 0)
• http://www.ausetute.com.au/calcelemf.html

E_cell = E_cathode – E_anode

E_cell must be > than 0 for the reaction to be spontaneous! So by knowing how to subtract what, you know WHICH “half-cell” is the cathode, and which is the anode.

*If both half-reactions appear to be reductions, the OPPOSITE direction of each is oxidation.

Electron movement (wire)

***ELECTRONS ALWAYS MOVE "AWAY" from ANODE, and "COME TO" the CATHODE.

ANODE = site of OXIDATION

CATHODE = site of REDUCTION

ION movement (salt bridge)

ANIONS (-) move to the ANODE

CATIONS (+) move to the CATHODE

***ANODE and CATHODE are just sites/locations in space (ex: California & New York), determined by the movement of IONS...do not confuse!!!

Galvanic (Voltaic) Cell: converts chemical energy of oxidants and reductants into electrical energy Electrodes: are conductors used to permit the flow of electrons in an electrochemical cell. One electrode is the anode, the other is the cathode. Anode: Oxidation occurs at anode Anode is negative Anode disintegrates Cathode:Reduction occurs at cathode Cathode is positive Solid deposits on cathode Salt bridge: allows for migration of ions to complete the electrical circuit Electron Flow: from anode to cathode (e- AWAY from ANODE, COME towards CATHODE)!!! Electrons flow from negative to positive Spontaneous Reaction: E0 for the galvanic cell is positive

Source: http://www.ausetute.com.au/voltcell.html

Electrochemical Cells

Galvanic and Electrolytic Cells

Oxidation-reduction or redox reactions take place in electrochemical cells. There are two types of electrochemical cells. Spontaneous reactions occur in galvanic (voltaic) cells; nonspontaneous reactions occur in electrolytic cells. Both types of cells containelectrodes where the oxidation and reduction reactions occur. Oxidation occurs at the electrode termed the anode and reduction occurs at the electrode called thecathode.

Electrodes & Charge

The anode of an electrolytic cell is positive (cathode is negative), since the anode attracts anions from the solution. However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell's electrons or negative charge. The cathode of a galvanic cell is its positive terminal. In both galvanic and electrolytic cells, oxidation takes place at the anode and electrons flow from the anode to the cathode.

Galvanic or Voltaic Cells

The redox reaction in a galvanic cell is a spontaneous reaction. For this reason, galvanic cells are commonly used as batteries. Galvanic cell reactions supply energy which is used to perform work. The energy is harnessed by situating the oxidation and reduction reactions in separate containers, joined by an apparatus that allows electrons to flow. A common galvanic cell is the Daniell cell, shown below.

Electrolytic Cells

The redox reaction in an electrolytic cell is nonspontaneous. Electrical energy is required to induce the electrolysis reaction. An example of an electrolytic cell is shown below, in which molten NaCl is electrolyzed to form liquid sodium and chlorine gas. The sodium ions migrate toward the cathode, where they are reduced to sodium metal. Similarly, chloride ions migrate to the anode and are oxided to form chlorine gas. This type of cell is used to produce sodium and chlorine. The chlorine gas can be collected surrounding the cell. The sodium metal is less dense than the molten salt and is removed as it floats to the top of the reaction container.

http://chemistry.about.com/library/weekly/aa082003a.htm