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ELECTROCHEMICAL CELLS. In redox reactions, there is a chemical reaction and an exchange of electrons between the particles being oxidized and reduced. An electrochemical cell involves a chemical reaction and a flow of electrons. PARTS OF AN ELECTROCHEMICAL CELL. electrodes
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ELECTROCHEMICAL CELLS • In redox reactions, there is a chemical reaction and an exchange of electrons between the particles being oxidized and reduced. An electrochemical cell involves a chemical reaction and a flow of electrons.
PARTS OF AN ELECTROCHEMICAL CELL • electrodes • anode - site where oxidation occurs • cathode – site where reduction occurs • half cells – separate containers in which oxidation and reduction half reactions occur • Electrons travel from the anodeto the cathode (from loss to gain, from oxidation to reduction) across the wire. • U-tube or salt bridge – lets ions travel between half cells to complete the circuit –ONLY for a voltaic cell RED CAT - REDuction = CAThode, AN OX” – OXidation = ANode
TYPES OF ELECTROCHEMICAL CELLS • VOLTAIC (or galvanic) cell (spontaneous reactions) uses chemical energy to produce electricity • ex include lead acid storage battery • (automobile battery), and breathalyzers http://www.youtube.com/watch?v=A0VUsoeT9aM&feature=related
What is Happening Here? • The oxidized metal (Al, anode) gets smaller as it turns into metal ions and dissolves into the solution. The reduced metal (Ag, cathode) gets larger as it gets coated with the reduced metal ions from solution. • 2) The negative (NO3 -1) anions go through the salt bridge from cathode to anode (from the Ag half-cell to the Al half-cell) • 3) The positive (Al+3) cations go through the salt bridge from anode to cathode (from the Al half-cell to the Ag half-cell) • 4) The salt bridge can contain a solution of any salt, as long as it does not contain Al or Ag. NaCl can be used. When the cell is first used, Na+1 passes into the cathode half-cell, and Cl-1 passes into the anode half-cell. Later, the ions mentioned in 3) and 4) take their place.
0 voltmeter Zn metal Anode Cu metal Cathode salt bridge Cu+2 Zn+2 SO4-2 SO4-2 Zn+2 Cu+2 • Electrode potential (voltage) is positive; when at zero, you have a dead cell or a dead battery • Cathode is (+), anode is (-) Music video: http://www.youtube.com/watch?v=-bxJXt_69yM&feature=related Let’s watch: Removing tarnish from silver http://www.youtube.com/watch?v=CsKLcc13WBo&feature=related
ELECTROLYTIC CELL (non spontaneous reactions) • uses electricity to cause a chemical reaction ex include: recharging a battery, electrolysis of a salt and electroplating (ie. jewelry) Electrode potential is negative (negative voltage reading) The polarities of the electrodes are reversed (Cathode = negative: anode = positive) Electrolysis of KI
Using Electrolysis in Metal (Electroplating) When electroplating, metal being used is on the anode and object being plated is on the cathode • In an electrolytic cell used for plating, the object to be plated (a bracelet) is the cathode. • The anode is the plating metal, in this case, a piece of silver. • At the anode, silver is oxidized to silver ions. • Silver ions in the electrolyte solution reduce to silver metal. • The silver metal forms a thin coating on the cathode, producing a plated bracelet.
Electrolytic cells requires electrical energy to produce chemical changes. This process is called electrolysis. • Signs of the electrodes switch! • Still ANOX REDCAT • Anode is positive ; cathode is negative
COMPARING ELECTROCHEMICAL CELLS When voltage is zero, the cell is dead No, requires an outside energy source; converts electrical energy to chemical energy spontaneous Yes; converts chemical energy to electrical energy
