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 anode to 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

Electrochemical Processes – Table J

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.

Electrode potential (voltage) is positive; when at zero, you have a dead cell or a dead battery  Cathode is (+), anode is (-) salt bridge voltmeter Zn metal Anode Cu metal Cathode Zn+2 SO4-2 Cu+2 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. Electroplating 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.

What is Happening Here? Silver is being oxidized at the + electrode as its electrons get stripped off and pulled into the + end of the DC power supply. The silver dissolves into solution as silver ions. Ag0 →Ag+1 + 1 e- Meanwhile, over at the – electrode, electrons are being pumped out of the DC power supply and into the ring. Metal atoms don’t gain electrons, so the electrons just hang out on the surface of the ring, giving the ring a negative charge. This attracts the positive silver ions in the solution, which migrate to the ring and touch it. As soon as a silver ion touches the ring, an electron jumps off the ring onto the silver ion, reducing it to solid silver metal, which freezes on to the ring. Ag+1+ 1 e- → Ag0 As more and more silver ions reduce and plate on to the ring, the thickness of the plating increases. The final thickness depends on how much current is being used, and how long you let the process continue.

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 Component Electrolytic cell Voltaic cell Electrodes Usually a metal or another conductor Anode (AN OX) Oxidation half reaction takes place Has a (+) charge Has a (-) charge Cathode (RED CAT) Reduction half reaction takes place Electrode potential (Voltage) negative Positive 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

Voltaic Cells -vs- Electrolytic Cells Electrochemical cells can be classified as voltaic or electrolytic. a) In a voltaic cell, energy is released from a spontaneous redox reaction. The system (cell) does work on the surroundings (lightbulb). b) In an electrolytic cell, energy is absorbed to drive a non-spontaneous reaction. The surroundings (battery or power supply) do work on the system (cell). Comparing and Contrasting How are voltaic and electrolytic cells similar? How are they different?