Electrochemistry

Electrochemical Cells  Electrons are transferred between the particles being oxidized and reduced  Two types –Spontaneous = Voltaic (or Galvanic) Cell  Uses a spontaneous redox reaction to produced electricity –Non-spontaneous = Electrolytic Cell  Uses electricity to force a redox reaction to occur

Voltaic Cells  Also called galvanic cells  A redox reaction produces electricity  Occurs spontaneously

Voltaic Cell

Half Cells Each ½ of the redox reaction occurs in a separate container –One for oxidation and one for reduction They are connected by a salt bridge –Salt Bridge: allows ions to flow between the two cells

Electrodes Metals which provide a surface for oxidation or reduction to occur –Solids –Oxidation Number = 0 –Anode –Cathode

ANODE –Oxidation occurs at the anode –Negative electrode CATHODE –Reduction occurs at the cathode –Positive electrode Red Cat – An Ox Reduction at the Cathode Oxidation at the Anode

Flow of Electrons The electrodes are connected by a wire Electrons flow from the anode to the cathode through the wire

Why does the cell produce electricity? There is a difference of electric potential between the two electrodes –E–E–E–Electrons will flow between the two electrodes until equilibrium is reached –A–A–A–At equilibrium the cell’s voltage would be zero

An Ox -oxidation takes place…electrons are lost. Red Cat -reduction takes place…electrons are gained. Zn Zn e - Cu e - Cu Electrons released here by oxidation Electrons needed here for reduction e- Zn + CuSO 4 Cu + ZnSO 4

Batteries Use a redox reaction which produces electricity spontaneously Batteries are recharged by reversing the reaction Dry Cell (Acid or Alkaline), Lead Storage (Car), Rechargeable (Ni/Cd)

Electrolytic Cells  Also called electrolysis  An electric current is used to produce a chemical reaction –An electric current is used to force a non-spontaneous reaction to occur

 Oxidation occurs at the anode  Reduction occurs at the cathode  Electrons flow from anode to cathode  The cathode is the negative electrode  The anode is the positive electrode –This is opposite of the chemical cell because the external current causes the polarities to switch

Electroplating Object to be plated is the CATHODE, negative Metal to be plated onto the object is the ANODE, positive Solution must contain ions of the metal to be plated

Silver Plating Cathode = Anode = Solution = What happens to the mass of each electrode during the reaction?

Electrolysis of Water The H + is reduced at the (-) cathode, producing H 2 (g), which is trapped in the tube The O 2 is oxidized at the (+) anode, yielding O 2 (g), which is trapped in the tube 2 H 2 O  2 H 2 + O 2

Hydrogen Fuel Cells Uses hydrogen gas as the fuel Spontaneous (Electrochemical Cell) 2 H 2 (g) + O 2 (g)  2 H 2 O(g) + energy

Corrosion Oxidation of a metal Metal combines with element (usually oxygen) Example: 4Fe + O 2 2Fe 2 O 3 (rust)

Prevention of Corrosion Cover the metal – paint, oil, another (more reactive) metal Cathodic Prevention –metal is placed in contact with a more reactive metal –That metal will be oxidized (acts as the anode), the original metal acts as the cathode Alloys – mixture of metals –Brass, stainless steel (Fe + Cr), cast iron (C + Si)