Electrochemistry

Electrochemical Cells/ Chemical Cells Also called voltaic or galvanic cells A redox reaction produces electricity Occurs spontaneously

Electrochemical 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

Reduction at the 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 Electrons will flow between the two electrodes until equilibrium is reached At equilibrium the cell’s voltage would be zero

- + Zn + CuSO4 Cu + ZnSO4 Zn Zn2+ + 2e- Cu2+ + 2e - Cu0 e- e- e- e- e- Red Cat -reduction takes place…electrons are gained. An Ox -oxidation takes place…electrons are lost. Zn Zn2+ + 2e- Cu2+ + 2e - Cu0 e- e- e- e- e- - e- + e- e- Electrons needed here for reduction Electrons released here by oxidation e- e- e-

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)

Corrosion Oxidation of a metal Metal combines with element (usually oxygen) Example: 4Fe + O2 2Fe2O3 (rust)

Prevention of Rust 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)

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 2 H2O  2 H2 + O2 The H+ is reduced at the (-) cathode, producing H2 (g), which is trapped in the tube The O2 is oxidized at the (+) anode, yielding O2 (g), which is trapped in the tube

Hydrogen Fuel Cells Uses hydrogen gas as the fuel 2 H2 + O2  2 H2O