Electrochemistry

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Electrochemistry
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Electrochemical Cells/Chemical Cells

• Also called voltaic or galvanic cells
• A redox reaction produces electricity
• Occurs spontaneously

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Electrochemical Cell
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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

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Electrodes

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

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• 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
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Flow of Electrons

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

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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 cells voltage would be zero

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Zn CuSO4 Cu ZnSO4
Red Cat -reduction takes placeelectrons are
gained.
An Ox -oxidation takes placeelectrons are lost.
Cu2 2e - Cu0
e-
e-
e-
e-
e-
-
e-

e-
e-
e-
e-
e-
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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)

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Corrosion

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

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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)

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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

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• 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

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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

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Silver Plating

• Cathode
• Anode
• Solution
• What happens to the mass of each electrode during
  the reaction?

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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

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Hydrogen Fuel Cells

• Uses hydrogen gas as the fuel

2 H2 O2 ? 2 H2O