Electrochem

1. Electrochem • The cathode is the electrode at which reduction occurs. • The cathode is positive in voltaic cells. • The anode is the electrode at which oxidation occurs. • The anode negative in voltaic cells. • 1 amp = 1 coulomb/second • In all voltaic cells, electrons flow spontaneously from the negative electrode (anode) to the positive electrode (cathode). • 1. Choose the appropriate half-reactions from a table of standard reduction potentials. • 2. Write the equation for the half-reaction with the more positive E0 value first, along with its E0 value. • 3. Write the equation for the other half-reaction by reverse the tabulated reduction half-reaction and change the sign of the tabulated E0 . • 4. Balance the electron transfer. Do not multiply the potentials. • 5. Add the reduction and oxidation half-reactions. • 6. Add their potentials. • This creates an E0cell which is positive, which indicates that the forward reaction is spontaneous. The Nernst Equation at 298 K The Gibbs Free Energy Relationship

2. Primary Voltaic Cells • As a voltaic cell discharges, its chemicals are consumed. • Once the chemicals are consumed, further chemical action is impossible. • The electrodes and electrolytes cannot be regenerated by reversing current flow through cell. • These cells are not rechargable. Secondary Voltaic Cells • Secondary cells are reversible, rechargeable. • The electrodes in a secondary cell can be regenerated by the addition of electricity. • These cells can be switched from voltaic to electrolytic cells. • One example of a secondary voltaic cell is the lead storage or car battery. The Hydrogen-Oxygen Fuel Cell • Fuel cells are batteries that must have their reactants continuously supplied in the presence of appropriate catalysts. • Hydrogen is oxidized at the anode. • Oxygen is reduced at the cathode. • Fuel cells are very efficient. • Energy conversion rates of 60-70% are common!