Electrochemistry

• A voltaic cell employs a spontaneous oxidation-reduction reaction as a source of energy.
• An electrochemical cell is a system consisting of electrodes that drip into an electrolyte and in which a chemical reaction either uses or generates an electric current.
• A voltaic (galvanic) cell is an electrochemical cell in which a chemical reaction uses or generates an electric current.
• An electrolytic cell is an electrochemical cell in which an external energy source drives an otherwise nonspontaneous reaction
• A voltaic cell consists of two half-cells that are electrically connected. Each half-cell is the portion of an electrochemical cell in which a half-reaction takes place
• A salt bridge is a tube of an electrolyte in a gel that is connected to the two half-cells of a voltaic cell; the salt bridge allows the flow of ions but prevents the mixing of the different solutions that would allow direct reaction of the cell reactants.
• The sum of the two half-reactions is called the cell reaction
• Electrons are given up by the anode (from the oxidation half-reaction) and thus flow from it, whereas electrons are used up by the cathode
• The anode in a voltaic cell has a negative sign because electrons flow from it. The cathode in a voltaic cell has a positive sign because electrons flow to it.
• When the half-reaction involves a gas, an inert material such as platinum serves as a terminal and as an electrode surface on which the half-reaction occurs.
• Work is needed to move electrons in a wire or to move ions through a solution to an electrode.
• Potential difference is the difference in electric potential (electrical pressure) between two points. Potential difference is measured in volts
• The electrical work expended in moving a charge through a conductor is : electrical work = charge x potential difference
• The Faraday constant, F, is the magnitude of the charge on one mole of electrons; it equals 9.6485 x 10^4 C per mole of electrons (96,485 C/mol e-)
• Cell voltage has its maximum value only when no current flows because it takes energy or work to drive a current through the cell.
• The maximum potential difference between the electrodes of a voltaic cell is referred to as the cell potential or electromotive force (emf) of the cell
• The anode of a voltaic cell has a negative polarity, and the cathode has a positive polarity.
• Ecell= oxidation potential + reduction potential or Ecell = Ecathode+ Eanode
• Reduction potential is a measure of the tendency for an oxidized species to gain electrons in the reduction half-reaction (in other words, a measure of the ability of the species to act as an oxidizing agent).
• The cell potential of a voltaic cell depends on the concentrations of substances and the temperature of the cell
• The standard cell potential, E8 cell, is the electromotive force (emf) of a voltaic cell operating under standard-state conditions
• The standard electrode potential, E, is the electrode potential when the concentrations of solutes are 1 M, the gas pressures are 1 atm, and the temperature has a specified value (usually 25C)
• The strongest oxidizing agents are the oxidized species corresponding to half-reactions with the largest (most positive) E values.
• The reduction half-reactions with lower (that is, more negative) electrode potentials have a greater tendency to go right to left
• The free-energy change ΔG for a reaction equals the maximum useful work. For a voltaic cell, this work is the electrical work, -nFEcell
• The process of producing a chemical change in an electrolytic cell is called electrolysis.