chapter 23

• electricity


  flow of electrons, in redox reactions we can control the flow of electrons between chemical species, this is the basis of cells and batteries
• voltaic cell


  converts chemical energy into electrical energy
• half cell


  contains the chemical species present in a redox half equation
• a voltaic cell can be made by connecting together 2 half cells, which then allows electrons to flow, in the cell the chemicals in the 2 half cells must be kept apart so electrons don't flow in an uncontrolled way and electrical energy is still released
• to form a circuit and measure the potential using a voltmeter, half cells must be connected by a salt bridge, a wire cannot be used to complete the circuit because that would create another half cell

  
• metal / metal ion half cells

  simplest half cell consists of a metal rod dipped into solution of its aqueous metal ion, at the phase boundary, where metal is in contact with its ions, an equilibrium is set up and by convection, the equilibrium will show the forward reaction being reduced and the backwards reaction being oxidised

  
• in an isolated half cell, there is no net transfer of electrons either into or out of the metal, when 2 half cells are connected, the direction of electron flow depends on the relative tendency of each electrode to release electrons
• ion half cells

  contain ions of the same element in different oxidation states e.g a half cell can be made from a mix of aqueous iron 2 and iron 3 ions, here there is no metal to transport electrons either in or out, so an inert metal electrode made out of platinum is used

  
• electrode potentials


  in an operating cell:

  • electrode with more reactive metal loses electrons and is oxidised = negative electrode
  • electrode with the less reactive metal gains electrons and is reduced = positive electrode
• standard electrode potential


  standard chosen is a half cell containing H2 gas and a solution containing H+ ions, an inert platinum electrode is used to allow electrons in and out of the half cell

  
• standard electrode potential conditions

  • solutions have to have a concentration of exactly 1 moldm-3
  • temperature is 298K / 25C
  • pressure is 100 kPa / 1 bar
• fuel cells

  uses energy from the reaction of a fuel with oxygen to create a voltage, operate continuously and do not need to be recharged, consist of the reactants flowing in
• hydrogen fuel cells

  hydrogen cell can have either an alkali or acid electrolyte, cell voltages can both be equal despite having different redox systems and half equations

  
• primary cells

  non rechargeable and are designed to be one use only, when in use electrical energy is produced by oxidation and reduction at the electrodes, but reactions cannot be reversed, when all the chemicals are used bu, the voltage will fall and the battery will go flat, the cell can be discarded or recycled
• primary cells still find use for low current, long storage devices such as wall clocks and smoke detectors, most modern primary cells are alkaline based on zinc and manganese dioxide and a potassium hydroxide alkaline electrolyte, E values:

  
• secondary cells

  are rechargeable, unlike primary cells the cell reaction producing electrical energy can be reversed during recharging, the chemicals in the cell are then regenerated and the cell can be used again
• common examples of secondary cells

  • lead acid batteries used in cars
  • nickel-cadmium, NiCd, cells and nickel metal hydride, NiMH, the cylindrical batteries used in radios and torches
  • lithium-ion and lithium-ion polymer cells used in laptops, phones and cameras