Electrode potentials and Electrochemical cells

• electrochemical cells use redox reactions as the electron transfer between products creates current which flows between electrodes in the cell
• electrochemical cells

  2 solutions with metal electrodes

  salt bridge eg. KCl, KNO3, NaCl, NaNO3 = tube of unreactive ions that move between the solutions to carry the flow of charge without interfering with the reaction

  
• conventional cell representation

  half cell that is most negative potential on the left

  most oxidised species from each half cell goes next to salt bridge

  salt bridge shown using double line

  include state symbols

  Platinum electrode when both are same state

  
• Standard Hydrogen Electrode

  measuring standard for half cell potentials

  cell potential of 0V, measured over standard conditions of 298K, 100kPa, 1 moldm3 solution of ions

  otherwise will change equilibrium position
• negative potential

  easily oxidised and lose electrons
• positive potential

  easily reduced and gain electrons
• SHE cell

  hydrochloric acid

  hydrogen gas

  platinum electrodes

  always on the left

  Pt I H2 I H+ II

  
• calculating electrode potential

  RHS - LHS

  positive value = spontaneous and favourable
• electrochemical series

  Standard electrode potentials listed

  half equations written as reduction reactions

  more positive = better oxidising agents, oxidise species more negative

  more negative = better reducing agents, reduce species more positive
• writing redox reactions

  more negative goes oxidation direction

  more positive stays in reduction direction
• effects of changing concentration/pressure

  increasing the concentration of the solutions used in the electrochemical cell makes the cell EMF more positive as fewer electrons produced

  increasing pressure of the cell makes cell EMF more negative as more electrons produced
• commercial applications of electrochemical cells

  commercial source of electrical energy

  produced to be rechargeable, non-rechargeable, fuelcells
• rechargeable cells

  use reversible reactions

  eg. lithium cells
• lithium cells

  lithium cobalt oxide electrode

  graphite electrode

  electrolyte is lithium salt in organic solvent
• lithium cell half equations

  
• fuel cells

  use to generate electric current

  dont need to be electrically recharged
• hydrogen oxygen fuel cells

  can be made in acidic or alkaline conditions

  uses supply of oxygen and hydrogen to generate electricity

  hydrogen produces electrons, oxygen accepts electrons

  
• pros and cons of hydrogen oxygen fuel cells

  greater proportion of energy is converted to useful energy

  only waste product is water

  CO2 released because fossil fuels are burned to produce electricity to generate hydrogen

  hydrogen highly flammable and explosive