C20 electrode potentials + electrochemical cells

• electrochemical cells are based on the fact that if you place two different metals in a salt solution and connect them together, a current flows as electrons move from the more reactive metal to the less reactive metal, creating a potential difference
• electric potential of one electrode cannot be measured by itself, instead connect two electrodes together and measure the difference in potential between them
• by measuring the potential difference between two electrodes, you can measure how good of a reducing agent the metals are, as reducing agents release electrons
• the circuit in an electrochemical cell is made up of two electrodes of different metals, in salt solutions of their own ions, joined together by a salt bridge between solutions
• a salt bridge is a tube of unreactive ions that can move between the solutions to carry the flow of charge, but will not affect the reaction
• each one of the solutions is a half cell, which has a cell potential, this can be used to work out if it will be oxidised or reduced when connected to another half cell
• conventional cell representation rules:

  • half cell with most negative potential goes on the left
  • most oxidised species from each half cell goes next to the salt bridge
  • salt bridge is shown with a double line
  • include state symbols
• the standard hydrogen electrode is used as a measuring standard for half cell potentials
• the standard hydrogen electrode has a potential of 0.00V measured under standard conditions
• standard conditions for the SHE are:

  • pressure of 100kPa
  • temperature of 298K
  • concentration of 1.00 moldm^-3
• the standard hydrogen electrode is made up of hydrochloric acid, hydrogen gas, and platinum electrodes, which are inert and conduct electricity so do not interfere with the reaction
• if you measure the potential of a half cell under standard conditions, you can compare this value with the SHE, to get a numerical value for the potential of the half cell
• negative potentials = more easily oxidised then the SHE, will lose electrons

  positive potentials = more easily reduced than the SHE, will gain electrons
• standard half cell potentials can be used to calculate the overall cell EMF
• EMF = E(right) - E(left)

  EMF = most positive E - most negative E
• if the overall EMF is positive, the reaction is spontaneous and favourable
• the more positive the EMF, the more favourable the reaction
• half-equations can be combined to give the overall cell reaction, always flip the half-equation with the more negative potential, electrons should be balanced, then combine
• positive potentials are better oxidising agents
• negative potentials are better reducing agents
• electrochemical cells can be used commercially as a source of energy
• the types of commercial electrochemical cell are:

  non-rechargeable, rechargeable, fuel
• rechargeable cells are based on a reversible reaction which allows the reactants to reform from the products, the reforming of reactants is the recharging
• an example of a rechargeable cell is the lithium ion cell, commonly used in phones, laptops and cars, they are made up of a lithium cobalt oxide anode, and a carbon cathode, with lithium salt in organic solvent
• at the anode of a lithium ion cell, the equation is:

  Li(+) + CoO2 +e- -> Li(+)[CoO2](-)
• at the cathode of a lithium ion cell, the equation is:

  Li -> Li(+) + e-
• in a lithium ion cell, the reverse of the reactions happen when the cell is recharging, this is caused by applying an external voltage greater than the EMF to drive the electrons in the opposite direction
• non-rechargeable cells are based on reactions which are not reversible, therefore they cannot be recharged by applying an external voltage
• fuel cells are used to generate a current without needing to be recharged
• an example of a fuel cell is the hydrogen fuel cell, which uses a supply of hydrogen and oxygen from the air, and the only waste product it produces is water
• at the anode of a hydrogen fuel cell, the equation is:

  O2 + 2H2O + 4e- -> 4OH-
• at the cathode of a hydrogen fuel cell, the equation is:

  2H2 + 4OH- -> 4H2O + 4e-
• the overall reaction in a hydrogen fuel cell is:

  H2 + 1/2(O2) -> H2O
• hydrogen fuel cells have disadvantages, hydrogen is highly flammable, and they are expensive to produce
• a fuel cell does not need to be electrically recharged because the reactants are supplied continuously
• an advantage of using a methanol fuel cell over a hydrogen fuel cell is that methanol is a liquid whereas hydrogen is a gas, so methanol is easier to transport
• when writing cell representations, if it involves the SHE that always goes on the left, and the species with the highest charge on each side goes next to the salt bridge, so the SHE is always Pt|H2|H+||
• the advantage of using hydrogen in a fuel cell over an ICE is that a higher proportion of energy from combustion is converted into kinetic energy