Electrode Potentials and Electrochemical Cells

• • a half cell is one half of an electrochemical cell
  • made from a metal dipped into its aqueous ions
  • or made from a platinum electrode dipped into the aqueous ions
• platinum is used because:

  • it is inert
  • conducts electricity well
• electrochemical cell:

  A) voltmeter

  B) salt bridge

  C) wire

  D) metal electrode

  E) solution of ions

  
• Ecell = E right - E left
• electrons flow from a more reactive metal to a less reactive one
• salt bridge:

  • completes the circuit
  • allows ions to flow between the two half-cells
  • made of filter paper saturated with KNO3 solution
• the electrode potential of a half cell tells us how easily it loses its electrons
• • the more negative half cell loses electrons - oxidation
  • the more positive half-cell gains electrons - reduction
• • the more negative half-cell is on the left
  • the equation needs to be flipped so it is backwards
• standard hydrogen electrode (SHE):

  • has an electrode potential of 0.00V
  • used as a reference - other half-cells are measured against this
  • under standard conditions: 100kPa, 298K, 1 mol dm-3 H+ ions
• standard hydrogen electrode:

  A) platinum electrode

  B) hydrogen gas bubbled through

  C) 1 mol dm-3 HCl

  
• oxidising agent:

  • gains electrons - more easily reduced
  • more positive electrode potential = stronger oxidising agent
  • more negative electrode potential = weaker oxidising agent
  • on the left hand side of the equation
• reducing agent:

  • lose electrons - more easily oxidised
  • on the right hand side of the equation
  • more negative electrode potential = stronger reducing agent
  • more positive electrode potential = weaker reducing agent
• cell notation:

  • most negative half cell is on the left
  • single line shows a change in state
  • double line shows the salt bridge
  • a comma shows they are in the same state
• • batteries are electrochemical cells
  • 2 forms - rechargeable and non-rechargeable
• • non-rechargeable batteries tend to be cheaper
  • rechargeable batteries are reversible and last longer
• lithium ion batteries:

  • rechargeable battery
  • used in wireless power tools, tablets, phones and electric cars
• lithium ion cell:

  • 3 components - electrode A, electrode B and the electrolyte
  • electrode A - lithium cobalt oxide
  • electrode B - graphite
  • electrolyte - lithium salt dissolved in an organic solvent
• the electrolyte is the part of a battery that acts as a conductive pathway for ions to move from one electrode to another
• rechargeable batteries:

  • plug them in to supply a current
  • current forces electrons to flow in the opposite way
  • reverse the overall discharge equation to show a battery recharging
• fuel cell:

  • electricity is supplied by a continuous external supply of chemicals rather than a 'ready store' in batteries
• alkaline hydrogen-oxygen fuel cell:

  1. hydrogen is fed here - 2H2 + 4OH- --> 4H2O + 4e-
  2. electrons produced in reaction 1 travel though a platinum electrode
  3. component - flow of electrons is used to power something
  4. oxygen is fed here - O2 + 2H2O + 4e- --> 4OH-
  5. electrons flow to the negative platinum electrode
  6. electrolyte of KOH solution carries the OH- from the cathode to the anode
  7. electrons flow from the positive platinum electrode
  8. water is released
  9. OH- ions are carried to the anode via the electrolyte

  
• in fuel cells - ion exchange membranes line the platinum electrodes and allow OH- ions to pass through but not hydrogen and oxygen gas
• fuel cell equations:

  A) 2H2

  B) 4OH-

  C) 4H2O

  D) 4e-

  E) O2

  F) 2H2O

  G) 4e-

  H) 4OH-

  I) 2H2

  J) O2

  K) 2H2O

  
• advantages of fuel cells:

  • more efficient than an internal combustion engine in machines - more energy is converted into kinetic energy, combustion engines waste a lot of energy as thermal energy
  • dont need to be recharged - just need a ready supply of oxygen and hydrogen
  • only waste product is water, no CO2 is directly emitted
• disadvantages of fuel cells:

  • hydrogen is highly flammable - must be stored and transported correctly
  • expensive to transport and store hydrogen - pressurised containers needed
  • energy is required to make hydrogen and oxygen in the first place - fossil fuels are usually used to pass water through an electrolysis process, contributing to CO2 emissions
• KNO3 is used for salt bridge because it is inert - does not react with most ions
• electrochemical series - a list of half cells in order of electrode potentials
• battery - multiple cells connected together
• zinc-carbon cell:

  A) carbon

  B) ammonium chloride paste

  C) zinc case

  
• zinc-carbon cell:

  • Zn (s) --> Zn2+ (aq) + 2e-
  • 2NH4+ (aq) + 2e- --> 2NH3 (g) + H2 (g)
  • 2NH4+ + Zn --> 2NH3 + H2 + Zn2+
• • the zinc-carbon cell might leak because as the cell discharges the zinc is used up and the walls of the zinc cannister become thin and prone to leakage
  • the ammonium chloride electrolyte is acidic and can become corrosive
• • zinc-carbon cell is ideal for doorbells
  • zinc-chloride cell is similar but uses zinc chloride as the electrolyte - e.g. used for radios
• • rechargeable batteries are recharged by reversing the cell reaction
  • apply an external voltage greater than the voltage of the cell to drive the electrons in the opposite direction
• rechargeable batteries:

  • lead-acid batteries used for the starter motor of cars
  • nickel-cadmium cells
  • lithium ion cells used for laptops/ smartphones
• advantages of the lithium ion cell:

  • it is light as lithium is the least dense metal
  • electrolyte is a solid polymer - can't leak as it isnt a paste or liquid
  • can be recharged at any time unlike other rechargeable batteries that can only be recharged efficiently when fully discharged
• hydrogen-oxygen fuel cell:

  A) NaOH electrolyte

  B) H2 in

  C) O2 in

  D) semi-permeable membrane

  E) porous platinum based electrodes

  
• the hydrogen-oxygen fuel cell has the same reaction as burning hydrogen and oxygen but the reaction in the fuel cell takes place at a lower temperature so no nitrogen oxides are formed
• hydrogen fuel cell:

  • used to generate electricity on spacecraft as the only by-product is water
  • terrestrial use is important as no CO2 is produced unlike other electrical energy sources
• the unreactive porous separator in non-rechargeable cells allows ions to pass through