3.1.11 Electrode potentials and electrochemical cells

• Standard electrode potential, EƟ conditions

  • 298K
  • 100 kPa
  • 1.00 mol dm−3 solution of ions
  • Pt electrode
• what is the purpose of the salt bridge and what is it soaked in?

  To allow the flow of ions

  • KNO3
• voltmeter characteristics

  high resistance - to prevent current from flowing in the circuit.

  • if we let current flow in this cell the voltage would eventually fall to 0 as reactants get used up. Need to measure max possible voltage
• how do we allow current to flow or complete the circuit?

  replace high resistance voltmeter with a bulb
• what does I represent

  a change in phase or state
• why do we use a Pt electrode

  • unreactive
  • conducts electricity
• • cannot measure half cell on its own
  • have to measure the potential difference between 2 half cells
• suggest why a cell often leaks after being used a long time

  Zn has reacted / been used up
• what happens to E cell when you increase conc of reactant

  increase
• what happens to E cell when you increase temp

  decrease

  • if E cell = positive , feasible
• hydrogen electrode at equilibrium

  H2 (g) <---> 2H+ (aq) + 2e-
• Ecell = E rhs - E lhs

  Ecell = E red - E ox
• the more negative half cell will always...

  oxidise (go backwards)

  • electrons are lost and travel to positive electrode
• the more positive half cell will always

  reduce ( go forwards)

  • electrons arrive at this electrode and are gained
• the most powerful reducing agents found at

  the most negative end of series on the right
• the most powerful oxidising agents found at

  the most positive end of series on the left
• a reaction is more likely to occur if the E cell is

  positive
• if concentration of reactants increase, E cell would

  increase
• Fuel cell

  Uses the energy from the reaction of a fuel with oxygen to create a voltage
• Hydrogen fuel cell in alkaline conditions

  4 e- + 4 H2O —-> 2 H2 + 4 OH- (-0.83)

  4 e- + 2 H2O + O2 ——> 4 OH- (+0.4)

  2 H2 + O2 ——> 2 H2O (1.23)
• Fuel cells will maintain a constant voltage over time because they are continuously fed with O2 and H2 so it maintains the concentration of the reactants. This differs from ordinary cells where voltage drops as reactant concentration drops.
• Increasing rate in fuel cells

  higher temperatures are used to increase rate but the reaction is exothermic so E falls. A higher pressure can counteract this
• Advantages of fuel cells

  • less pollution and CO2. Pure hydrogen emits only water
  • Greater efficiency
• limitations of fuel cells

  • expensive
  • hard to store and transport hydrogen
  • Limited lifetime and high production costs
  • Use of toxical chemicals in their production
• Hydrogen source for fuel cells

  • readily available by the electrolysis of water
  • to be a green fuel the electricity needed would need to be produced from renewable resources.
• Hydrogen can be stored in fuel cells as:

  • a liquid under pressure
  • Adsorbed on the surface of a solid material
  • Absorbed within a solid material
• Ethanol fuel cell equations

  • oxygen electrode: 4 e- + 4 H+ + O2 ——> 2 H2O
  • Ethanol electrode: C2H5OH + 3H2O ——> 2 CO2 + 12H+ + 12e-
  • Overall equation: C2H5OH + 3 O2 ——> 2 CO2 + 3 H2O
• The simplified electrode reactions in a lithium cell:

  • Positive electrode: Li+ + CoO2 + e– → Li+[CoO2]-
  • Negative electrode: Li → Li+ + e–
• method for measuring the emf of an e cell?

  1. Clean the 2 metal foils with emery before use and degrease the metal using some cotton wool and propanone.
  2. Place one metal strip (X) into a 100 cm³ beaker with about 50 cm³ of a 1.0mol dm⁻³ X salt solution
  3. Place one metal strip (Y) into a 100 cm³ beaker with about 50 cm³ of a 1.0mol dm⁻³ Y salt solution
  4. Use a strip of filter paper soaked in saturated potassium nitrate solution for the salt bridge
  5. Connect the half- cells by connecting the metals using the crocodile clips and leads provided to the voltmeter
  6. Then record the emf value
• meaning of electrochemical series

  electrode potentials in numerical order
• describe a standard hydrogen electrode

  • H2 gas
  • 1.0 mol dm–3 H+
  • At 298K and 100kPa
  • Pt electrode
• explain the function of a salt bridge

  • The ions in the ionic substance in the salt bridge move through the salt bridge
  • To complete the circuit
• advantage of using hydrogen fuel cell

  • A fuel cell converts more of the available energy from combustion of hydrogen into kinetic energy of the car / an internal combustion engine wastes more (heat) energy
• deduce half equations for the electrode reactions in a hydrogen-oxygen fuel cell

  • H2 + 2OH- → 2H2O + 2e-
  • O2 + 4e- + 2H2O → 4OH-
• how is an electric current generated in a hydrogen-oxygen fuel cell? (2)

  • hydrogen electrode produces electrons
  • oxygen electrode accepts electrons
• explain why a fuel cell doesn't need to be charged (1)

  reactants supplied continuously
• Suggest the main advantage of using hydrogen in a fuel cell rather than in an internal combustion engine. (1)

  In the fuel cell, a greater proportion of the energy available from the hydrogen–oxygen reaction is converted into useful energy
• Solar cells generate an electric current from sunlight. These cells are often used to provide electrical energy for illuminated road signs. Explain why rechargeable cells are connected to these solar cells. (2)

  • Solar cells do not supply electrical energy all the time
  • Rechargeable cells can store electrical energy for use when the solar cells are not working
• Suggest one reason why many waste disposal centres contain a separate section for cells and batteries.

  • Prevent pollution of the environment by toxic or dangerous substances
  • recycling of valuable components
• Explain why the current in the external circuit of this cell falls to zero after the cell has operated for some time.

  Eventually the ions in each electrode will be at the same concentration