Electrode potentials and cells

• What are the advantages of fuel cells?

  More efficient than internal combustion engines, as they convert more available energy into kinetic energy to get car moving, water is only waste product (no toxic chemicals need to be disposed of and no C02 emissions) and they don't need to be recharged like batteries do.
• What are the disadvantages of fuel cells?

  Need energy to produce supply of H2, so process not carbon neutral, H2 highly flammable so need to handle carefully when stored or transported, and infrastructure to provide hydrogen fuel for cars doesn't exist on large scale yet, so refuelling stations very rare.
• Name a metal as a suitable electrode in half-cells involving solutions of 2 aqueous ions of the same element?


  Platinum, as it is inert and can conduct electricity.
• What is the standard electrode potential of a half-cell?

  Voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.
• Standard conditions used when measuring electrode potentials?


  Pressure 100kPa, 298 K, solutions of ions have conc. 1.00 mol dm-3.
• How are standard hydrogen electrodes set up?

  Hydrogen gas bubbled through solution of aqueous H+ ions. Platinum electrode platform for reactions.
• What 3 factors can influence electrode potentials?

  Changes in temperature, pressure and concentration.
• What does an electrochemical cell look like?

  2 different metals dipped in salt solutions of their own ions and connected by a wire (external circuit). Redox process within cell. Solutions connected with salt bridge. Electrons flow from most reactive metal to least.
• What is EMF?

  Cell potential (voltage between 2 half-cells). Use voltmeter in external circuit to measure this. Cell potential = reduced - oxidised.
• When is a reaction feasible?

  When EMF has a positive value.
• What is an electrochemical series?

  List of electrode potentials for different electrochemical half-cells. In numerical order from most negative to most positive.
• Suitable salt bridge:


  Strip of filter paper soaked in KNO3
• In a conventional representation, where do oxidised forms go?

  In the middle of the diagram
• What is a non-rechargeable cell?

  Uses irreversible reactions. E.g, dry cell alkaline battery. Not practical to reverse reactions- can be made to run backwards under right conditions, but battery can explode or leak. (Zn electrode forms casing of battery, so becomes thinner as Zn is oxidised).
• What are rechargeable cells?

  Use reversible reactions. Current supplied to force electrons to flow in opposite direction around circuit & reverse reactions. Possible as none of substances in rechargeable battery escape or are used up.
• What is a fuel cell?

  Chemicals stored separately outside cell & fed in when electricity required.
• How does an alkaline hydrogen-oxygen fuel cell work?


  H2 & O2 fed into 2 separate platinum-containing electrodes. (Made by coating a porous ceramic material with thin layer platinum- cheaper than solid platinum rods & larger surface area for reactions). Electrodes separated by anion-exchange membrane that allows anions (OH-) & water to pass through it, but stops H2 & O2. Aqueous alkaline (KOH) solution= electrolyte. H2 fed to negative electrode.
• Overall equation in alkaline fuel cell:

  2H2 + O2 -> 2H2O
• What happens in an alkaline hydrogen-oxygen fuel cell?

  O2 in positive electrode reacts with H20 & electrons to form OH-. They cross anion-exchange membrane & travel though electrolyte to negative electrode & react with H2 to form H20 & electrons. Electrons travel round external circuit towards positive electrode, generating electricity.