Redox + Electrode Potentials

• What is an oxidising agent?

  A species that is reduced in a reaction
• How does an oxidising agent function in a reaction?

  It causes another species to be oxidised
• What is a reducing agent?

  A species that is oxidised in a reaction
• What role does a reducing agent play in a reaction?

  It causes another species to be reduced
• What is oxidation defined as?

  Loss of electrons
• What does an increase in oxidation number indicate?

  It indicates oxidation has occurred
• What is reduction defined as?

  Gain of electrons
• What does a decrease in oxidation number signify?

  It signifies reduction has occurred
• What occurs in a redox reaction?

  Electrons are transferred between species
• What happens to one element in a redox reaction?

  One element is oxidised while another is reduced
• Why is the reaction 2CrO4$^{2-}$ + 2H$^{+}$ → Cr2O7$^{2-}$ + H2O not a redox reaction?


  Hydrogen remains the same oxidation state
• What is the definition of standard electrode potential?

  The e.m.f. of a half cell compared to hydrogen
• Under what conditions is standard electrode potential measured?

  At 298 K, 1 mol dm-3, 100 kPa
• What happens when a metal rod is dipped into its own ion solution?

  An equilibrium is set up between metal and ions
• What is the half-equation for zinc (s) to zinc (II)?

  Zn (s) ⇌ Zn$^{2+}$ (aq) + 2e-
• What is the half-equation for copper (II) to copper (III)?

  Cu$^{2+}$ (aq) ⇌ Cu$^{3+}$ (aq) + e-
• What are the components of a standard hydrogen half cell?

  • Hydrochloric acid 1 mol dm-3
  • Hydrogen gas at 100 kPa
  • Inert platinum electrode
• Why is the hydrogen half cell used as a standard half cell?

  It is easy to control its purity
• How do you make a simple salt bridge?

  Soak filter paper in KNO3 or NH4NO3
• Why are salt bridges necessary in electrochemical cells?

  They complete the circuit between half cells
• What is the purpose of drawing a standard hydrogen half cell?

  To illustrate the standard reference conditions
• Why might other standard electrodes be used occasionally?

  They are cheaper and easier to use
• What does a more negative E$^{\circ}$ value indicate?


  It indicates a better reducing agent
• What does a more positive E$^{\circ}$ value indicate?


  It indicates a better oxidising agent
• How do you calculate the emf of a cell from E$^{\circ}$ values?


  E$^{\circ}$cell = E$^{\circ}$ positive - E$^{\circ}$ negative
• When would you use a platinum electrode?

  When both forms are in aqueous solution
• Why is platinum chosen for electrodes?

  It is inert and a good conductor
• How would you predict if a reaction would occur?

  By comparing standard electrode potentials
• How do you calculate the emf of a cell from Eo values?

  Eo cell = Eo positive - Eo negative
• When would you use a Platinum electrode?

  When both oxidised and reduced forms are in solution
• How would you predict if a reaction would occur?

  Calculate Eo value; if > 0.4V, it occurs
• What are the 3 main types of electrochemical cells?

  • Non rechargeable cells
  • Rechargeable cells
  • Fuel cells
• Describe how non rechargeable cells work.

  They provide energy until chemicals are used up
• Describe how rechargeable cells work.

  Reactions can be reversed when recharging
• Give some examples of rechargeable cells.

  • Nickel and cadmium batteries
  • Lithium ion batteries
  • Lithium polymer batteries
• Explain why lithium is used in laptop batteries.

  Lithium is light and very reactive
• What are the drawbacks of using lithium batteries?

  They are toxic and can cause fires
• Describe how fuel cells work.

  They use external fuel and oxidant supplies

  
• What are modern fuel cells based on?

  • Hydrogen
  • Hydrogen rich fuels (e.g., methanol)
• What are the reactions in an alkaline hydrogen fuel cell?

  2H2 + 4OH- → 4H2O + 4e-