Electrode Potentials

• What are the two components of an electrochemical cell?

  Two half-cells
• Why must the two half-cells be connected with a salt bridge?

  To complete the circuit and allow ions to flow

  
• What materials are typically used to make a salt bridge?

  Filter paper soaked in salt solution
• Why should the salt in a salt bridge be unreactive with the electrodes?

  To avoid forming complexes with ions
• What is the reaction that occurs in the zinc half-cell?

  Zn → Zn$^{2+} +$$2e^{-}$
• What does a potential difference between electrodes indicate?

  There is a voltage created
• Why is a high resistance voltmeter used?

  To measure maximum potential difference
• What happens when current flows in the circuit?

  The reactions occur at each electrode
• What is the most positive electrode's role in a cell?

  It undergoes reduction
• What is the most negative electrode's role in a cell?

  It undergoes oxidation
• What is the structure of a cell diagram for electrochemical cells?

  • Oxidised form next to double line
  • Solid vertical line for phase boundaries
  • Double line for salt bridge
  • Voltage produced indicated
  • More positive half-cell on the right

  
• What must be used if a system does not include a metal electrode?

  A platinum electrode must be used
• What is the equilibrium reaction for the standard hydrogen electrode?

  H$_{2}(g) ⇌ 2H^{+}(aq) +$$2e^{-}$
• What conditions are required for a standard hydrogen electrode?

  100 kPa, 1 mol dm$^{-3}$, 298K
• What are the components of a standard hydrogen electrode?

  • Hydrogen gas at 100 kPa
  • 1.0 mol dm$^{-3}$ H+ solution
  • Temperature at 298K
  • Platinum electrode
• What is a secondary standard in electrochemistry?

  A standard calibrated against the SHE
• What are common secondary standards?

  Silver/silver chloride and calomel electrode
• What happens to the redox equilibrium with concentration changes?

  It shifts towards the more concentrated side
• What are the standard conditions for measuring electrode potentials?

  • All ion solutions at 1 mol dm$^{-3}$
  • Temperature at 298 K
  • Gases at 100 kPa pressure
  • No current flowing
• How do you calculate the EMF of a cell?

  Ecell = Ered - Eox
• What does a spontaneous change in a redox reaction indicate?

  It has a positive Ecell
• How do you determine which half-cell will oxidize or reduce?

  More negative half-cell oxidizes, positive reduces
• What is the overall reaction when combining half-reactions?

  Add half-reactions, cancel electrons
• What is the trend in standard electrode potentials?

  • More positive E indicates stronger oxidizing agents
  • More negative E indicates stronger reducing agents
• What happens to hydrogen peroxide when it is reduced?

  It forms water
• What is the reaction of fluorine with water?

  Fluorine reacts to form oxygen and hydrogen ions
• What is the reaction of chlorine with water?

  Chlorine undergoes a redox reaction with water
• What is the significance of the half-equations in redox reactions?

  They show oxidation states changes
• What is the formula to calculate Ecell from standard electrode potentials?

  Ecell = Ered – Eox
• What is the standard electrode potential for the reaction of O2 and H2O?

  Eo +1.23V
• Why does fluorine react with water?

  Fluorine has a higher standard electrode potential
• Write the equation for the reaction of fluorine with water.

  2F2(g) + 2H2O(I) → 4F–(aq) + O2(g) + 4H+(aq)
• Why should chlorine undergo a redox reaction with water?

  Chlorine has a higher standard electrode potential
• Write the equation for the reaction of chlorine with water.

  2Cl2(g) + 2H2O(I) → 4Cl–(aq) + O2(g) + 4H+(aq)
• What happens to hydrogen peroxide when it is reduced?

  It is converted to water
• Write the half-equation for the reduction of hydrogen peroxide.

  H2O2(aq) + 2H+(aq) + 2e– → 2H2O(I)
• How does the standard electrode potential of H2O2 compare to O2?

  H2O2 has a higher standard electrode potential
• What is the overall reaction when hydrogen gas is bubbled into iron(II) and iron(III) ions?

  Fe3+ is reduced to Fe2+
• Write the equation for the reaction of hydrogen gas with iron ions.

  2Fe3+ (aq) + H2(g) → 2Fe2+ (aq) + 2H+(aq)
• What does it mean for H2O2 to disproportionate?

  It changes oxidation states during the reaction