Electrode potential and electrochemical cells

• What are the two components of an electrochemical cell?

  Two half-cells
• Why do the two half-cells need to be connected with a salt bridge?

  To complete the circuit and allow charge conduction

  
• What does a simple half-cell consist of?

  A metal and a solution of a compound containing that metal
• What happens when the two half-cells are connected into a circuit?

  They produce a small voltage
• What is the function of the salt bridge?

  To connect the circuit and conduct charge
• What material is typically used to make a salt bridge?

  A piece of filter paper soaked in a salt solution
• Why should the salt used in the salt bridge be unreactive with the electrodes?

  To prevent unwanted chemical reactions
• What is the reaction that occurs at the zinc half-cell?

  Zn → Zn$^{2+}$ + 2e$^{-}$
• What is created between the zinc and copper electrodes?

  A potential difference
• What is the symbol for the potential difference measured in the cell?

  E
• Why is a high resistance voltmeter used?

  To stop current from flowing and measure maximum potential difference
• What happens if current is allowed to flow in the circuit?

  The reactions occur separately at each electrode
• What is the most positive electrode expected to undergo?

  Reduction
• What is the half-equation for the reduction at the copper electrode?

  Cu$^{2+}$ (aq) + 2e$^{-}$ → Cu(s)
• What is the half-equation for the oxidation at the zinc electrode?

  Zn(s) → Zn$^{2+}$ (aq) + 2e$^{-}$
• What is the structure of a cell diagram for an electrochemical cell?

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

  
• What must be used if a system does not include a metal that can act as an electrode?

  A platinum electrode
• Why is a platinum electrode used in electrochemical cells?

  It is unreactive and can conduct electricity
• How is the hydrogen electrode represented in a cell diagram?

  Pt | H$_{2}$ (g) | H$^{+}$ (aq)
• What are the conditions for a standard hydrogen electrode?

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

  A standard electrode calibrated against the standard hydrogen electrode
• What is the standard electrode potential of the silver/silver chloride electrode?

  +0.22 V
• What is the standard electrode potential of the calomel electrode?

  +0.27 V
• What are the standard conditions for measuring standard electrode potentials?

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

  E$_{cell}$ = E$_{red}$ - E$_{ox}$
• What does a positive E$_{cell}$ indicate?


  A spontaneous change in the redox reaction
• How do you calculate the E$_{cell}$ for the cell Mg(s) | Mg$^{2+}$ (aq) || Cu$^{2+}$ (aq) | Cu(s)?


  E$_{cell}$ = 0.34 - (-2.37) = +2.71 V
• What does the more negative half-cell do in a redox reaction?

  It will oxidise
• What does the more positive half-cell do in a redox reaction?

  It will reduce
• What is the overall reaction when combining the half-reactions of Zn and Cu$^{2+}$?


  Zn + Cu$^{2+}$ → Cu + Zn$^{2+}$
• What happens to the oxidation state of hydrogen peroxide when it is reduced?

  It decreases
• What is the half-equation for the reduction of hydrogen peroxide?

  H$2$O$2$ (aq) + 2H$^{+}$ (aq) + 2e$^{-}$ → 2H$2$O(l)
• What is the half-equation for the oxidation of oxygen gas?

  O$2$ (g) + 4H$^{+}$ (aq) + 4e$^{-}$ → 2H$2$O(l)
• How do standard electrode potentials relate to redox reactions?

  • More positive E$_{o}$ indicates a tendency to reduce
  • More negative E$_{o}$ indicates a tendency to oxidise
  • Used to predict spontaneous reactions
• What is the standard electrode potential of fluorine gas?

  +2.87 V
• What is the standard electrode potential of chlorine gas?

  +1.36 V
• Why does fluorine react with water?

  Because it has a high standard electrode potential
• Why does chlorine undergo a redox reaction with water?

  Because it has a positive standard electrode potential
• What happens to the oxidation state of hydrogen in hydrogen peroxide when it is reduced?

  The oxidation state decreases
• What is the half-equation for the reduction of oxygen gas?

  O$2$ (g) + 4H$^{+}$ (aq) + 4e$^{-}$ → 2H$2$O(l)