Physical 2 Section 1 - Unit 20: Electrode potentials and electrochemical cells
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Explain the function of a salt bridge (2 marks)
• The ions in the ionic substance in the salt bridge move through the salt bridge • To maintain charge balance / complete the circuit
Does the weakest reducing agent have a high or low EMF (1)
High
Does the weakest oxidising agent have a high or low EMF (1)
Low
State the advantage of using hydrogen in a fuel cell rather than hydrogen in an internal combustion engine to power an electric motor (1 mark)
• A fuel cell converts more of the available energy from combustion of hydrogen into kinetic energy of the car OR • An internal combustion energy wastes more heat energy
Explain why the metal used as an electrode in an electrochemical cell is rubbed before use (1 mark)
To remove the oxide layer formed on the metal
Explain why the current in an external circuit falls to zero after the cell has operated for some time (1 mark)
• Eventually the ions in each electrode will be at the same concentration
Suggest why the recharging of a lithium cell may lead to release of carbon dioxide into the atmosphere (1 mark)
Electricity for recharging the cell may come from power stations burning (fossil) fuel
State the half equations for the electrode reactions in a hydrogen-oxygen fuel cell (2 marks)
• H₂ + 2OH- → 2H₂O + 2e- & • O₂ + 4e- + 2H₂O → 4OH-
Explain why a fuel cell does not need to be recharged (1 mark)
• The reactants are continuously supplied
Suggest one major hazard associated with the use of a hydrogen–oxygen fuel cell in a vehicle (1 mark)
• Hydrogen is explosive
Explain why rechargeable cells are often connected to solar cells (2 marks)
• 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 (1 marks)
• To prevent pollution of the environment by toxic or dangerous substances OR • To recycle valuable components
Suggest why platinum electrodes are sometimes used (2)
• It is unreactive - Can conduct electricity so provides a conductive surface for electron transfer
Standard conditions in a standard hydrogen electrode (4)
• Platinum electrode • 298K • 100kPa • 1.0 mol dm-3
+ve standard electrode potential
• More likely to be reduced - Better oxidising agents
-ve standard electrode potential
• More likely to be oxidised - Better reducing agents
Effect of concentration on Ecell (1)
As concentration increases, Ecell increases
Effect of temperature on Ecell (1)
• Most cells are exothermic so increasing temperature decreases Ecell
How would you determine the feasibility of an electrochemical cell (1)
Feasible = +ve Ecell
Define fuel cells (1)
• Use the energy from the reaction of a fuel with oxygen to create a voltage
State the half equations for the electrode reactions in a hydrogen-oxygen fuel cell (2 marks)
• H₂ + 2OH- → 2H₂O + 2e- - O₂ + 4e- + 2H₂O → 4OH-
State the overall reaction that occurs in a hydrogen-oxygen fuel cell (1 mark)
• 2H₂ + O₂ → 2H₂O
State why the electrode potential for the standard hydrogen electrode is equal to 0.00V (1 mark)
• Hydrogen only has one electron so it is assumed that there is not • By definition
Why do fuel cells maintain a constant voltage over time (2)
• They are continuously fed with fresh oxygen and hydrogen - So the concentration of reactants is maintained