Physical 2 Section 1 - Unit 20: Electrode potentials and electrochemical cells

Question 1

Explain the function of a salt bridge (2 marks)

Answer 1

• The ions in the ionic substance in the salt bridge move through the salt bridge
• To maintain charge balance / complete the circuit

Question 2

Does the weakest reducing agent have a high or low EMF (1)

Answer 2

High

Question 3

Does the weakest oxidising agent have a high or low EMF (1)

Answer 3

Low

Question 4

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)

Answer 4

• 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

Question 5

Explain why the metal used as an electrode in an electrochemical cell is rubbed before use (1 mark)

Answer 5

To remove the oxide layer formed on the metal

Question 6

Explain why the current in an external circuit falls to zero after the cell has operated for some time (1 mark)

Answer 6

• Eventually the ions in each electrode will be at the same concentration

Question 7

Suggest why the recharging of a lithium cell may lead to release of carbon dioxide into the atmosphere (1 mark)

Answer 7

Electricity for recharging the cell may come from power stations burning (fossil) fuel

Question 8

State the half equations for the electrode reactions in a hydrogen-oxygen fuel cell (2 marks)

Answer 8

• H₂ + 2OH- → 2H₂O + 2e-
  &
• O₂ + 4e- + 2H₂O → 4OH-

Question 9

Explain why a fuel cell does not need to be recharged (1 mark)

Answer 9

• The reactants are continuously supplied

Question 10

Suggest one major hazard associated with the use of a hydrogen–oxygen fuel cell in a vehicle (1 mark)

Answer 10

• Hydrogen is explosive

Question 11

Explain why rechargeable cells are often connected to solar cells (2 marks)

Answer 11

• 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

Question 12

Suggest one reason why many waste disposal centres contain a separate section for cells and batteries (1 marks)

Answer 12

• To prevent pollution of the environment by toxic or dangerous substances
  OR
• To recycle valuable components

Question 13

Suggest why platinum electrodes are sometimes used (2)

Answer 13

• It is unreactive

- Can conduct electricity so provides a conductive surface for electron transfer

Question 14

Standard conditions in a standard hydrogen electrode (4)

Answer 14

• Platinum electrode
• 298K
• 100kPa
• 1.0 mol dm-3

Question 15

+ve standard electrode potential

Answer 15

• More likely to be reduced

- Better oxidising agents

Question 16

-ve standard electrode potential

Answer 16

• More likely to be oxidised

- Better reducing agents

Question 17

Effect of concentration on Ecell (1)

Answer 17

As concentration increases, Ecell increases

Question 18

Effect of temperature on Ecell (1)

Answer 18

• Most cells are exothermic so increasing temperature decreases Ecell

Question 19

How would you determine the feasibility of an electrochemical cell (1)

Answer 19

Feasible = +ve Ecell

Question 20

Define fuel cells (1)

Answer 20

• Use the energy from the reaction of a fuel with oxygen to create a voltage

Question 21

State the half equations for the electrode reactions in a hydrogen-oxygen fuel cell (2 marks)

Answer 21

• H₂ + 2OH- → 2H₂O + 2e-

- O₂ + 4e- + 2H₂O → 4OH-

Question 22

State the overall reaction that occurs in a hydrogen-oxygen fuel cell (1 mark)

Answer 22

• 2H₂ + O₂ → 2H₂O

Question 23

State why the electrode potential for the standard hydrogen electrode is equal to 0.00V (1 mark)

Answer 23

• Hydrogen only has one electron so it is assumed that there is not
• By definition

Question 24

Why do fuel cells maintain a constant voltage over time (2)

Answer 24

• They are continuously fed with fresh oxygen and hydrogen

- So the concentration of reactants is maintained

Question 25

Advantages of Fuel cells (3)

Answer 25

• Less pollution and CO₂
• Only water is emitted
• Greater efficiency

Question 26

Limitations of Hydrogen Fuel cells (7)

Answer 26

• Expensive
• Hydrogen has a low density so it is expensive to transport
• Transporting a pressurised liquid is unfeasible
• Limited life of cell
• Limited life cycle of a solid absorber
• High production costs
• Toxic chemicals used in production

Question 27

State the half equations for the electrode reactions in a lithium rechargeable cell (2 marks)

Answer 27

• Li → Li+ + e-

- Li+ + CoO₂ + e- →Li+(CoO₂)-

Question 28

Define electrochemical series (1 mark

Answer 28

List of electrode potentials in order (or half cells in order of electrode potential)