14: Redox II

Question 1

Describe the terms ‘oxidation’ and ‘reduction’ in terms of electron transfer.

Answer 1

Oxidation is loss of electrons.
Reduction is gain of electrons.

Question 2

Describe the terms ‘oxidation’ and ‘reduction’ in terms of changes in oxidation number.

Answer 2

Oxidation is an increase in oxidation number.
Reduction is a decrease in oxidation number.

Question 3

Describe what is meant by the term ‘standard electrode potential’, E^⦵

Answer 3

The potential obtained when a metal is placed in a solution of its ions under standard conditions.

Question 4

State the conditions referred to by the standard electrode potential, E^⦵.

Answer 4

298 K temperature,
100 kPa pressure of gases,
1.00 mol dm^-3 concentration of ions

Question 5

Describe the features of the standard hydrogen electrode.

Answer 5

Platinum electrode in solution of 1.00 mol dm^-3 hydrogen ions.
100 kPa Hydrogen gas.
298 K temperature.

Question 6

State why a reference electrode is necessary to determine standard electrode potentials.

Answer 6

Cannot measure the potential of an individual cell, so we need to measure potential difference.

Question 7

Describe a method used to measure standard electrode potentials of metals or non-metals in contact with their ions in aqueous solution.

Answer 7

Solid electrodes in 1.00 mol dm^-3 solutions of ions.
Electrodes connected with a high resistance voltmeter.
Solutions connected with a salt bridge of KNO₃.

Question 8

Describe a method used to measure standard electrode potentials of ions of the same element with different oxidation numbers.

Answer 8

Inert platinum electrode in 1.00 mol dm^-3 solution of ions.

Question 9

State the equation to calculate a standard emf, E^⦵_cell.

Answer 9

E^⦵_cell = E^⦵_right - E^⦵_left

Question 10

State how to predict the thermodynamic feasibility of a reaction using standard electrode potentials.

Answer 10

A reaction if thermodynamically feasible if E^⦵_cell is positive.

Question 11

State the relationship between E^⦵_cell and the total entropy change.

Answer 11

Directly proportional

Question 12

State the relationship between E^⦵_cell and ln(k).
(k = equilibrium constant)

Answer 12

Directly Proportional

Question 13

Describe the limitations of predictions made using standard electrode potentials.

Answer 13

• Indicates the energetic feasibility of a reaction, not the kinetics
• Refers to standard conditions

Question 14

Describe the application of electrode potentials to storage cells.

Answer 14

• Using energy from the cell discharges the cell.
• Applying a potential to the cell can reverse the electron flow, recharging the cell.

Question 15

State how the energy released on the reaction of a fuel with oxygen is utilised in a fuel cell.

Answer 15

To generate a voltage

Question 16

Give an example of fuel used in fuel cells.

Answer 16

Methanol and other hydrogen-rich fuels

Question 17

Give equations for the electrode reactions that occur in a hydrogen-oxygen fuel cell with acidic electrolytes.

Answer 17

H₂ -> 2H⁺ + 2e^-

1/2 O₂ + 2H⁺ + 2e^- -> H₂O

Question 18

Give equations for the electrode reactions that occur in a hydrogen-oxygen fuel cell with alkaline electrolytes.

Answer 18

H₂ + 2OH^- -> 2H₂O + 2e^-

1/2 O₂ + H₂O + 2e^- -> 2OH^-

Question 19

Describe the method used to determine the concentration of reducing agents.

Answer 19

Titration with potassium manganate(VII) and H₃PO₄,
End point: Solution stays pink (all the reducing agent has reacted)

Question 20

Describe the method used to determine the concentration of oxidising agents.

Answer 20

Add excess iodide (produces I₂),
Titrate with thiosulphate and starch indicator,
End point: Starch turns from blue to colourless (all the I₂ has reacted)