14: Redox II
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Describe the terms ‘oxidation’ and ‘reduction’ in terms of electron transfer.
Oxidation is loss of electrons. Reduction is gain of electrons.
Describe the terms ‘oxidation’ and ‘reduction’ in terms of changes in oxidation number.
Oxidation is an increase in oxidation number. Reduction is a decrease in oxidation number.
Describe what is meant by the term ‘standard electrode potential’, E⦵
The potential obtained when a metal is placed in a solution of its ions under standard conditions.
State the conditions referred to by the standard electrode potential, E⦵.
298 K temperature, 100 kPa pressure of gases, 1.00 mol dm-3 concentration of ions
Describe the features of the standard hydrogen electrode.
Platinum electrode in solution of 1.00 mol dm-3 hydrogen ions. 100 kPa Hydrogen gas. 298 K temperature.
State why a reference electrode is necessary to determine standard electrode potentials.
Cannot measure the potential of an individual cell, so we need to measure potential difference.
Describe a method used to measure standard electrode potentials of metals or non-metals in contact with their ions in aqueous solution.
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.
Describe a method used to measure standard electrode potentials of ions of the same element with different oxidation numbers.
Inert platinum electrode in 1.00 mol dm-3 solution of ions.
State the equation to calculate a standard emf, E⦵cell.
E⦵cell = E⦵right - E⦵left
State how to predict the thermodynamic feasibility of a reaction using standard electrode potentials.
A reaction if thermodynamically feasible if E⦵cell is positive.
State the relationship between E⦵cell and the total entropy change.
Directly proportional
State the relationship between E⦵cell and ln(k). (k = equilibrium constant)
Directly Proportional
Describe the limitations of predictions made using standard electrode potentials.
• Indicates the energetic feasibility of a reaction, not the kinetics • Refers to standard conditions
Describe the application of electrode potentials to storage cells.
• Using energy from the cell discharges the cell. • Applying a potential to the cell can reverse the electron flow, recharging the cell.
State how the energy released on the reaction of a fuel with oxygen is utilised in a fuel cell.
To generate a voltage
Give an example of fuel used in fuel cells.
Methanol and other hydrogen-rich fuels
Give equations for the electrode reactions that occur in a hydrogen-oxygen fuel cell with acidic electrolytes.
H2 -> 2H+ + 2e- 1/2 O2 + 2H+ + 2e- -> H2O
Give equations for the electrode reactions that occur in a hydrogen-oxygen fuel cell with alkaline electrolytes.
H2 + 2OH- -> 2H2O + 2e- 1/2 O2 + H2O + 2e- -> 2OH-
Describe the method used to determine the concentration of reducing agents.
Titration with potassium manganate(VII) and H3PO4, End point: Solution stays pink (all the reducing agent has reacted)
Describe the method used to determine the concentration of oxidising agents.
Add excess iodide (produces I2), Titrate with thiosulphate and starch indicator, End point: Starch turns from blue to colourless (all the I2 has reacted)