Redox 2

Question 1

What is oxidation?

Answer 1

Loss of electrons, increase in oxidation number

Question 2

What is reduction?

Answer 2

Gain of electrons, decrease in oxidation number

Question 3

What is oxidation number?

Answer 3

Shows the total number of electrons which have been removed or added to an element

Question 4

What are standard conditions?

Answer 4

298K, 100 kPa, 1.0 moldm^-3

Question 5

What is a standard hydrogen electrode?

Answer 5

A redox electrode which all other electrode potentials are measured relative to at 298K. It contains HCl at 1.0 moldm^-3, hydrogen gas at 100kPa and a solid platinum electrode

Question 6

What is an electrochemical cell?

Answer 6

A device that that creates an electrical current from a reaction. Consists of 2 half cells; electrons flow from the negative one to the positive one through an external circuit

Question 7

What is a cell diagram?

Answer 7

A shorthand way of representing an electrochemical cell. The parts of each half cell are separated by solid lines and the half cells are separated by 2 dotted lines

Question 8

What is a half cell?

Answer 8

One half of an electrochemical cell. Either negative (oxidation takes place) or positive (reduction takes place). The two half cells are linked by a salt bridge

Question 9

What is a salt bridge?

Answer 9

A piece of filter paper, soaked in a soluble salt (like potassium nitrate) that is used to complete the circuit in an electrochemical cell

Question 10

What is standard electrode potential?

Answer 10

Potential of a cell acting as a cathode and the standard hydrogen electrode acting as an anode. Reduction always takes place at the cathode, and oxidation at the anode

Question 11

What is cell potential?

Answer 11

The cell potential is the way in which we can measure how much voltage exists between the two half cells - the emf that electrons feel

Question 12

What is thermodynamic feasibility?

Answer 12

A reaction that is thermodynamically favourable. A feasible reaction will occur spontaneously - without continuous input of energy

Question 13

What is kinetic stability?

Answer 13

Reactants are kinetically stable if there is a high activation energy preventing them from reacting i.e. reactions can be thermodynamically feasible but not actually ever happen

Question 14

What is disproportionation?

Answer 14

A reaction in which an element in a species is simultaneously oxidised and reduced in the same reaction

Question 15

What is a fuel cell?

Answer 15

The energy released on reaction of a fuel with oxygen is utilised in a fuel cell to generate a voltage (e.g. a hydrogen-oxygen fuel cell (2H2 + O2 -> 2H2O))

Question 16

What is a redox titration?

Answer 16

The concentration of a reaction can be calculated from the results of a redox titration. The number of moles of electrons involved must be taken into account

Question 17

What does a more positive value for standard electrode potential indicate?

Answer 17

Reduction is more likely to happen

Question 18

What does a more negative value for standard electrode potential indicate?

Answer 18

Oxidation is more likely to happen

Question 19

Why is a platinum electrode used in the standard hydrogen electrode?

Answer 19

It is unreactive and it acts as a connection to complete the circuit

Question 20

What are the 3 possibilities for electrode setups?

Answer 20

• Metals in contact with their ions in aqueous solution (the metal itself is a solid so it can be used as a connection to the external circuit)
• Non-metals in contact with their ions in aqueous solution (no solids present so platinum must be used as a connection to the external circuit)
• Ions of the same element with different oxidation numbers (no solids present so platinum must be used as a connection to the external circuit)

Question 21

Why is a high resistance voltmeter used in electrochemical cells?

Answer 21

It stops the flow of current. If no electrons can flow between the electrodes, no reaction can happen but we measure the emf that is trying to push the electrons

Question 22

Why is a salt bridge used in electrochemical cells?

Answer 22

Ions move through the paper to complete the circuit and it is usually filter paper soaked in potassium or sodium nitrate

Question 23

What are the rules for half cell notation?

Answer 23

• Two dashed vertical lines are used to represent the salt bridge/external circuit between the electrodes
• Single vertical lines represent phase boundaries
• The most reduced species are placed on the outside, furthest from the 2 dashed lines

Question 24

What important things is Ecell directly proportional to?

Answer 24

ΔStotal and lnK

Question 25

Why might a reaction have a positive Ecell value but not actually happen?

Answer 25

- Reactants could be kinetically stable, so a high activation energy prevents the reaction from happening - The standard electrode potentials used to calculate Ecell are measured under standard conditions, so a reaction may not happen under standard conditions which would make the prediction inaccurate

Question 26

Why are hydrogen-oxygen fuel cells not used a lot?

Answer 26

They can be hard to store and are flammable

Question 27

What are the reactions of an acidic hydrogen-oxygen fuel cell?

Answer 27

2H^+ (aq) + 2e^- -> H2 (g) 4H^+ (aq) + O2 (g) + 4e^- -> 2H2O (l) Overall reaction becomes O2 + 2H2 -> 2H2O

Question 28

What are the reactions of an alkaline hydrogen-oxygen fuel cell?

Answer 28

4H2O (l) + 4e^- -> 4OH^- (aq) + 2H2 (g) 2H2O (l) + O2 (g) + 4e^- -> 4 Oh^- Overall reaction becomes O2 + 2H2 -> 2H2O

Question 29

Why can methanol fuel cells be preferable to hydrogen-oxygen fuel cells?

Answer 29

Methanol is liquid at room temperature so it's easier to store

Question 30

What is the overall reaction for a methanol fuel cell?

Answer 30

CH3OH + 1.5O2 -> CO2 + 2H2O

Question 31

Using acidified potassium manganate in redox titrations (with Fe^2+ as an example)

Answer 31

KMnO4 added from the burette will decolourise as it is added. When the end point of the titration is reached with the first drop of excess KMnO4, the purple colour will remain if there is no Fe^2+ left to react with it

Question 32

Using sodium thiosulphate in redox titrations

Answer 32

Typically used to find iodine concentration. Thiosulphate ions reduce iodine into iodide ions 2S2O3^2- -> S4O6^2- + 2e^- I2 + 2e^- -> 2I^- Overall reaction: 2S2O3^2- + I2 -> S4O6^2- + 2I^- A starch indicator is added when the titration is close to its end point, otherwise an irreversible blue-black complex forms and the titration never ends. The titration is complete when the blue-black colour of the starch-iodine complex disappears because all of the iodine has been converted into iodine