🟩 TOPIC 14 - Redox II

Question 1

What is a salt bridge? Give an example.

Answer 1

A porous substance that connects two half-cells soaked with a solution of an inert (so doesn’t interfere with the components of the half cells), strong electrolyte (eg. Filter paper soaked in KNO3 (aq)). These salt ions flow through the bridge to complete the cell and keep the balance of positive and negative ions in each half cell without them mixing together (not interfering with the reaction)

Question 2

What is a half-cell?

Answer 2

Half of an electrolytic or voltaic cell where either oxidation or reduction occurs

Question 3

What does standard electrode potential mean?

Answer 3

(E^circle with line through)
The voltage produced produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions (which are 100kPa pressure of gases, 298K temperature, 1moldm^-3 concentration of ions in solution)
(They are connected by connecting the electrodes with wires and the half-cells with a salt bridge to allow ion movement)

Question 4

What is the standard hydrogen electrode? Describe what it’s made up of.

Answer 4

It is the measuring standard for half-cell potentials as it has a cell potential of 0.00V, measured under standard conditions.
The cell consists of hydrochloric acid solution, hydrogen gas and platinum electrodes. The hydrogen gas (1atm) is flowed over the (porous) platinum, which is immersed in a beaker of hydrochloric acid solution (1moldm^-3) (Platinum electrodes are chosen as they are metallic so will conduct electricity but inert so will not interfere with the reaction)

Question 5

What does a positive (or MORE positive) potential (E value) mean?

Answer 5

• substance (on left-hand side of half-equation) is more easily reduced and will gain electrons
• the +ve electrode
• EQUILIBRIUM POSITION is further to the right
• the forwards reaction (reduction reaction) is favoured (ie. the equation is NOT flipped)

Question 6

What does a negative (or MORE negative - may be +ve) potential (E value) mean?

Answer 6

• the substance (on right hand side of half-equation) is more easily oxidised and has greater tendency to lose electrons to become more stable
• the -ve electrode
• EQUILIBRIUM POSITION is further to the left (not necessarily on the left)
• the reverse reaction (oxidation reaction) is favoured (ie. the equation is flipped)

Question 7

Why can the salt bridge not be replaced by an unreactive metal wire?

Answer 7

The salt bridge allows the flow of ions but not the flow of electrons.
The wire does not allow the flow of ions, but allows the flow of electrons.
A flow of ions between the half-cells is needed to complete the circuit

Question 8

What does it mean if the overall cell potential (emf/ Ecell) is a positive value?

Answer 8

The reaction taking place is spontaneous and favourable / feasible (likely to occur)
(The more positive the potential, the more favourable the reaction)

Question 9

Why are the conditions important when measuring the standard cell potential?

Answer 9

Because changing the conditions will change the emf value obtained
(A reaction that is not thermodynamically feasible under standard conditions may become feasible when the conditions are altered (and v/v))

Question 10

How does increasing the concentration of the solutions used in the electrochemical cell affect the emf value obtained? Why?

Answer 10

It makes the cell emf more positive as fewer electrons are produced in the reaction
(Half-equations are in equilibria so changes in concentration will shift the position of equilibrium, which alters the electron transfer)

Question 11

How does increasing the pressure of the cell affect the cell emf? Why?

Answer 11

It makes the cell emf more negative as more electrons are produced.

Question 12

What two things is the standard emf of a cell (Ecell) directly proportional to?

Answer 12

• total entropy change (🔺Stotal) of the reaction —> if Ecell is positive, 🔺Stotal will be positive so reaction is thermodynamically feasible
• lnK (where K is the thermodynamic equilibrium constant of the reaction)

Question 13

Why may a reaction, in practice, not actually occur even when the emf value tells you it is feasible (ie. a +ve value)?

Answer 13

• It does not take into account the kinetics of the reaction (ie. high activation energies) - it may occur at such a slow rate that, in practice, it does not actually occur.
• Also in reality the system may not have standard conditions, but the standard cell potential relies on conditions being standard throughout
• (Also not all reactions occur in aqueous solutions)

Question 14

Why are the species on the right-hand side of half-cell reactions all capable of behaving as reducing agents?

Answer 14

Because they can lose electrons (they are themselves oxidised)

Question 15

Reactions generally occur if the redox system of the oxidising agent has a more positive E value that the redox system of the reducing agent. What other requirement generally has to be met for the reaction to take place?

Answer 15

The difference between E values should be large, preferably greater than 0.4V
(However if the difference is below 0.4V, a reaction may still occur if conditions are altered from standard conditions)

Question 16

What real life uses do electrochemical cells have?

Answer 16

They are a useful source of energy for commercial use - they can be produced to be non-rechargeable, rechargeable or fuel cells.

Question 17

What are fuel cells, and how do they work?

Answer 17

They are a type of electrochemical cell used to generate an electrical current without needing to be recharged.
In fuel cells, a fuel (usually hydrogen - hydrogen-oxygen fuel cell - but methanol and other hydrogen-rich fuels can be used instead) undergoes combustion in oxygen and the energy released is used to generate a voltage.
(Chemicals are stored separately outside the cell [externally] and fed in when electricity is required)

Question 18

What is the most common type of fuel cell, and how does it work?

Answer 18

The hydrogen fuel cell - uses a continuous supply of hydrogen (the fuel) and oxygen from the air to generate a continuous current. (The cell can operate as long as the fuel supply is maintained)

Question 19

Why is the hydrogen fuel cell seen as being relatively environmentally friendly, and what are the other advantages of hydrogen fuel cells?

Answer 19

• The reaction that takes place in a hydrogen fuel cell produces water as the only waste product (products do not include pollutants such as CO, CO2, NOx)
  Also,
• an alternative to direct use of fossil fuels such as petrol and diesel
• lighter and more efficient than engines that use fossil fuels (more efficient than just burning hydrogen)

Question 20

What are the disadvantages to hydrogen fuel cells? (Meaning they are not yet commonly used)

Answer 20

• high flammability of hydrogen + hydrogen is explosive, so great care has to be taken when transporting it
• expensive to produce (most hydrogen is produced from methane (a fossil fuel) which is a finite resource, so using a method with renewable sources is expensive)

Question 21

What is a storage cell?

Answer 21

(A battery/secondary cell) - A cell that can be RECHARGED when the current is passed in the opposite way to the current generated by a chemical reaction in the cell (ie. reverses the reaction that occurs)
Eg/ nickel-cadmium (nickel compound for +ve electrode, cadmium for -ve electrode and aqueous potassium hydroxide as the electrolyte), lead-acid batteries
(Essentially an electrochemical cell?)
- the emf (Ecell) is the voltage output of the cell

Question 22

Why is potassium manganate (KMnO4) a powerful oxidising agent, and what conditions are required and why?

Answer 22

The manganate (VII) ions (MnO4-) are readily reduced to form manganese ions (Mn2+) in acidic conditions (cannot be used in alkaline conditions as manganese (IV) oxide (MnO2) will form which is a brown precipitate which will interfere with the end point colour)

Question 23

In an iron (II) and potassium manganate titration, what is oxidised and what is reduced?

Answer 23

• the Fe2+ ions are oxidised (to Fe3+)
• the MnO4- (VII) ions are reduced (to Mn2+)

Question 24

In an iron (II) and potassium manganate titration, what is often being calculated, so which solutions go in which equipment?

Answer 24

A calculation can be carried out to find the concentration of iron in a solution:
- burette: KMnO4 (MnO4- ions) - known conc, unknown vol.
- conical flask: iron solution (Fe2+ ions) - unknown conc, known vol. [+ dilute sulphuric acid as acidic conditions are required]

Question 25

Describe what happens (in terms of colour changes) in an iron (II) and potassium manganate titration

Answer 25

• when the MnO4- is added from the burette to the conical flask, there will be an immediate colour change (purple —> colourless) as the Fe2+ is oxidised to Fe3+ and the MnO4- is reduced to Mn2+ (although these have colours (pale pink), at the dilution used, they give a colourless solution)
• after continuing to add the MnO4-, once all the Fe2+ ions have reacted, there will be an excess of MnO4- ions in the conical flask, so the colour change to colourless will stop and the solution will STAY purple/pink which is the end point (acts as it’s own indicator)

Question 26

In a thiosulfate and iodine titration, what is oxidised and what is reduced?

Answer 26

Thiosulfate ions are a reducing agent, so:
- iodine is reduced to iodide ions
- thiosulfate ions (S2O3^2-) are oxidised (to S4O6^2-)

Question 27

What is the indicator used in the thiosulfate and iodine titration, and how does it indicate the end-point?

Answer 27

• starch solution - with iodine (in the conical flask) it produces a deep blue-black colour, which disappears at the end-point as all the iodine has reacted

Question 28

Describe what happens (in terms of colour changes) in a thiosulfate and iodine titration

Answer 28

• add sodium thiosulfate solution from the burette to the iodine solution until the original brown colour of the iodine changes to pale yellow
• add a few drops of the starch solution to produce a blue/black colouration
• add the sodium thiosulfate solution drop by drop until the blue/black solution turns colourless

[to state a colour change remember to say what the initial colour was - _______ to _______]

Question 29

Why does the starch solution have to be added at a specific time in a thiosulfate and iodine titration?

Answer 29

• if the starch is added too early, it adsorbs some of the iodine and reduces the accuracy of the titration
• if starch was not added, it would be very difficult to detect the end point (the colour of the solution becomes very faint towards the end)

Question 30

What is iodometry?

Answer 30

The use of iodine to find the number of moles of another substance