Topic 14: Redox II

Question 1

Describe what happens in electrochemical cells

Answer 1

• made from two different metals dipped in salt solutions of their own ions and connected by a wire (the external circuit)
• it is a redox process because two reactions are occurring
• oxidation always happens at the anode (positive electrode) and reduction always happens at the cathode
• electrons flow from the anode to the cathode
• more reactive metal is oxidised, so it is the anode. Less reactive metal is reduced so becomes the cathode

Question 2

What happens in a zinc/copper electrochemical cell?

Answer 2

• zinc loses electrons more easily than copper, so it is oxidised to form Zn²⁺ ions
• Zn_(s) → Zn²⁺_(aq) + 2e-
• in the other half-cell, the same number of electrons are taken frm the external circuit, reducing the Cu²⁺ ions to copper atoms
• Cu²⁺_(aq) + 2e- → Cu_(s)

Question 3

What is cell potential or EMF, E_cell ?

Answer 3

• the maximum voltage produced by an electrochemical cell

Question 4

Why do we need a salt bridge?

Answer 4

• to complete the electrical circuit to ensure there is not a build up of charge at each electrode

Question 5

What can be used as a salt bridge?

Answer 5

• unreactive ions that do not form precipitates
• highly soluble
• potassium nitrate is used

Question 6

What is the electrode potential of a half cell?

Answer 6

• a measure of how easily the substance in the half-cell is oxidised

Question 7

What is the standard electrode potential, E^ø?

Answer 7

• the standard electrode potential, E^Ø , of a half-cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode

Question 8

What are the standard conditions?

Answer 8

• solutions of the ions must have a concentration of 1.00 mol dm^-3
• temperature must be 298K (25°C)
• pressure must be 100kPa

Question 9

What is the standard hydrogen electrode and why is it used?

Answer 9

• it is a reference electrode
• compare electrode potential of the half cell it is connected to as hydrogen half-cell has a value of 0.00V

Question 10

How do you work out the cell potential E^Ø ?

Answer 10

• E^ø_cell = (E^ø_reduction - E^ø_oxidation)
• cell potential will always be postive

Question 11

How do you draw the shorthand way of drawing electrochemical cells?

Answer 11

• half-cell with more negative potential goes on left
• oxidised forms go in the centre and reduced forms go on the outside
• double verticle lines show the salt bridge and single vertical lines separate species into different physical states
• comma separate species that are in the same half-cell and same physical state
• if platinum, lead or other inert electrodes are used, show these on the outside

Question 12

What chemicals have a more negative standard electrode potential and why?

Answer 12

• more reactive metals
• it lose electrons more easily to form a positive ion

Question 13

What chemicals have a more positive standard electrode potential?

Answer 13

• more reactive non-metals
• it gains electrons more easily to form a negative ion

Question 14

How do you know if a reaction is thermodynamically feasible?

Answer 14

• if the overall potential is positive

Question 15

Why are predictions about thermodynamic feasibility sometimes wrong?

Answer 15

• the conditions are not standard
• reaction kinetics are not favourable:
• the rate of reaction may be slow so that reaction may not appear to happen
• if a reaction has high activation energy, it may stop reaction from happening

Question 16

Give the two formulas which show the relationship between cell potential, entropy and equilibrium constant

Answer 16

Question 17

What are energy storage cells?

Answer 17

• batteries
• they work just like electrochemical cell s
• you can work out the voltage produced by these cells by using the electrode potentials of the substances used in the cell

Question 18

Explain the alkaline hydrogen-oxygen fuel cell

Answer 18

• chemicals are stored separately outside the cell and fed in when electricity is required
• H and O₂ gases are fed into two separate platinum-containing electrodes
• the electrodes are separated by an anion-exchange membrane that allows anions (OH-) and water to pass through it, but not H and O₂ gas
• the electrolyte is an aqueous alkaline (KOH) solution

Question 19

Describe hydrogen-oxygen fuel cells working in acidic conditions

Answer 19

• at the anode, the platinum catalyst splits the H₂ into protons and electrons
• the polymer electrolyte membrane (PEM) only allows the H+ across and this forces the e^- to travel around the circuit to get to the cathode
• an electric current is created in the circuit, which is used to power something
• at the cathode, O₂ combines with the H⁺ from the anode and the e^- from the circuit to make H₂O
• this is the only waste product

Question 20

What are some fuel cells that don’t just use hydrogen?

Answer 20

• hydrogen-rich fuels are being developed
• these have a high percentage of H in their molecules and can be converted into H₂ by a reformer
• these fuels include methanol and ethanol
• there is also a new generation that requires no reforming at all

Question 21

How do you perform a redox titration?

Answer 21

Question 22

Why don’t you need an indicator in some redox titrations?

Answer 22

• as transition metals change oxidation state, they also often change colour so it is easy to spot when reaction is finished
• a sharp colour change should be watched out
• do reaction in front of a white surface

Question 23

How do you calculate the concentration of a reagent from redox titration results?

Answer 23

Question 24

How do you calculate the percentage of iron in iron tablets?

Answer 24

Question 25

What are iodine-sodium thiosulfate titrations used for?

Answer 25

• they are a way to finding the concentration of an oxidising agent
• the more concentrated an oxidising agent is, the more ions will be oxidised by a certain volume of it

Question 26

How do you find the concentration of a solution of oxidising agent potassium iodate(V)?

Answer 26

Question 27

How can you use titration to find the percentage of copper in an alloy?

Answer 27