3.1.11 Electrode potentials and electrochemical cells

Question 1

what is meant by ‘standard electrode potential’?

Answer 1

the voltage meausred under standard conditions when a half-cell is connected to SHE (standard hydrogen electrode)

Question 2

how do you calculate standard cell potential? (Eocell)

Answer 2

Eocell = Eo(reduction)-Eo(oxidation)

it is ALWAYS a positive value

Question 3

how do you draw a standard electrode potential diagram (both solids)?

Answer 3

• draw 2 poles showing each element; each pole should be dipped in an aqueous solution with the same ion
• external wire connects the 2 poles with a high resistance voltmeter in between
• salt bridge dipped in both solutions

Question 4

how do you draw a standard electrode potential diagram (when it is both aqueous)?

Answer 4

• external wire is connected to a THIN pole of platinum (Pt)
• dipped in an equimolar solution of both ions e.g. Fe(2+) and Fe(3+)

Question 5

how do you display cell conventions?
e.g. Sn(s) -> Sn2+(aq) + 2e-
Ag+(aq) + e- -> Ag(s)

Answer 5

Sn(s)|Sn2+(aq)||Ag+(aq)|Ag(s)
- the left shows oxidation; the right shows reduction

Question 6

how to tell which species is oxidised or reduced using standard electrode potential values?

Answer 6

• the smaller value gets oxidised; the larger value gets reduced

Question 7

which side does the oxidised species go on the diagram?

Answer 7

left

Question 8

which side does the reduced species go on the diagram?

Answer 8

right

Question 9

what are the standard conditions for SHE (standard hydrogen electrode)?

Answer 9

• temperature - 298K
• concentration - 1moldm(-3)
• pressure - 1atm/100kPa

Question 10

how do you predict the feasibility of a reaction?

Answer 10

if the standard cell potential is negative, the reaction is NOT feasible

Question 11

what is meant by ‘oxidising agent’ in terms of electrons?

Answer 11

electron acceptor

Question 12

what is meantby ‘reducing agent’ in terms of electrons?

Answer 12

electron donor

Question 13

what are the characteristics of a non-rechargeable cell?

Answer 13

• irreversible reaction
• battery is flat when all the reactants have been used up

Question 14

give an example of a non-rechargeable battery

Answer 14

electrochemical cell

Question 15

what are the characteristics of a rechargeable battery?

Answer 15

• reversible reaction
• when the battery is flat, electricity is supplied to force the electrons to move in the opposite direction so the reaction is reversed

Question 16

what are the characteristics of fuel cells?

Answer 16

chemicals are stored separately outside of the cell and is fed into the cell when electricity is required

Question 17

what is the general equation of a lithium battery (rechargable)?

Answer 17

Li(s) + CoO2(aq) -> Li+[CoO2]-(aq)

Question 18

what is meant by ‘electrolyte’?

Answer 18

a lithium salt in an organic solvent

Question 19

why aren’t aqueous electrolytes used?

Answer 19

Li is a powerful reducing agent and reduces water to OH- ions, resulting in an alkaline solution

Question 20

overall reaction between lithium and water?

Answer 20

H2O + Li -> Li + H2 + 2OH-

Question 21

how does an ‘alkaline-hydrogen-oxygen’ cell work?

Answer 21

• H2 is fed into the anode (oxidation occurs) - H2 -> 2H+ + 2e-
• O2 is fed into the cathode (reduction occurs) - O2 + 4H+ + 4e- -> 2H2O
• electrons pass through the external circuit from the anode to the cathode
• an ion exchange membrane allows smaller size ions e.g. H+, OH- but not H2O and H2

Question 22

what are the uses of non-rechargeable cells?

Answer 22

doorbells, torches

Question 23

what are the uses of rechargeable cells?

Answer 23

phones, laptop, other mobile devices

Question 24

what are the uses of fuel cells?

Answer 24

electric cars, spacecraft

Question 25

what are the advantages of non-rechargeable cells?

Answer 25

• cheap
• easy to use + portable

Question 26

what are the disadvantages of non-rechargeable cells?

Answer 26

• casing eventually dissolves and leaks acidic ammonium chloride paste
• made from finite materials; unsustainable
• toxic chemicals can leach from landfill sites to water sources after disposal

Question 27

what are the advantages of rechargeable cells?

Answer 27

• light metal, won’t leak as electrolyte is solid polymer
• can be recharged without having to run down completely first

Question 28

what are the disadvantages of rechargeable cells?

Answer 28

• costly, sensitive to high temperatures, may explode on heating

Question 29

what are the advantages of fuel cells?

Answer 29

clean emissions

Question 30

what are the disadvantages of fuel cells?

Answer 30

• sourcing raw materials, storage space of gases, explosion hazard

Question 31

example of a non-rechargeable cell

Answer 31

zinc-carbon

Question 32

example of a rechargeable cell

Answer 32

lithium ion

Question 33

example of a fuel cell

Answer 33

alkaline hydrogen-oxygen