5.2.3 Redox, Electrode Potential and Fuel Cells

Question 1

what is a redox reaction

Answer 1

where one or more elements change oxidation state

Question 2

what is a disproportionation reaction

Answer 2

when one species is simultaneously oxidised and reduced

Question 3

what is an oxidising agent

Answer 3

causes a species to lose electrons

Question 4

what is a reducing agent

Answer 4

causes a species to gain electrons

Question 5

describe the thiosulfate redox titration and give the equation and colour changes

Answer 5

A starch indicator is added is added near the end point and iodine fades to a pale yellow and this causes the colour change to be from blue/black to colourless
2S2O3^2- + I2 –> 2I- + S4O6^2-

Question 6

describe the manganate redox titration and give the equation and colour changes

Answer 6

If the manganate is in the burette then the end point of the titration will be the first permanent pink colour so colour change is colourless to purple
MnO4- + 8H+ + 5Fe^2+ –> Mn^2+ + 4H2O + 5Fe^3+

Question 7

what do insufficient volumes of sulfuric acid or weak acids cause to happen in the manganate redox titration

Answer 7

the solution is not acidic enough so MnO2 to be produced instead of Mn^2+ which is brown and masks the colour change causing greater volumes of manganate to be used in the titration

Question 8

Why can conc HCl not be not be used in the manganate redox titration

Answer 8

the Cl- ions would be oxidised to Cl2

Question 9

describe an electrochemical cell

Answer 9

a cell has two half cells which must be connected by a salt bridge
the cells will produce a small volatge if connected in a circuit

Question 10

why does a voltage form

Answer 10

• the LHS anode has more of a tendency to oxidise and release electrons that build up on the anode
• a potential difference is created between the two electrodes
• this is measured with a high resistance meter and has the symbol E^o

Question 11

what are the standard conditions in an electrochemical cell when measuring potential difference

Answer 11

• 298k
• 100kPa
• 1.00 moldm^-3 conc
• no current flowing

Question 12

what metal is used if there is no solid metal and why

Answer 12

Pt because it isn’t very reactive

Question 13

why is a high resistance voltmeter used

Answer 13

it needs to stop the current flowing in the circuit so maximum potential difference can be calculated

Question 14

what is the function of the salt bridge and how does it work

Answer 14

• connects the circuit with free moving ions that conduct the charge
• the salt is KNO3 as this is unreactive with the electrodes eg KCl would form complexes with some ions
• a wire would set up its own electrode system

Question 15

what happens at each side of the cell

Answer 15

LHS/anode- oxidation, negative

RHS/cathode- reduction, positive

Question 16

what is standard cell potential

Answer 16

potential of a cell composed of two electrodes under standard conditions

Question 17

what voltage does a standard hydrogen electrode have

Answer 17

0.00 volts

Question 18

why are standard conditions needed in a standard hydrogen electrode

Answer 18

position of redox equilibrium will change with conditions

SHE is difficult to use so a secondary standard is used where other electrodes are calibrated against SHE

Question 19

what is standard electrode potential

Answer 19

potential difference measured when an electrode system is connected to the hydrogen electrode system and standard conditions apply

Question 20

what happens to potential if pressure of H2 increases

Answer 20

equilibrium shifts to the left top oppose

potential is more negative

Question 21

what happens to potential if surface area of electrode increases

Answer 21

faster rate of electron transfer but has no effect on potential

Question 22

what happens to potential if temp increases

Answer 22

if forward reaction is exothermic equilibrium shifts to the left to oppose and potential is negative

Question 23

how is EMF calculated and what makes a reaction feasible

Answer 23

EMF= Ered-Eox (RHS-LHS)

if EMF is positive it is feasible

Question 24

what happens to EMF is the voltmeter is replaced with a bulb

Answer 24

EMF falls to zero and reactant concentrations drop

Question 25

what is the effect of concentration on EMF

Answer 25

increasing conc of reactants would increase EMF and decreasing them would decrease EMF

Question 26

what is the effect of temperature on EMF

Answer 26

most E cells are exothermic in the forwards direction so increase of temp would decrease EMF

Question 27

how can a substance act as a catalyst in e cells

Answer 27

the electrode potential must lie between the electrode potentials of the two reactants eg Fe^3+ in a reaction between S2O8^2- and I2

Question 28

why are some cells non-rechargeable and give examples

Answer 28

chemicals used over time so EMF drops and cannot be recharged and have to be disposed of e.g zinc-carbon and alkaline which has a higher cost but longer life

Question 29

why are cells rechargeable and give examples

Answer 29

reactions are reversible e.g lithium ion, lead-acid, nickel-calcium
the recharge reaction is the discharge reaction flipped

Question 30

what are fuel cells and give an example

Answer 30

they have a continuous supply of chemicals so doesn’t run out or need recharging
e.g hydrogen fuel cells

Question 31

give the equations at anode and cathode for an alkaline hydrogen fuel cell

Answer 31

anode H2 + 2OH- –> 2H2O +2e-

cathode O2 + 2H2O + 4e- –> 4OH-

Question 32

give the equations at anode and cathode for an acidic hydrogen fuel cell

Answer 32

anode H2 –> 2H+ + 2e-

cathode O2 + 4H+ + 4e- –> 2H2O

Question 33

give the overall equation for a hydrogen fuel cell and EMF

Answer 33

2H2 + O2 –> 2H2O EMF 1.23V

Question 34

give two advantages of fuel cells

Answer 34

• less pollution and CO2 as only water is produced

- greater efficiency as in constant use

Question 35

give three disadvantages of fuel cells

Answer 35

• hard to transport hydrogen as has high pressure and is flammable
• limited lifetime as needs regular replacement
• toxic chemicals used in production

Question 36

give an advantage and disadvantage of cells

Answer 36

+ portable

- waste issues

Question 37

give an advantage of non-rechargeable cells

Answer 37

+ cheap

Question 38

give three advantages of rechargeable cells

Answer 38

+ less waste
+ cheaper overall
+ lower environmental impact

Question 39

give three advantages of hydrogen fuel cells

Answer 39

+ only waste is water
+ don’t need recharging
+ efficient