electrochem

Question 1

what is hydrogen’s oxidation number

Answer 1

in non-metals: +1
in metals: -1

Question 2

oxidation number of oxygen

Answer 2

normally: -2
in peroxides: -1
with fluorine: +2

Question 3

how to explain whether a redox reaction has occured

Answer 3

detail whether there has been a change in oxidation number

Question 4

reduction of dichromate ion

Answer 4

6e + 14H+ + Cr2O7 2- –> 2Cr3+ + 7H2O

Question 5

what type of redox is combustion of a fuel

Answer 5

oxidation

Question 6

reduction of manganese ion

Answer 6

5e + 8H+ + MnO4- –> Mn2+ + 4H2O

(different for basic conditions)

Question 7

direct redox reaction

Answer 7

oxidation and reduction occur in the same vessel/ reactants are mixed together

generally results in release of heat

Question 8

indirect redox reaction

Answer 8

reactants are in two separate beakers

electrical nrg produced hw some heat nrg

Question 9

explanation of ‘no spontaneous reaction’

Answer 9

reductant species is a weaker reductant than the conjugate reductant of the oxidant species which is stonger reductant hence no spontaneous reaction will be observed

italics= substitute with chemical species

‘reductant’ can be replaced with oxidant

Question 10

explanation of ‘no spontaneous reaction’

Answer 10

reductant species is a weaker reductant than the conjugate reductant of the oxidant species which is stonger reductant hence no spontaneous reaction will be observed

italics= substitute with chemical species

‘reductant’ can be replaced with oxidant

Question 11

limitations of predictions in terms of electrochemical series

Answer 11

impacted by:
changes in temp
conc higher/lower than 1M
conditions and experimantal design

Question 12

potential impacts and observations due to limitaions of predictions

Answer 12

reaction may not be observed bc too slow
another product may be formed at electrode
different voltage produced

Question 13

function of membrane in cells

Answer 13

transport ions bw electrodes
prevent oxidant and reductant coming into contact
separate the 2 half cells

Question 14

factors that limit the life of cells

Answer 14

corrosion of electrode (be specific which one)
side reactions

Question 15

oxidation

Answer 15

loss of electrons, increase in oxidiation number, loss of hydrogen, gain of oxygen

Question 16

salt bridge

Answer 16

allow flow of ions between two half cells to maintain electrical neutrality of the cell

Question 17

typical problems with using galvanic cells

Answer 17

• a reactant or product is reactive with either water or air -> fire
• product could react with another reactant or product

Question 18

ESC is only accurate for

Answer 18

reactions occuring at standar lab conditions

Question 19

fuel cell

Answer 19

fuel is constantly pumped in and electricity is constantly generated

w/a galvanic = fixed quantity of fuel -> generate finite amount of elec

Question 20

electrolyte in fuel cells

Answer 20

• carry current between electrodes by movement of cations to the cathode and anions to the anode
• complete the circuit allowing the flow of charged particles through the cell

Question 21

electrodes in fuel cell

Answer 21

electrodes are porous
-> greater SA for reactions to take place -> reaction can occur at a faster rate and electrode may also contain a cataylst.
-> pores allow reacting gases to access electrolyte

Question 22

solid oxide fuel cell

Answer 22

• solid electrolyte
• oxide ions = gaseous
  ie O2(g) + 4e- -> 2O2-(g)

Question 23

advantages of fuel cells

Answer 23

• quiet operation
• high energy conversion efficiency (vs combustion)
• low running costs
• low chemical pollution (vs combustion)
• doesn’t need to be recharged

Question 24

disadvantages of fuel cells

Answer 24

• manufacturing process and materials are expensive
• electrodes are expensive bc they also have to function as catalysts
• need reliable, continual supply of fuel
• distribution, storage and transport of H2 is difficult

Question 25

galvanic vs fuel cells

Answer 25

galvanic: - electrodes are inert or contain reactants/products - closed system - operate at lower temperatures - half cells in separate vessels

Question 26

primary cells

Answer 26

non-rechargable cells that convert chemical energy to electrical energy | amount of energy supplied is limited to amount of reactants available

Question 27

secondary cells

Answer 27

rechargable electrochemical cells where reactants can be formed from products of discharge reaction

Question 28

factors impacting cell selection

Answer 28

- mass: portable devices -> lightness is a priority hw transport vehicles can accomodate heavier batteries - voltage provided ie aqueous electrolytes cannot provide >2V - current: greater SA of electrodes -> greater current - shelf life: bc some cells may discharge over time without being used

Question 29

factors affecting battery life

Answer 29

- temp: increase temp -> increase rate of deterioration and rate of side reactions that comprimise battery functionality decreasing battery life hw too low temps -> decrease capaicity of batttery -> lower discharge rate and lower electricity supplied - if reactants and products detach from electrodes - reactants and products are converted into inactive forms via side reactions - leakage of electrolyte

Question 30

if concentration of reactants are high and the two reactants have similar electrode potentials, which reactant is oxidised

Answer 30

REACTANT WITH HIGHER CONC IS OXIDISED

Question 31

what voltage must be given to a cell during recharge

Answer 31

voltage higher than voltage produced when discahrgeing

Question 32

faraday's first law of electrolysis

Answer 32

the amount of any substance dischared at an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the cell

Question 33

faraday's second law of electrolyisis

Answer 33

the amount (mol) of any substance discharged at an electrode during electroysis is the reciprical of the charge on the ion

Question 34

best temperature range for the largest number of charge cycles for a lithium-ion battery

Answer 34

9-21 degrees | around room temp (~22) and no higher than

Question 35

advantages of batteries over fuel cells

Answer 35

- low maintainence - dont need large amounts of fuel

Question 36

what allows a galvanic cell to produce electricity

Answer 36

- 1/2 cell connected via salt bridge completes the circuit and allows e to flow through connecting wire - 1/2 cells are separated so e can flow through connecting wire