electrochem Flashcards
what is hydrogen’s oxidation number
in non-metals: +1
in metals: -1
oxidation number of oxygen
normally: -2
in peroxides: -1
with fluorine: +2
how to explain whether a redox reaction has occured
detail whether there has been a change in oxidation number
reduction of dichromate ion
6e + 14H+ + Cr2O7 2- –> 2Cr3+ + 7H2O
what type of redox is combustion of a fuel
oxidation
reduction of manganese ion
5e + 8H+ + MnO4- –> Mn2+ + 4H2O
(different for basic conditions)
direct redox reaction
oxidation and reduction occur in the same vessel/ reactants are mixed together
generally results in release of heat
indirect redox reaction
reactants are in two separate beakers
electrical nrg produced hw some heat nrg
explanation of ‘no spontaneous reaction’
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
explanation of ‘no spontaneous reaction’
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
limitations of predictions in terms of electrochemical series
impacted by:
changes in temp
conc higher/lower than 1M
conditions and experimantal design
potential impacts and observations due to limitaions of predictions
reaction may not be observed bc too slow
another product may be formed at electrode
different voltage produced
function of membrane in cells
transport ions bw electrodes
prevent oxidant and reductant coming into contact
separate the 2 half cells
factors that limit the life of cells
corrosion of electrode (be specific which one)
side reactions
oxidation
loss of electrons, increase in oxidiation number, loss of hydrogen, gain of oxygen
salt bridge
allow flow of ions between two half cells to maintain electrical neutrality of the cell
typical problems with using galvanic cells
- a reactant or product is reactive with either water or air -> fire
- product could react with another reactant or product
ESC is only accurate for
reactions occuring at standar lab conditions
fuel cell
fuel is constantly pumped in and electricity is constantly generated
w/a galvanic = fixed quantity of fuel -> generate finite amount of elec
electrolyte in fuel cells
- 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
electrodes in fuel cell
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
solid oxide fuel cell
- solid electrolyte
- oxide ions = gaseous
ie O2(g) + 4e- -> 2O2-(g)
advantages of fuel cells
- quiet operation
- high energy conversion efficiency (vs combustion)
- low running costs
- low chemical pollution (vs combustion)
- doesn’t need to be recharged
disadvantages of fuel cells
- 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
galvanic vs fuel cells
galvanic:
- electrodes are inert or contain reactants/products
- closed system
- operate at lower temperatures
- half cells in separate vessels
primary cells
non-rechargable cells that convert chemical energy to electrical energy
amount of energy supplied is limited to amount of reactants available
secondary cells
rechargable electrochemical cells where reactants can be formed from products of discharge reaction
factors impacting cell selection
- 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
factors affecting battery life
- 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
if concentration of reactants are high and the two reactants have similar electrode potentials, which reactant is oxidised
REACTANT WITH HIGHER CONC IS OXIDISED
what voltage must be given to a cell during recharge
voltage higher than voltage produced when discahrgeing
faraday’s first law of electrolysis
the amount of any substance dischared at an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the cell
faraday’s second law of electrolyisis
the amount (mol) of any substance discharged at an electrode during electroysis is the reciprical of the charge on the ion
best temperature range for the largest number of charge cycles for a lithium-ion battery
9-21 degrees
around room temp (~22) and no higher than
advantages of batteries over fuel cells
- low maintainence
- dont need large amounts of fuel
what allows a galvanic cell to produce electricity
- 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
