electrochem Flashcards
oxidation (and at which electrode)
oxidation is the
-gain of oxygen
-loss of elctrons
-loss of hydrogen
-increase in ox number
(it occurs at anode)
reduction(and at which electrode)
reduction is
-gain of electrons
-loss of oxygen
-gain of hydrogen
-decrease in ox number
(occurs at the cathode)
direction of flow of electrons
anode to cathode
balancing(+alkaline from acidic)
KOHES
-add (-OH) to both sides as many as there are (H+) ions
-both ions form water eg.( 2OH- + 2H+ –> 2H2O)
-cancel out
direct vs indirect
direct: both reactants in same container causing a spontaneous reaction which primarily produces heat(not wanted in batteries)
indirect: reaction where reactants are in seperate containers usually connected by wire(for transfer of e-) and hence mostly electrical energy produced(wanted for batteries)
when will spontaneous reaction occur re. electrochem series
top left(strongest oxidant)bottom right(strongest reductant)
why no spontaneous
(Reductant species) is a weaker reductant than the conjugate reductant of the (oxidant species), which is (stronger reductant) , hence no spontaneous reaction will be observed.
-vise versa for oxidants
limitation of prediction
not at standard:
changes in temperature:reaction may be not observed-reaction at slower rate
higher/lower conc than 1M:another product from at electrode or no reaction observed due to low concentration
conditions and exp design: diff voltage produced
electrolyte
is the source of ions which are used to allow for the flow of electric charge within the cell
salt bridge
structure which allows the flow of ions between two half cells(between electrodes) to maintain cell neutrality.
fuel cell characteristic
-porus elctrodes
-continous supply of fuel –>continous energy supply
-either alklaine or acidic electrolyte
-fuel at anode oxygen at cathode
-overall equation=combustion equation
- electrolyte:allows movement of charged ions so redox reaction can occur
porus electrode function
-composed of many holes (pores) that allow greater SA for reactions to take place
-also may contain a catalyst that aids increasing ror
-pores allow gas to access electrolyte
fuel cell vs regular combustion
fuel cell has:
higher energy conversion efficiency
therefore less CO2
quieter operation
lower running cost
higher set up cost
disadv + adv of fuel cell
adv:
high energy conversion efficiency
low chem pollution
quiet operation
no need for recharge
unlimited run when fuel supplied
low runnign cost
diasdv:
- expensive elctrodes b/c prous and also function as catalyst
- high set up cost
-need continual supply
how to obtain H2 + saftey issue
-electrolysis of water(renewable means)or fossil fuel
-highly flammable
electrode polarity glavanic
anode: - cathode:+
electrode polarity electrolytic(2nd battery when recharging)
anode: + cathode: -
primary vs secondary cells/battery
non-rechargeable electrochem cell which converts electrical energy from chemical vs rechargeable one
impact cell selection
operating cost, size and shape,mass, voltage provided, current, shelf life, ease of disposal+enviro factors.
recharge conditions
-products of discharge reactions must stay in contact with the electrodes
-voltage higher than voltage produced when discharging is required to return cell to full charge
+ve terminal to +ve electrode ect.
self discharge
the loss of electrical capacity of battery due to the deterioration of battery components( due to side reactions when not used( therefore cold temp slows this done))
battery life factors
temp: high= deteriote quicker b/c quicker side reactions low= lower electric potential(lower capacity)
side reactions: self discharge + disrupt function
electrode contact: if not reduced recharge capacity
memory effect: due to incomplete discharge before recharge–> reduced cycles
molten vs aqeous
molten: no water aqeous: with water
electroplate
object at cathode metal at anode
electrolysis of water
done with low conc of H2SO4 or KNO3
iron mesh screen
seperate products or products from reactants to prevent a direct spont reaction.
alimunium production
AL2O3 (alumina) dissolved in cryolite ( NA3AlF6) lowers MP from approx 2000 c to 1000c
- carbon rods= anode ( carbon + oxygen ions from CO2)
- carbon lined box= cathode (AL3+ reduced)
therefore Al form at bottom and is drawn out
electrorefining
-blister copper at anode
-pure copper at cathode
-ni and pb pref oxidised but b/c Cu is main species present=oxidised the most
-Cu2+ ions are strongest oxidant and are pref reduced at cathode.
-Au Ag and Pt are weaker reductants and are not oxidised fall to bottom to form ANODIC SLUDGE
faraday law 1
amount of any substance discharged at an electrode during electrolysis is directly proportional to quantity of electric charge passed through cell
-therefore eg. mass lost at anode=mass gained at cathode during electroplating
faraday law 2
amount of any substance (mol) discharged at an electrode during electrolysis is the reciprocal of the charge on the ion.
electrode characteristics(fuel cell)
porus
conductive
can be inert
how to determine electrolysis reactions
1)identify where species are going to (+ve charged species to cathode and -ve charged ones to anode(water to both))
2) determine which will be preferentially oxidised/reduced
