electrochemistry Flashcards
secondary cell examples
lead acid accumulator, nicad cell, nimh cells
discharge
conversion of chem energy to electrical as a result of a spontaneous reaction
secondary cells
rechargeable cells or accumulators
- when secondary cell discharges acts as a galvanic cell which converts chemical energy to electrical energy
- when cell is recharged it will act as an electrolytic cell and electrical energy is converted to chemical energy - will require voltage for reaction bc non spontaneous
what must happen for a secondary cell to be recharged
-products of the discharge must remain in contact with the electrodes
-voltage higher than voltage produced in discharged is required
what is positive terminal connected to in recharge
the positive terminal of cell connects to positive terminal of power supply
recharging 2dary cells polarity
cathode is negative while anode is postitive
discharge - galvanic cells polarity
anode is neg and cathode is pos
NiCd battery discharge
oxidation- Cd+2OH- -> Cd(OH)2 + 2e-
reduction - NiO(OH) + H2O + e- -> Ni(OH)2 + OH-
recharge will be the other way around
lead acid battery
- secondary cells used in car and truck batteries
- need 12 V above total potential
- product of the discharge is PbSO4 which forms a solid on electrodes and allows battery to be recharged
lead acid battery discharge oxidation
anode oxidation
Pb+ H2SO4 -> PbSO4 + 2H+ + 2e-
lead acid battery discharge reduction
PbO2 + H2SO4 + 2H+ + 2e- -> PbSO4 + 2H2O
battery life
- used to describe battery cell performance
- technical specification of battery life refers to number of charge and discharge cycles before a battery becomes unstable
temp affect on battery life
temperature
- increase temp will mean battery deteriorates quicker
- as temp increases ROR of side reactions increase and decrease battery life
-temp decrease capacity of battery decreases and cell ROR decreases - less energy generated (electrical)
battery life factors
-reactants and products ay become detached from their electrodes
- reactants and products may be converted into inactive forms through side reactions
-corrosion of internal components
- leakage of electrolyte solution
-impurities in cell could react with products or reactants
self discharge
loss of electrical capacity of a battery due to deriation of battery components
- caused by side reaction , low temp and slow ROR
how can battery life be improved for non rechargeable cells
increase amount of _______ present
reduce SA of ___ anode and____cathode
store at consistent temp - avoid high temp
NiMH cells
electrolysis
electrical energy is converted to chemical energy via a nonspontaneous reaction using a power source
what are some uses of electrolysis
- convert ions into metals
-produce reactive materials eg aluminium and sodium - electrorefining - purifying metals
- recharging secondary cells
electrolysis
- ox at the positive anode
reduction at the negative cathode
anode to cathode electrons move
what does the external power source do
it will force the reaction to occur - will force the ions to seperate
- external power source pushes electrons to cathode
what state is molten
liquid
comparing molten with aqueous
molten will just seperate the ions but the aqueous also has water ( competition at the electrodes)
why is there a barrier or seperator in electrolysis
so the products and ions will not react with each other and will not interfere in the half cell reactions - allows for a spontaneous reaction to occur
what will react at the electrodes in electrolysis
strongest oxidising agent is reduced at cathode
stronger reducing agent is oxidised at anode
can be water
in a galvanic cell why must the reactants/ half cells be seperated
they do not want to have a direct reaction because it will reduce electrical energy generated and will produce thermal energy - need indirect to produce electricity and a voltage
what is needed for electrolysis of water
an electrolyte eg . KNO3
electrolysis of water equation
2H2O -> 2H2 + O2
electrolysis of water
- electrolyte KNO3 or other is added in low conc
- cathode becomes basic and anode becomes acidic
what are safety considerations of OH-
it is caustic
what are the safety implications of H+
corrosive
safety considerations of electrolysis of water
-H2 is flammable - keep away from naked flames
-H2 is hard to detect leaks - need detectors
- H2 is highly reactive - wear safety equiptment and store at high pressures
electroplating
application of electrolyisis when metal ions are reduced to form a solid metal coating on an object
what is being plated in electroplating
the cathode - it will gain ions from the anode to form solid metal coating
what is the polarity of where the deposits are in electroplating
negative because it is at the cathode
why is it deposited at the cathode
when cell is electrolysed the cathode is negative. the positive cations will move toward the cathode and they will be reduced to formed the metal which will coat the desired substance.
downs cell - what is it used for ?
it is used for production of sodium metal by using molten sodium chloride
down cell equations
- the Cl is oxidised
- the Na is reduced
why is a carbon used instead of other electrodes
it is inert and will not participate or interfere with the reaction
why is cryolite added
decrease the MP required - energy and cost effective
aluminium production - hall heroult process
Al2O3 is dissolved in cryolite which lowers MP
anode - C + 2O(2-) -> CO2 + 4e-
cryolite formula
Na3AlF6
membrane cell - use?
used to produce sodium hydroxide and generate both chlorine and hydrogen gas
brine
concentrated salt solution eg . NaCl - must be 5M or higher
why is a porous electrode used
it will increase surface area, increase rate of reaction and increase effectiveness of membrane cell
membrane cell - half cell reactions
ox - Cl- is oxidised to Cl2
red - H2O is reduced to H2 gas and hydroxide ions
electrorefining - purification
why are fuel cell electrodes expensive
they are porous
formed from high quality metals
how to improve battery life
-store at consistent temps and avoid high temps
-increase amount of reactants present
-reduce surface area of anode and cathode so longer lasting
safety measures / guidlines
- keep away from naked flames
- wear safety equiptment - lab coat, glasses, dilute spills
- use a fume cupboard
what are similarities with galvanic and electrolytic cells
oxidation still at anode
reduction still at cathode
difference (5) bw galvanic and electrolytic cells
- in g the anode is negative while in e the anode is positive
- in g the cathode is positive while in e the anode is positive
- g is a spontaneous reaction while e is a nonspontaneous reaction
- in galvanic the half cells are indirect while in e they are not seperated into diff vessels
- g converts chemical energy to electrical while e will convert electrical to chemical energy
when electroplating occurs does conc of metal change
no. molar ratios of reduction and oxidation equations are the same, so concentration remains constant. the metal ions used must be a stronger oxidant than water
features in electrolytic cell
their must be two conductive ekectrodes, electrolyte to be used to allow flow of ion, a power supply
limitations of ESC
-only applies to reactions in aq. sols
-can only apply to SLC
-does not provide indication of reaction rate
-does not tell us if a precipitate is formed
oxidation occurs at the
anode
reduction occurs at the
cathode
salt bridge
a soluble strip which will allow cations and anions and spectator ions to flow
- maintains the cell neitrality
- completes internal circut
cations move to the
cathode
anions move to the
anode
oxidation
the loss of electron
- will increase oxidation number , lose H and gain O
reduction
gain of electrons
(reactants side )
- will decrease oxidation number , gain H, lose O
reductant
it will undergo oxidation
- it will cause reduction by donating the electrons
oxidant
it will undergo reduction
- it will accept electrons and cause oxidation
direct
when the reactants ad products reaction will be within the same vessel so can interact with one another directly spontaneously - will release thermal energy
indirect
will use seperate vessels to ensure half reactions can occur and will produce electrical energy
does indirect produce moanily thermal energy
no it does not . it will produce electrical energy
observing a redox reaction
the reaction rate may be too slow to observe , other side reactions might occur , colour changes or change in mass of electrode
galvanic cell
when chemical energy is converted to electrical energy via an indirect spontaneous reaction
function of the electrolyte
it is a source of cations and anions for both ahlf cells and will allow for a flow of ions in order to maintain cell neautrality
electric potential equation
E cathode/ pos - E anode/ neg = ____V
acidifed permanganate equation
Cr2O(2-)7 + 14H+ -> 2Cr(3+) + 7H2O
MnO4(-) -> Mn(2+)
MnO4(_) + 8H+ + 5e -> Mn(2+) + 4H2O
reductant - non spontaneous reaction explanation
reductant species is a weker reductant than conjugate reductant of oxidant , stronger reductant -> no spontaneous reaction
oxidant - explanation why non spontaneous reaction occurs
oxidant species is a weaker oxidant than conj oxidant of reductant which is stronger oxidant -> non spontaneous reaction
limitations of ESC
diff voltage, reaction may be too slow, another product may form
ESC predictions are impacted by
changes in temp , concentrations differing from 1M, conditions
fuel cells
- a type of galvanic cell
- chemical to electrical energy
- continuous supply of reactants
WHAT ALWAYS HAPPENS AT FUEL CELL
fuel is at anode and O2 is always at cathode
electrolyte
allows movement of charged ions - allows redox to occur
does galvanic cell have finite capcaity
yes … not a continuous supply of reactants
electrodes in fuel cells
porous , increase SA , increase ROR
- may also have catalyst
advantage of fuel cells
high energy conversion efficiency because direct reaction and less energy transformations , fuel flexibility, constant supply of reactants,
disadvantage of fuel cells
need continual supply of fuel , more expensive to operate and get porous electrodes , difficult to transport gases etc.
similarities bw galvanic and fuel
both convert chemical to electrical energy
difference bw g and fuel cells
- porous electrodes at f while inert electrodes or contain reactants/products
- ongoing supply of reactants at f while limited supply in g
- high temps f while g is at low temps
- half cells in sperate vessels g while fuel cells have same vessel
factor impacting cell selection
initial and operating cost, size and shape, mass, memory effect, voltage provided, current, shelf life , ease of disposal , environment concerns
examples of primary cells
alkaline, lelanche, zinc air or silver oxide button cell
KOHES IN BASIC AND ACIDIC
acidic - H+
basic - OH-
