Electrochemistry Flashcards
oxidation occurs at the
anode
reduction occurs at the
cathode
define oxidation
-loss of electrons
-increase in oxidation number
-gain of oxygen
-loss of hydrogen
define reduction
-gain of electrons
-decrease in oxidation number
-loss of oxygen
-gain if hydrogen
reductant AKA
reducing agent
oxidant AKA
oxidising agent
oxidant undergoes
reduction
reductant undergoes
oxidation
How does H oxidation state change
+1 in compounds with non metals eg HCl
-1 in compounds with metal hydrides eg NaH
what must the sum of oxidation numbers be
in a neutral compound oxidation numbers should sum to zero
in a polyatomic ion the oxidation number must
sum to the charge of the ion
How do oxidation numbers work in compounds without oxygen or hydrogen
the more electronegative element has a negative oxidation number, equivalent to what the charge of its ion would be
oxygen usually has an oxidation number of
-2
Fluorines oxidation number is always
-1
when does oxygen have a different oxidation number
-in peroxides it becomes -1 eg. H2O2
-when bonded to fluorine it becomes +2
KOHES stands for
balance Key element
balance Oxygen using water
balance Hydrogen by adding H+
balance charges by adding Electrons
add States
equation for Cr2O72- into Cr3+
Cr2O72- + 14H+ + 6e- –> 2Cr3+ + 7H2O
equation for Mn04- into Mn2+
MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O
Whats a direct redox reaction
reactants are directly mixed in a single beaker and a spontaneous reaction occurs which typically releases thermal energy
Whats an indirect redox reaction
reactants are in two separate beakers and an indirect redox reaction occurs which typically produces electrical energy
How do you explain whether a reaction will occur in terms of reductants
(reactant species) is a WEKAER REDUCTANT than the CONJUGATE REDUCTANT of the (oxidant species), which is (stronger reductant) hence reaction will/ will not occur
How do you explain whether a reaction will occur in terms of oxidants
(oxidant species) is a WEKAER OXIDANT than the CONJUGATE OXIDANT of the (reductant species), which is (stronger oxidant) hence reaction will/ will not occur
Limitations of predicting using ECS
changes in temperature –> reaction may not be observed as the rate of reaction is too slow
Higher/lower concentration than 1M–> another product may form at an electrode OR if concentration too low the reaction may to be observed
Different conditions/experimental design –> different voltage produced
When can a weaker oxidant or reductant preferentially react
-It is possible when the weaker oxidant/ reductant has concentration greater than 1.0M eg 4-5M
-it is possible when the stronger oxidant/reductant has concentration less than 1M eg 0.01 M
What are SLC
25 degrees celsius , 100kPa and 1M solutions
define galvanic cell
electrochemical cells in which chemical energy from spontaneous redox reactions are converted into electrical energy
define anode
negatively charged in galvanic cell where oxidation occurs
define cathode
positively charged in galvanic cels where reduction occurs
define salt bridge
structure which allows for transfer of ions between two half cells to maintain electrical neutrality of cell and complete the circuit
define electrolyte
source of ions which are used to allow for flow of electric charge within a cell
define electrode
electrical conductor that is either unreactive or involved in reaction
define standard half cell
cells consisting of both members of conjugate redox pair and an electrode at SLC
define standard electrode potential
potential of the half reaction (reduction), given in volts, relative to standard H electrode
energy conversion is galvanic cells
chemical –> electrical
Cations of salt bridge move towards
cathode
anions of salt bridge move towards
anode
electrons flow from
anode to cathode
how to calculate electric potential
E(top in ECS) - E(bottom in ECS)
Fuel cell vs galvanic cell
-fuel cell requires constant supply of reactants and electricity is continuously generated whereas in a galvanic cell a fixed amount of fuel is used to generate a finite amount of electricity
-fuel cells use porous electrodes whereas galvanic cells use inert or contain reactants/products
-fuel cells operate at higher temperatures
-fuel cells separate half cells in same vessels whereas galvanic cells have half cells in seperate vessels
-Both convert chemical into electrical energy
in fuel cells , fuel reacts at ___
and ___ reacts at cathode
anode
oxygen
function of electrolyte in fuel cell
-allows for movement of charged ions/source of ions which are needed for reaction to proceed
-transports H+ or OH- in acidic and alkaline fuel cells respectively
function of electrodes in fuel cells
-site of oxidation /reduction
-porous, increase SA and rate of reaction
-may contain catalysts which lower activation energy and provide alternative reaction pathway
-pores allow gases to access electrode
how to convert KOHES into alkaline conditions
add OH- to both sides, OH- and H+ cancel out to form water
list 3 advantages of fuel cells
-high energy efficiency
-fuel flexibility
-quiet operation
-no need to be recharged
-low maintenance due to lack of moving parts
-low running costs
disadvantages of fuel cells
-infrastructure and manufacturing is expensive
-need reliable and continual supply of fuels
-difficult to store, transport and distribute H2
compare fuel cell and combustion engine
-fuel cells are much more energy efficient than combustion engines because combustion engines require many intermediate steps whereas fuel cells convert chemical directly into electrical energy.
-fuel cells can have overall less impacts on environment (because more efficient)
-fuel cells more expensive because H very expensive to store and obtain
-fuel cells quieter
intermediate steps for combustion engines
chemical to thermal to mechanical to electrical
Safety problems with hydrogen gas
-highly flammable
-colourless, tasteless and odourless so difficult to detect
-must be compressed and stored at very low temperatures so expensive
When can fuel cells be beneficial to the environment
if the fuel supplied eg hydrogen gas is obtained via renewable resources then there is a negligible change to CO2 levels
rechargeable batteries aka
secondary cells
primary cells are
used until the supply of reactants run out (not rechargeable)
Define memory effect for batteries
some cells have decreased capacity to be fully charged. This occurs when rechargeable batteries are not fully discharged before recharging
When do mass of cells matter
in portable devices lightness is prioritised whereas vehicles can often accommodate heavier and bulkier batteries
what two reactions do rechargeable cells undergo
discharge and recharge
when a secondary cell discharges it coverts ___ into ___ and acts as a ___ cell
chemical
electrical
galvanic
when a secondary cell recharges it coverts ___ into ___ and acts as a ___ cell
electrical
chemical
electrolytic
What are the conditions for a secondary cell to be recharged
-the products of the discharge reaction must stay in contact with the electrodes
-a voltage higher than the voltage supplied during discharge is required
during recharge the positive terminal of the power supply connects to the ___ an the negative terminal of the power supply connects to the ___
positive electrode
negative electrode
polarity of electrode during recharge
anode is positive
cathode is negative
what occurs at anode and cathode in recharge
oxidation at anode and reduction at cathode
what dictates the polarity of electrode in recharge
electrons are pushed to the cathode and pulled away from the anode by the power supply therefore reduction at cathode and oxidation at anode
define battery life
number of charge/discharge cycles before a battery becomes unusable
what factors effect battery life
-high temperatures will deteriorate the cell at a greater rate ,increase the rates of side reactions and compromise the battery’s functionality (parts)
-low temperatures decrease the electric potential
-fluctuations in temperature can also impact battery life
-reactants and products may fail to adhere to electrodes
-reactants and products may be converted to inactive forms
-other chemicals may interfere with internal mechanism of batteries
-internal components may corrode
-impurities could react with reactants or products
-electrolyte may leak decreasing contact between electrodes and electrolyte
-memory effect
how to increase battery life of rechargeable cells
store batteries at lower temperatures eg 10-15 degrees celsius
how to increase the battery life of primary cells
-same principles as secondary cells but could also increase amount of reactants
polarities of electrodes in electrolysis
positive anode and negative cathode
what type of reactions occur in electrolysis
non spontaneous redox reactions
functions of electrolysis
-electroplating
-produce reactive metals
-electrorefining
-recharge of secondary cells
-electrolysis of water to produce oxygen and hydrogen gas
describe electron movement in electrolytic cells
electrons are pushed to the cathode and pulled away from the anode by the power supply therefore reduction at cathode and oxidation at anode
liquid/ molten vs aqueous
aqueous contains WATER
energy conversion in electrolytic cells
electrical to chemical
in electroplating what is where
-object to be plated at cathode
-anode is the metal which will be plate on cathode eg silver metal
what ions should always comprise the electrolyte
highly soluble ones
eg NO3- or SO42-
in electrolysis of water what happens where
at cathode water is reduced to four hydrogen gas
at anode water is oxidised to from oxygen gas
why is Aluminium dissolved in cryolite
lowers melting point of alumina from 2000 to 1000 degrees celsius approx.
cryolite formula
Na3AlF6
what is faradays first law
amount of any substance discharged at an electrode during electrolysis is directly proportional to quantity of electrical charge passed through the cell (linear relationship)
the greater the charge of the ion the ___ mass deposited on cathode
less
because of molar ratios eg 1 mole of electrons will produce 1/2 mole of Mg
what is Faradays second law
amount (in mol) of any substance discharged at an electrode during electrolysis is the reciprocal of the charge of ion
at which electrode is the impure metal during electrorefining (eg.blister copper)
at the anode
onto which electrode does the pure metal collect in electrorefining
the cathode
explain the events occuring at the anode during electrorefining
the impure metal(eg. blister copper) is placed at the anode where the strongest reductants are oxidised(metals such as Pb and Ni).
the reductants weaker than the metal being refined(eg. Ag, Au and Pt) are not oxidised and hence fall to the bottom of the anode forming a muddy mixture known as anodic sludge.
features of daniell(galvanic) cell which allows electrcity production
The two half-cells are separated so that electrons can flow through the connecting wire.
The two half-cells are connected via a salt bridge to complete the circuit and allow electrons to
flow through the connecting wire.
