electrochemistry Flashcards
redox reaction
a reaction involving the transfer of electrons
oxidation
loss of electrons
reduction
gain of electrons
oxidising agent
substance that accepts electrons
(is reduced)
reducing agent
substance that donates electrons
(it is oxidised)
spontaneous redox reaction
self sustaining
stronger reactants preferentially
non-spontaneous redox reaction
does not happen on its own
2 relatively weak agents
requires potential
galvanic cell
electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy
has capacity to deliver current until rxn reaches eqm or has run to completion - voltage of cell = 0
cathode
electrode where reduction takes place
(RED CAT)
plating
anode
electrode where oxidation takes place
anode
electrode where oxidation takes place
function of salt bridge
completes circuit (allows ions to flow)
maintains electrical neutrality in half cells
cell notation
anode metal | anode soln || cathode soln||cathode metal
or RA reactant| RA product || OA reactant | OA product
electrolyte
substance that can conduct electricity due to the presence of free (mobile) ions when molten or dissolved in solution (in salt bridge)
KNO3 (aq ) - strong,very soluble - no precipitate
also KCl, AgNO3, NaCl
slower slay bridge will slow down rxn- lower current
choosing electrodes
inert - Pt
conductive non metal - graphite
Standard electrode potential
indication of pos of eqm
more negative - eqm lies left
more positive- eqm lies right
standard conditions
temp of 25 degrees celsius (room temp)
concentration of ion soln- 1 mol.dm-3
gas pressure of 1 atm
calculating emf
Ecell = Ecathode - Eanode
OR
Ecell= Eoxidising agent - Ereducing agent
pos,big gap- spontaneous rxn
neg,small gap- non-spontaneous
standard hydrogen electrode
reference electrode
connected as anode (may not spontaneously act as anode)
written as anode in cell notation
ie Pt| H2 | H+
neg emf will indicate that rxn does not occur spontaneously in direction given in cell notation
SHE connected as and reacted as anode
Voltmeter +
notation shows spontaneous rxn
H2 (g) is stronger reducing agent
spontaneous e- flow
SHE connected as and reacted as anode
Voltmeter +
notation shows spontaneous rxn
H2 (g) is stronger reducing agent
spontaneous e- flow
E - pos
SHE connected as anode BUT reacting as cathode
voltmeter -
notation non spontaneous
hydrogen ions bring reduced
E neg
non spontaneous e- flow
rxns must be written spontaneously
emf
indicates how close system is to reaching eqm
max- far
low- close
zero- at eqm
LCP and emf
increase [reactants] or decrease [products] - fwd favoured - emf increased
decrease [reactants] or increase [products] - rvs favoured - emf decreased
increase in temp favours endothermic rxn (rvs rxn) - decreased emf
decrease in temp favours exothermic rxn (fwd rxn) - increased emf
as anode metal is oxidised , anode ions increase - LCP- favour rxn to use - rate of reduction favoured - eqm shifts right -E more positive
as cathode ions reduced , ion concentration decreases - LCP -favour production - rate of oxidation favoured - eqm shift left
E more neg
if factors change in way that oxidation at cathode and reduction at anode favoured
emf decreases
if factors change in way that oxidation at anode and reduction at cathode favoured
emf increase
internal resistance
opposition to the flow of charge through electrodes and electrolytes in a cell
increased surface area of electrodes …
increases rate of rxn by lowering internal resistance and increasing max current that cell can deliver (does not affect emf of cell )
wider shorter or more conductive salt bridge
lowers internal resistance and increased max current cell can deliver (does not affect emf of cell )
rate changes
no effect on pos of eqm - no impact on emf
faradays constant
q =nF
mole ratio applies to e-
if no of moles of e- is determined - faradays constant can be used to determine quantity of charge
can link to q=It once moles of e- have been converted to charge
electrolytic cells
electrochemical cell that uses electrical every four cause a non spontaneous redox reaction to occur ,thus, electrical every is converted into chemical energy.
consists of 2 inert electrodes dipped into electrolyte , DC supply, e- flow from neg terminal of battery to electrode, pos and neg ions carry CURRENT through electrolyte, e- glow from second electrode back to battery
electrolysis
when electricity flows through molten state or through soln containing ions and ions undergo chemical rxn
types of substances that undergo electrolysis
ionic salts in soln
molten ionic salts
aqueous solns
identifying cathode in electrolytic cell
electrode connected to negative terminal becomes negatively charged
pos. charged ions will be attracted to neg. charged electrode where ions gain electrons (due to repulsion from cell) - thus being reduced therefore CATHODE
typically plating
identifying anode in electrolytic cell
electrode connected to pos. terminal of cell - positive electrode
negative ions attracted - ions will lose e- due to attraction - thus oxidised
ANODE
typically bubbles
Cu colour
blue
MnO4- colour
purple
Cr207, colour
orange
Cr3+ colour
green
Cl2 colour
pale green
Br2 colour
orange / brown
I2 colour
purple
I2 colour
purple
S colour
yellow
identify predominant reactions at electrodes in electrolysis of aqueous soln
the specific half cell reactions that will occur during electrolysis are those with the smallest difference in their standard electrode potentials
- the ones with the closest E values will take place bcus they require less energy to drive the non spontaneous rxn
role of concentration in predicting predominant anode half rxn during electrolysis
dilute soln- inherent reactivity will determine which half reaction occurs at anode as it would oxidise innately faster
concentrated soln- increased concentration of ions - favour rxn to use them- increasing rate - adjusting that electrode potential
under which circumstances will anode be oxidised in preference to ions in soln
relative strengths of reducing agents determine which reaction predominates
brine
concentrated sodium chloride soln
best conductor
largest current
non electrolyte
no current - no products
elements that don’t plate
weaker oxidising agents than water ( above water on left- Mn2+ upwards )
metals above Cu in electro refining of copper
stronger reducing agents than cu
will be oxidised and form
ions in soln
weaker oxidising agents than Cu
ions will remain in soln
metals below Cu in electro refining of copper
weaker reducing agents - won’t be oxidised- remain as unreacted metal
falls off anode as it is eroded- collect in pile- anode sludge
membrane cell requirements
for electrolysis of brine
- supply of electricity
- inert electrodes
- purified, concentrated brine
- only water supplied to cathode side
- positive ion permeable membrane separate anode and cathode compartments- only allow movement of Na+ cations
