redox II Flashcards
what do electrochemical cells do
use redox reactions to create a flow of electrons
potential difference is produced between two electrodes
this can be measured using a voltmeter
what do most electrochemical cells consist of
2 solutions
metal electrodes
salt bridge
what is a salt bridge
a tube of unreactive ions that can move between the solutions
carries the flow of charge without interfering with the reaction
what does a positive cell potential mean
substance is more easily reduced
will gain electrons
oxidising agent
what does a negative cell potential mean
substance is more easily oxidised
will lose electrons
reducing agent
what is the standard hydrogen electrode
measuring standard for half-cell potentials
has a cell potential of 0.00V
measured under standard conditions
what are the standard conditions for a half cell
1.0 moldm-3 solution
298K
100 KPa
what does a hydrogen cell consist of
hydrochloric acid solution
hydrogen gas
platinum electrodes
why are platinum electrodes used
metallic - so will conduct electricity
inert - so will not intefere with reaction
what are the 4 rules for conventional cell representation
- the half cell with the most negative potential goes to the left
- the most oxidised species from each half cell goes next to the salt bridge (ROOR)
- a salt bridge is shown using a double line
- state symbols are always included
example of conventional cell representation with Zn and Cu
Zn(s) l Zn2+(aq) ll Cu2+(aq) l Cu(s)
how to calculate cell Emf
Emf (cell) = cell potential (right) - cell potential (left)
or
most positive potential - most negative potential
what does it mean if the Emf value is positive
reaction taking place is spontaneous and favourable
what does increasing the concentration of solutions do to the Emf value (why)
cell Emf becomes more positive
because fewer electrons are produced in the reaction
what does increasing the pressure do to the Emf value (why)
cell Emf becomes more negative
because more electrons are produced in the reaction
what two things is the standard Emf of a cell directly proportional to
lnK (K is equilibrium constant)
total entropy change (deltaS total)
what is the limitation of determining feasibility from standard cell potential (2)
although it will tell you if reaction is thermodynamically feasible, it does not take into account the kinetics of a reaction
standard cell potential relies on conditions being standard, when in reality, system may deviate from standard conditions
how to determine if a disproportionation reaction is feasible
if the overall cell potential is positive
how can electrochemical cells have commercial use
they can be produced to be non-rechargeable, rechargeable or fuel cells
what is a fuel cell
a type of electrochemical cell used to generate an electrical current without needing to be recharged
how do fuel cells work
a fuel undergoes combustion in oxygen
the energy released is used to generate a voltage
how does a hydrogen fuel cell work
uses a continuous supply of hydrogen and oxygen from the air
to generate a continuous current
what are the pros and cons of hydrogen fuel cells
pro - water is the only waste product - environmentally friendly
con - hydrogen is highly flammable - dangerous to transport
con - expensive to produce
what is reduced/oxidised in the redox titration between iron ions and manganate ions
iron ions are oxidised
manganate ions are reduced
what is the endpoint of the iron and potassium manganate titration
pale pink
what is reduced/oxidised in the redox titration between iodine and sodium thiosulphate
thiosulphate ions are oxidised
iodine is reduced
